CHAPTER II.

VOCABULARY OF PHOTOGRAPHIC CHEMICALS.

ACETIC ACID.

Symbol, C{4}H{3}O{3} + HO. Atomic weight, 60.

Acetic Acid is a product of the _oxidation_ of Alcohol. Spirituous liquids, when perfectly pure, are not affected by exposure to air; but if a portion of yeast, or Nitrogenous organic matter of any kind, be added, it soon acts as a _ferment_, and causes the spirit to unite with oxygen derived from the atmosphere, and so to become sour from formation of Acetic Acid, or "vinegar."

Acetic Acid is also produced on a large scale by heating _wood_ in close vessels: a substance distils over which is Acetic Acid contaminated with empyreumatic and tarry matter; it is termed Pyroligneous Acid, and is much used in commerce.

The most concentrated Acetic Acid may be obtained by neutralizing common vinegar with Carbonate of Soda, and crystallizing out the Acetate of Soda so formed; this Acetate of Soda is then distilled with Sulphuric Acid, which removes the Soda and liberates Acetic Acid: the Acetic Acid being volatile, distils over, and may be condensed.

_Properties of Acetic Acid._--The strongest acid contains only a single atom of water; it is sold under the name of "Glacial Acetic Acid," so called from its property of solidifying at a moderately low temperature. At about 50° the crystals melt, and form a limpid liquid of pungent odour and a density nearly corresponding to that of water; the specific gravity of Acetic Acid however is no test of its real strength, which can only be estimated by analysis.

The commercial _Glacial_ Acetic Acid is often diluted with water, which may be suspected if it does not solidify during the cold winter months. Sulphurous and Hydrochloric Acids are also common impurities. They are injurious in Photographic Processes, from their property of precipitating Nitrate of Silver. To detect them proceed as follows:--dissolve a small crystal of Nitrate of Silver in a few drops of water, and add to it about half a drachm of the Glacial Acid; the mixture should remain quite clear even when exposed to the light. Hydrochloric and Sulphurous Acid produce a white deposit of Chloride or Sulphite of Silver; and if _Aldehyde_ or volatile tarry matter be present in the Acetic Acid, the mixture with Nitrate of Silver, although clear at first, becomes discoloured by the action of light.

Glacial Acetic Acid sometimes has a smell of garlic. In this state it probably contains an organic Sulphur Acid, and is unfit for use.

Many employ a cheaper form of Acetic Acid, sold by druggists as "Beaufoy's" acid; it should be of the strength of the Acetic Acid fortiss. of the London Pharmacopœia, containing 30 per cent, real acid. It will be advisable to test it for Sulphuric Acid (see Sulphuric Acid), and other impurities, before use.

ACETATE OF SILVER. _See_ Silver, Acetate of.

ALBUMEN.

Albumen is an organic principle found both in the animal and vegetable kingdom. Its properties are best studied in the _white of egg_, which is a very pure form of Albumen.

Albumen is capable of existing in two states; in one of which it is soluble, in the other insoluble, in water. The aqueous solution of the soluble variety gives a slightly alkaline reaction to test-paper; it is somewhat thick and glutinous, but becomes more fluid on the addition of a small quantity of an alkali, such as Potash or Ammonia.

Soluble Albumen may be converted into the _insoluble_ form in the following ways:--

1. _By the application of heat._--A moderately strong solution of Albumen becomes opalescent and coagulates on being heated to about 150° Fahrenheit, but a temperature of 212° is required if the liquid is very dilute. A layer of _dried_ Albumen cannot easily be coagulated by the mere application of heat.

2. _By addition of strong acids._--Nitric Acid coagulates Albumen perfectly without the aid of heat. Acetic Acid however acts differently, appearing to enter into combination with the Albumen, and forming a compound soluble in warm water acidified by Acetic Acid.

3. _By the action of metallic salts._--Many of the salts of the metals coagulate Albumen completely. Nitrate of Silver does so; also the Bichloride of Mercury. Ammoniacal Oxide of Silver however does not coagulate Albumen.

The white precipitate formed on mixing Albumen with Nitrate of Silver is a chemical compound of the animal matter with Protoxide of Silver. This substance, which has been termed Albuminate of Silver, is soluble in Ammonia and Hyposulphite of Soda; but after exposure to light, or heating in a current of Hydrogen gas, it assumes a brick-red colour, being probably reduced to the condition of an organic compound of a _Suboxide_ of Silver. It is then almost insoluble in Ammonia, but enough dissolves to tinge the liquid wine-red. The _red coloration_ of solution of Nitrate of Silver employed in sensitizing the Albuminized photographic paper is probably produced by the same compound, although, often referred to the presence of Sulphuret of Silver.

Albumen also combines with Lime and Baryta. When Chloride of Barium is used with Albumen, a white precipitate of this kind usually forms.

_Chemical composition of Albumen._--Albumen belongs to the _Nitrogenous_ class of organic substances (see page 325). It also contains small quantities of Sulphur and Phosphorus.

ALCOHOL.

Symbol, C{4}H{6}O{2}. Atomic weight, 46.

Alcohol is obtained by the careful distillation of any spirituous or fermented liquor. If wine or beer be placed in a retort, and heat applied, the Alcohol, being more volatile than water, rises first, and is condensed in an appropriate receiver; a portion of the vapour of water however passes over with the Alcohol, and dilutes it to a certain extent, forming what is termed "Spirits of Wine." Much of this water may be removed by redistillation from Carbonate of Potash, in the manner described at page 196 of this work; but in order to render the Alcohol thoroughly _anhydrous_, it is necessary to employ _Quicklime_, which possesses a still greater attraction for water. An equal weight of this powdered lime is mixed with strong Alcohol of ·823, and the two are distilled together.

_Properties of Alcohol._--Pure anhydrous Alcohol is a limpid liquid, of an agreeable odour and pungent taste; sp. gr. at 60°, ·794. It absorbs vapour of water, and becomes diluted by exposure to damp air; boils at 173° Fahr. It has never been frozen.

Alcohol distilled from Carbonate of Potash has a specific gravity of ·815 to ·823, and contains 90 to 93 per cent, of real spirit.

The specific gravity of ordinary rectified Spirits of Wine is usually about ·840, and it contains 80 to 83 per cent, of absolute Alcohol.

AMMONIA.

Symbol, NH{3} or NH{4}O. Atomic weight, 17.

The liquid known by this name is an aqueous solution of the volatile gas Ammonia. Ammoniacal gas contains one atom of Nitrogen combined with three of Hydrogen: these two elementary bodies exhibit no affinity for each other, but they can be made to unite under certain circumstances, and the result is Ammonia.

_Properties of Ammonia._--Ammoniacal gas is soluble in water to a large extent; the solution possessing those properties which are termed alkaline (see page 308). Ammonia however differs from the other alkalies in one important particular--it is volatile: hence the original colour of turmeric-paper affected by Ammonia is restored on the application of heat. Solution of Ammonia absorbs Carbonic Acid rapidly from the air, and is converted into Carbonate of Ammonia; it should therefore be preserved in stoppered bottles. Besides Carbonate, commercial Ammonia often contains Chloride of Ammonium, recognized by the white precipitate given by Nitrate of Silver after acidifying with pure Nitric Acid.

The strength of commercial Ammonia varies greatly; that sold for pharmaceutical purposes under the name of Liquor Ammoniæ, contains about 10 per cent, of real Ammonia. The sp. gr. of aqueous Ammonia diminishes with the proportion of Ammonia present, the Liquor Ammoniæ being usually about ·936.

Ammonia, although forming a large class of salts, appears at first sight to contrast strongly in composition with the alkalies proper, such as Potash and Soda. Mineral bases generally are _protoxides of metals_, as already shown at page 308, but Ammonia consists simply of Nitrogen and Hydrogen united without Oxygen. The following remarks may perhaps tend somewhat to elucidate the difficulty:--

_Theory of Ammonium._--This theory supposes the existence of a substance possessing the properties of a _metal_, but differing from metallic bodies generally in being _compound_ in structure: the formula assigned to it is NH{4}, one atom of Nitrogen united with four of Hydrogen. This hypothetical metal is termed "Ammonium;" and Ammonia, associated with an atom of water, may be viewed as its _Oxide_, for NH{3} + HO plainly equals NH{4}O. Thus, as Potash is the Oxide of _Potassium_, so Ammonia is the Oxide of _Ammonium_.

The composition of the _salts_ of Ammonia is on this view assimilated to those of the alkalies proper. Thus, Sulphate of Ammonia is a Sulphate of the Oxide of Ammonium; Muriate or Hydrochlorate of Ammonia is a Chloride of Ammonium, etc.

AMMONIO-NITRATE OF SILVER. _See_ Silver, Ammonio-Nitrate of.

AQUA-REGIA. _See_ Nitro-Hydrochloric Acid.

BARYTA, NITRATE OF. _See_ Nitrate of Baryta.

BICHLORIDE OF MERCURY. _See_ Mercury, Bichloride of.

BROMINE.

Symbol, Br. Atomic weight, 78.

This elementary substance is obtained from the uncrystallizable residuum of sea-water, termed _bittern_. It exists in the water in very minute proportion, combined with Magnesium in the form of a soluble Bromide of Magnesium.

_Properties._--Bromine is a deep reddish-brown liquid of a disagreeable odour, and fuming strongly at common temperatures; sparingly soluble in water (1 part in 23, Löwig), but more abundantly so in Alcohol, and especially in Ether. It is very heavy, having a specific gravity of 3·0.

Bromine is closely analogous to Chlorine and Iodine in its chemical properties. It stands on the list intermediately between the two; its affinities being stronger than those of Iodine, but weaker than Chlorine (see Chlorine).

It forms a large class of salts, of which the Bromides of Potassium, Cadmium, and Silver are the most familiar to Photographers.

BROMIDE OF POTASSIUM.

Symbol, KBr. Atomic weight, 118.

Bromide of Potassium is prepared by adding Bromine to Caustic Potash, and heating the product, which is a mixture of Bromide of Potassium and Bromate of Potash, to redness, in order to drive off the Oxygen from the latter salt. It crystallizes in anhydrous cubes, like the Chloride and Iodide of Potassium; it is easily soluble in water, but more sparingly so in Alcohol; it yields red fumes of Bromine when acted upon by Sulphuric Acid.

BROMIDE OF SILVER. _See_ Silver, Bromide of.

CARBONATE OF SODA.

Symbol, NaO CO{2} + 10 Aq.

This salt was formerly obtained from the ashes of seaweeds, but is now more economically manufactured on a large scale from common salt. The Chloride of Sodium is first converted into Sulphate of Soda, and afterwards the Sulphate into Carbonate of Soda.

_Properties._--The perfect crystals contain ten atoms of water, which are driven off by the application of heat, leaving a white powder--the anhydrous Carbonate. _Common Washing Soda_ is a neutral Carbonate, contaminated to a certain extent with Chloride of Sodium and Sulphate of Soda. The Carbonate used for effervescing draughts is either a Bicarbonate with 1 atom of water, or a Sesquicarbonate, containing about 40 per cent, of real alkali; it is therefore nearly double as strong as the washing Carbonate, which contains about 22 per cent, of Soda. Carbonate of Soda is soluble in twice its weight of water at 60°, the solution being strongly alkaline.

CARBONATE OF POTASH. See Potash, Carbonate of.

CASEINE. _See_ Milk.

CHARCOAL, ANIMAL.

Animal Charcoal is obtained by heating animal substances, such as bones, dried blood, horns, etc., to redness, in close vessels, until all volatile empyreumatic matters have been driven off, and a residue of Carbon remains. When prepared from bones it contains a large quantity of inorganic matter in the shape of Carbonate and Phosphate of Lime, the former of which produces _alkalinity_ in reacting upon Nitrate of Silver (see p. 89). Animal Charcoal is freed from these earthy salts by repeated digestion in Hydrochloric Acid; but unless very carefully washed it is apt to retain an acid reaction, and so to liberate free Nitric Acid when added to solution of Nitrate of Silver.

_Properties._--Animal Charcoal, when pure, consists, solely of Carbon, and burns away in the air without leaving any residue: it is remarkable for its property of decolorizing solutions; the organic colouring substance being separated, but not actually _destroyed_, as it is by _Chlorine_ employed as a bleaching agent. This power of absorbing colouring matter is not possessed in an equal degree by all varieties of Charcoal, but is in great measure peculiar to those derived from the animal kingdom.

CHINA CLAY, OR KAOLIN.

This is prepared, by careful levigation, from mouldering granite and other disintegrated felspathic rocks. It consists of the _Silicate of Alumina_,--that is, of Silicic Acid or _Flint_, which is an Oxide of Silicon, united with the base Alumina (Oxide of Aluminum). Kaolin is perfectly insoluble in water and acids, and produces no decomposition in solution of Nitrate of Silver. It is employed by Photographers to decolorize solutions of Nitrate of Silver which have become brown from the action of Albumen or other organic matters.

Commercial Kaolin may contain chalk, in which state it produces alkalinity in solution of Nitrate of Silver. The impurity, detected by its effervescence with acids, is removed by washing the Kaolin in diluted vinegar and subsequently in water.

CHLORINE.

Symbol, Cl. Atomic weight, 36.

Chlorine is a chemical element found abundantly in nature, combined with metallic Sodium in the form of Chloride of Sodium, or Sea-salt.

_Preparation._--By distilling common Salt with Sulphuric Acid, Sulphate of Soda and Hydrochloric Acid are formed. Hydrochloric Acid contains Chlorine combined with Hydrogen; by the action of nascent Oxygen (see Oxygen), the Hydrogen may be removed in the form of water, and the Chlorine left alone.

_Properties._--Chlorine is a greenish-yellow gas, of a pungent and suffocating odour; soluble to a considerable extent in water, the solution possessing the odour and colour of the gas. It is nearly 2-1/2 times as heavy as a corresponding bulk of atmospheric air.

_Chemical properties._--Chlorine belongs to a small natural group of elements which contains also Bromine, Iodine, and Fluorine. They are characterized by having a strong affinity for Hydrogen, and also for the metals; but are comparatively indifferent to Oxygen. Many metallic substances actually undergo _combustion_ when projected into an atmosphere of Chlorine, the union between the two taking place with extreme violence. The characteristic bleaching properties of Chlorine gas are explained in the same manner:--Hydrogen is removed from the organic substance, and in that way the structure is broken up and the colour destroyed.

Chlorine is more powerful in its affinities than either Bromine or Iodine. The salts formed by these three elements are closely analogous in composition and often in properties. Those of the Alkalies, Alkaline Earths, and many of the Metals, are soluble in water; but the Silver salts are insoluble; the Lead salts sparingly so.

The combinations of Chlorine, Bromine, Iodine, and Fluorine, with Hydrogen, are acids, and neutralize Alkalies in the usual, manner, with formation of Alkaline Chloride and water (see page 311).

The test by which the presence of Chlorine is detected, either free or in combination with bases, is _Nitrate of Silver_; it gives a white curdy precipitate of Chloride of Silver, insoluble in Nitric Acid, but soluble in Ammonia. The solution of Nitrate of Silver employed as the test must not contain Iodide of Silver, as this compound is precipitated by dilution.

CHLORIDE OF AMMONIUM.

Symbol, NH{4}Cl. Atomic weight, 54.

This salt, also known as Muriate or Hydrochlorate of Ammonia, occurs in commerce in the form of colourless and translucent masses, which are procured by _sublimation_, the dry salt being volatile when strongly heated. It dissolves in an equal weight of boiling, or in three parts of cold water. It contains more Chlorine in proportion to the weight used than Chloride of Sodium, the atomic weights of the two being as 54 to 60.

CHLORIDE OF BARIUM.

Symbol, BaCl + 2 HO. Atomic weight, 123.

Barium is a metallic element very closely allied to Calcium, the elementary basis of Lime. The Chloride of Barium is commonly employed as a test for Sulphuric Acid, with which it forms an insoluble precipitate of Sulphate of Baryta. It is also said to affect the colour of the Photographic image when used in preparing Positive paper, which may possibly be due to a chemical combination of Baryta with Albumen; but it must be remembered that this Chloride, from its high atomic weight, contains less Chlorine than the alkaline Chlorides (see page 124).

_Properties of Chloride of Barium._--Chloride of Barium occurs in the form of white crystals, soluble in about two parts of water, at common temperature. These crystals contain two atoms of water of crystallization, which are expelled at 212°, leaving the anhydrous Chloride.

CHLORIDE OF GOLD. See Gold, Chloride of.

CHLORIDE OF SODIUM.

Symbol, NaCl. Atomic weight, 60.

Common Salt exists abundantly in nature, both in the form of solid rock-salt and dissolved in the waters of the ocean.

Properties of the pure Salt.--Fusible without decomposition at low redness, but sublimes at higher temperatures; the melted salt concretes into a hard white mass on cooling. Nearly insoluble in absolute alcohol, but dissolves in minute quantity in rectified spirit. Soluble in three parts of water, both hot and cold. Crystallizes in cubes, which are anhydrous.

_Impurities of Common Salt._--Table Salt often contains large quantities of the Chlorides of Magnesium and Calcium, which, being deliquescent, produce a dampness by absorption of atmospheric moisture: Sulphate of Soda is also commonly present. The salt may be purified by repeated re-crystallization, but it is more simple to prepare the pure compound _directly_, by neutralizing Hydrochloric Acid with Carbonate of Soda.

CHLORIDE OF SILVER. _See_ Silver, Chloride of.

CITRIC ACID.

This acid is found abundantly in lemon-juice and in lime-juice. It occurs in commerce in the form of large crystals, which are soluble in less than their own weight of water at 60°.

Commercial Citric Acid is sometimes mixed with Tartaric Acid. The adulteration may be discovered by making a concentrated solution of the acid and adding _Acetate of Potash_; crystals of Bitartrate of Potash will separate if Tartaric Acid be present.

Citric Acid is tribasic. It forms with Silver a white insoluble salt, containing 3 atoms of Oxide of Silver to 1 atom of Citric Acid. When the Citrate of Silver is heated in a current of Hydrogen gas, a part of the acid is liberated and the salt is reduced to a Citrate of Suboxide of Silver; which is of a red colour. The action of white light in reddening Citrate of Silver is shown by the Author to be of a similar nature.

CYANIDE OF POTASSIUM.

Symbol, KC{2}N, or KCy. Atomic weight, 66.

This salt is a compound of Cyanogen gas with the metal Potassium. Cyanogen is not an elementary body, like Chlorine or Iodine, but consists of Carbon and Nitrogen united in a peculiar manner. Although a compound substance, it reacts in the manner of an element, and is therefore (like Ammonium, previously described) an exception to the usual laws of chemistry. Many other bodies of a similar character are known.

Properties of Cyanide of Potassium.--These have been sufficiently described at page 44, to which the reader is referred.

ETHER.

Symbol, C{4}H{5}O. Atomic weight, 37.

Ether is obtained by distilling a mixture of Sulphuric Acid and Alcohol. If the formula of Alcohol (C{4}H{6}O{2}) be compared with that of Ether, it will be seen to differ from it in the possession of an additional atom of Hydrogen and of Oxygen: in the reaction the Sulphuric Acid removes these elements in the form of water, and by so doing converts one atom of Alcohol into an atom of Ether. The term Sulphuric applied to the commercial Ether has reference only to the manner of its formation.

Properties of Ether.--The properties of Ether have been described to some extent at pages 85 and 195. The following particulars however may be added. It is neither acid nor alkaline to test-paper. Specific gravity, at 60°, about ·720. Boils at 98° Fahrenheit. The vapour is exceedingly dense, and may be seen passing off from the liquid and falling to the ground: hence the danger of pouring Ether from one bottle to another if a flame be near at hand.

Ether does not mix with water in all proportions; if the two are shaken together, after a short time the former rises and floats upon the surface. In this way a mixture of Ether and Alcohol may be purified to some extent, as in the common process of washing Ether. The water employed however always retains a certain portion of Ether (about a tenth part of its bulk), and acquires a strong ethereal odour; washed Ether also contains water in small quantity.

Bromine and Iodine are both soluble in Ether, and gradually react upon and decompose it.

The strong alkalies, such as Potash and Soda, also decompose Ether slightly after a time, but not immediately. Exposed to air and light. Ether is oxidized and acquires a peculiar odour (page 85).

Ether dissolves fatty and resinous substances readily, but inorganic salts are mostly insoluble in this fluid. Hence it is that Iodide of Potassium and other substances dissolved in Alcohol are precipitated to a certain extent by the addition of Ether.

FLUORIDE OF POTASSIUM.

Symbol, KF. Atomic weight, 59.

_Preparation._--Fluoride of Potassium is formed by saturating Hydrofluoric Acid with Potash, and evaporating to dryness in a platinum vessel. Hydrofluoric Acid contains Fluorine combined with Hydrogen; it is a powerfully acid and corrosive liquid, formed by decomposing Fluor Spar, which is a Fluoride of Calcium, with strong Sulphuric Acid; the action which takes place being precisely analogous to that involved in the preparation of Hydrochloric Acid.

_Properties._--A deliquescent salt, occurring in small and imperfect crystals. Very soluble in water: the solution acting upon glass in the same manner as Hydrofluoric Acid.

FORMIC ACID.

Symbol, C{2}HO{3}. Atomic weight, 37.

This substance was originally discovered in the _red ant_ (_Formica rufa_), but it is prepared on a large scale by distilling Starch with Binoxide of Manganese and Sulphuric Acid.

_Properties._--The strength of commercial Formic Acid is uncertain, but it is always more or less dilute. The strongest acid, as obtained by distilling Formiate of Soda with Sulphuric Acid, is a fuming liquid with a pungent odour, and containing only one atom of water. It inflames the skin in the same manner as the sting of the ant.

Formic Acid reduces the Oxides of Gold, Silver, and Mercury to the metallic state, and is itself oxidized into Carbonic Acid. The alkaline formiates also possess the same properties.

GALLIC ACID.

Symbol, C{7}H{3}O{5} + H{3}O. Atomic weight, 94.

The chemistry of Gallic Acid is sufficiently described at page 27, to which the reader is referred.

GELATINE.

Symbol, C{13}H{10}O{5}N{2}. Atomic weight, 156.

This is an organic substance somewhat analogous to Albumen, but differing from it in properties. It is obtained by subjecting bones, hoofs, horns, calves' feet, etc., to the action of boiling water. The jelly formed on cooling is termed size, or, when dried and cut into slices, _glue_. Gelatine, as it is sold in the shops, is a pure form of Glue. _Isinglass_ is gelatine prepared, chiefly in Russia, from the air-bladders of certain species of sturgeon.

_Properties of Gelatine._--Gelatine softens and swells up in cold water, but does not _dissolve_ until heated: the hot solution, on cooling, forms a tremulous jelly. One ounce of cold water will retain about three grains of Isinglass without gelatinizing; but much depends upon the temperature, a few degrees greatly affecting the result.

When long boiled in water, and especially in presence of an acid, such as the Sulphuric, Gelatine undergoes a peculiar modification, and the Solution loses either partially or entirely its property of solidifying to a jelly.

GLYCERINE.

Fatty bodies are resolved by treatment with an alkali into an Acid--which combines with the alkali, forming a _soap_,--and Glycerine, remaining in solution.

Pure Glycerine, as obtained by Price's patent process of distillation, is a viscid liquid of sp. gr. about 1·23; miscible in all proportions with water and Alcohol. It is peculiarly a neutral substance, exhibiting no tendency to combine with acids or bases. It has little or no action upon Nitrate of Silver in the dark, and reduces it very slowly even when exposed to light.

GLYCYRRHIZINE.

Glycyrrhizine, obtained from the fresh root of Liquorice, is a substance intermediate in properties between a sugar and a resin. Sparingly soluble in water but very soluble in Alcohol. It precipitates strong solution of Nitrate of Silver white, but the deposit becomes reddened by exposure to light. Its preparation is described in the larger works on organic chemistry.

GOLD, CHLORIDE OF.

Symbol, AuCl{3}. Atomic weight, 303.

This salt is formed by dissolving pure metallic Gold in Nitro-hydrochloric Acid, and evaporating at a gentle heat. The solution affords deliquescent crystals of a deep orange colour.

Chloride of Gold, in a state fit for Photographic use, may easily be obtained by the following process:--Place a half-sovereign in any convenient vessel, and pour on it half a drachm of Nitric Acid mixed with two and a half drachms of Hydrochloric Acid and three drachms of water; digest by a gentle heat, but do not _boil_ the acid, or much of the Chlorine will be driven off in the form of gas. At the expiration of a few hours add fresh Aqua-Regia in quantity the same as at first, which will probably complete the solution, but if not, repeat the process a third time.

Lastly, neutralize the liquid by adding Carbonate of Soda until all effervescence ceases, and a green precipitate forms; this is _Carbonate of Copper_, which must be allowed several hours to separate thoroughly. The Chloride of Gold is thus freed from Copper and Silver, with which the metallic Gold is alloyed in the standard coin of the realm. The solution so prepared will be _alkaline_, and consequently prone to a reduction of metallic Gold: a slight extra quantity of Hydrochloric acid should therefore be added, sufficient to redden a piece of immersed litmus-paper.

The weight of a half-sovereign is about 61 grains, of which 56 grains are pure Gold. This is equivalent to 86 grains of Chloride of Gold, which will be the quantity contained in the solution.

The following process for preparing Chloride of Gold is more perfect than the last:--Dissolve the Gold coin in Aqua-Regia as before; then boil with excess of Hydrochloric Acid, to destroy the Nitric Acid,--dilute largely with distilled water, and add a filtered aqueous solution of common Sulphate of Iron (6 parts to 1 of Gold); collect the precipitated Gold, which is now free from copper; redissolve in Aqua-Regia, and evaporate to dryness on a water bath.

Avoid using _Ammonia_ to neutralize Chloride of Gold, as it would occasion a deposit of "Fulminating Gold," the properties of which are described in the next page.

_Properties of Chloride of Gold._--As sold in commerce it usually contains excess of Hydrochloric Acid, and is then of a bright yellow colour; but when neutral and somewhat concentrated, it is dark red (_Leo ruber_ of the alchemists). It gives no precipitate with Carbonate of Soda unless heat be applied; the free Hydrochloric Acid present forms, with the alkali. Chloride of Sodium, which unites with the Chloride of Gold, and produces a double salt, Chloride of Gold and Sodium, soluble in water.

Chloride of Gold is decomposed with precipitation of metallic Gold by Charcoal, Sulphurous Acid, and many of the vegetable acids; also by Protosulphate and Protonitrate of Iron. It tinges the cuticle of an indelible purple tint. It is soluble in Alcohol and in Ether.

GOLD, FULMINATING.

This is a yellowish-brown substance, precipitated on adding Ammonia to a strong solution of Chloride of Gold.

It may be dried carefully at 212°, but explodes violently on being heated suddenly to about 290°. Friction also causes it to explode when dry; but the moist powder may be rubbed or handled without danger. It is decomposed by Sulphuretted Hydrogen.

Fulminating Gold is probably an Aurate of Ammonia, containing 2 atoms of Ammonia to 1 atom of Peroxide of Gold.

GOLD, HYPOSULPHITE OF.

Symbol, AuO S{2}O{2}. Atomic weight, 253.

Hyposulphite of Gold is produced by the reaction of Chloride of Gold upon Hyposulphite of Soda (see page 133).

The salt sold in commerce as Sel d'or is a double Hyposulphite of Gold and Soda, containing one atom of the former salt to three of the latter, with four atoms of water of crystallization. It is formed by adding one part of Chloride of Gold, in solution, to three parts of Hyposulphite of Soda, and precipitating the resulting salt by Alcohol: the Chloride of Gold must be added to the Hyposulphite of Soda, and not the Soda salt to the Gold (see page 250).

Properties.--Hyposulphite of Gold is unstable and cannot exist in an isolated state, quickly passing into Sulphur, Sulphuric Acid, and metallic Gold. When combined with excess of Hyposulphite of Soda in the form of Sel d'or, it is more permanent.

Sel d'or occurs crystallized in fine needles, which are very soluble in water. The commercial article is often impure, containing little else than Hyposulphite of Soda, with a trace of Gold. It may be analyzed by adding a few drops of strong Nitric Acid (free from Chlorine), diluting with water, and afterwards collecting and igniting the yellow powder, which is metallic Gold.

GRAPE SUGAR.

Symbol, C{24}H{28}O{28}. Atomic weight, 396.

This modification of Sugar, often termed _Granular Sugar_, or _Glucose_, exists abundantly in the juice of grapes and in many other varieties of fruit. It forms the saccharine concretion found in honey, raisins, dried figs, etc. It may be produced artificially by the action of fermenting principles and of dilute mineral acids, upon Starch.

_Properties._--Grape Sugar crystallizes slowly and with difficulty from a concentrated aqueous solution, in small hemispherical nodules, which are hard, and feel gritty between the teeth. It is much less sweet to the taste than Cane Sugar, and not so soluble in water (1 part dissolves in 1-1/2 of cold water).

Grape Sugar tends to absorb Oxygen, and hence it possesses the property of decomposing the salts of the noble metals, and reducing them by degrees to the metallic state, even without the aid of light. _Cane_ Sugar does not possess these properties to an equal extent, and hence it is readily distinguished from the other variety. The product of the action of Grape Sugar upon Nitrate of Silver appears to be a very low form of Oxide of Silver combined with organic matter.

HONEY.

This substance contains two distinct kinds of Sugar, Grape Sugar, and an uncrystallizable substance analogous to, or identical with, the Treacle found associated with common Sugar in the cane-juice. The agreeable taste of Honey probably depends upon the latter, but its reducing power on metallic oxides is due to the former. Pure Grape Sugar can readily be obtained from inspissated Honey, by treating it with Alcohol, which dissolves out the syrup, but leaves the crystalline portion.

Much of the commercial article is adulterated, and, for Photographic use, the Virgin Honey should be obtained direct from the comb.

HYDROCHLORIC ACID.

Symbol, HCl. Atomic weight, 37.

Hydrochloric Acid is a volatile gas, which may be liberated from most of the salts termed Chlorides by the action of Sulphuric Acid. The acid, by its superior affinities, removes the base; thus,--

NaCl + HO SO{3} = NaO SO{3} + HCl.

Properties.--Abundantly soluble in water, forming the liquid Hydrochloric or Muriatic Acid of commerce. The most concentrated solution of Hydrochloric Acid has a sp. gr. 1·2, and contains about 40 per cent, of gas; that commonly sold is somewhat weaker, sp. gr. 1·14 = 28 per cent, real acid.

Pure Hydrochloric Acid is colourless, and fumes in the air. The yellow colour of the commercial acid depends upon the presence of traces of Perchloride of Iron, or of organic matter; commercial Muriatic Acid also often contains a portion of free Chlorine and of Sulphuric Acid.

HYDRIODIC ACID.

Symbol, HI. Atomic weight, 127.

This is a gaseous compound of Hydrogen and Iodine, corresponding in composition to the Hydrochloric Acid. It cannot however, from its instability, be obtained in the same manner, since, on distilling an Iodide with Sulphuric Acid, the Hydriodic Acid first formed is subsequently decomposed into Iodine and Hydrogen. An aqueous solution of Hydriodic Acid is easily prepared by adding Iodine to water containing Sulphuretted Hydrogen gas; a decomposition takes place, and Sulphur is set free: thus, HS + I = HI + s.

Properties.--Hydriodic Acid is very soluble in water, yielding a strongly acid liquid. The solution, colourless at first, soon becomes brown from decomposition, and liberation of free Iodine. It may be restored to its original condition by adding solution of Sulphuretted Hydrogen.

HYDROSULPHURIC ACID.

Symbol, HS. Atomic weight, 17.

This substance, also known as Sulphuretted Hydrogen, is a gaseous compound of Sulphur and Hydrogen, analogous in composition to the Hydrochloric and Hydriodic Acid. It is usually prepared by the action of dilute Sulphuric Acid upon Sulphuret of Iron, as described at page 373; the decomposition being similar to that involved in the preparation of the Hydrogen acids generally:--

FeS + HO SO{3} = FeO SO{3} + HS.

Properties.--Cold water absorbs three times its bulk of Hydrosulphuric Acid, and acquires the peculiar putrid odour and poisonous qualities of the gas. The solution is faintly acid to test-paper, and becomes opalescent on keeping, from gradual separation of Sulphur. It is decomposed by Nitric Acid, and also by Chlorine and Iodine. It precipitates Silver from its solutions in the form of black Sulphuret of Silver; also Copper, Mercury, Lead, etc.; but Iron and other metals of that class are not affected, if the liquid contains free acid. Hydrosulphuric Acid is constantly employed in the chemical laboratory for these and other purposes.

HYDROSULPHATE OF AMMONIA.

Symbol, NH{4}S HS. Atomic weight, 51.

The liquid known by this name, and formed on passing Sulphuretted Hydrogen gas into Ammonia, is a double Sulphuret of Hydrogen and Ammonium. In the preparation, the passage of the gas is to be continued until the solution gives no precipitate with Sulphate of Magnesia, and smells strongly of Hydrosulphuric Acid.

_Properties._--Colourless at first, but afterwards changes to yellow, from liberation and subsequent solution of Sulphur. Becomes milky on the addition of any acid. Precipitates, in the form of Sulphuret, all the metals which are affected by Sulphuretted Hydrogen, and, in addition, those of the class to which Iron, Zinc, and Manganese belong.

Hydrosulphate of Ammonia is employed in Photography to darken the Negative image, and also in the preparation of Iodide of Ammonium, the separation of Silver from Hyposulphite solutions, etc.

HYPOSULPHITE OF SODA.

Symbol, NaO S{2}O{2} + 5 HO. Atomic weight, 125.

The chemistry of Hyposulphurous Acid and the Hyposulphite of Soda has been sufficiently described at pages 43, 129, and 137 of the present Work. The crystallized salt includes five atoms of water of crystallization.

HYPOSULPHITE OF GOLD. _See_ Gold, Hyposulphite of.

HYPOSULPHITE OF SILVER. _See_ Silver, Hyposulphite of.

ICELAND MOSS.

_Cetraria Islandica._--A species of Lichen found in Iceland and the mountainous parts of Europe; when boiled in water, it first swells up, and then yields a substance which gelatinizes on cooling.

It contains Lichen Starch, a bitter principle soluble in Alcohol, termed "Cetrarine," and common Starch; traces of Gallic Acid and Bitartrate of Potash are also present.

IODINE.

Symbol, I. Atomic weight, 126.

Iodine is chiefly prepared at Glasgow, from _kelp_, which is the fused ash obtained on burning seaweeds. The waters of the ocean contain minute quantities of the Iodides of Sodium and Magnesium, which are separated and stored up by the growing tissues of the marine plant.

In the preparation, the mother-liquor of kelp is evaporated to dryness and distilled with Sulphuric Acid; the Hydriodic Acid first liberated is decomposed by the high temperature, and fumes of Iodine condense in the form of opaque crystals.

_Properties._--Iodine has a bluish-black colour and metallic lustre; it stains the skin yellow, and has a pungent smell, like diluted Chlorine. It is extremely volatile when moist, boils at 350°, and produces dense violet-coloured fumes, which condense in brilliant plates. Specific gravity 4·946. Iodine is very sparingly soluble in water, 1 part requiring 7000 parts for perfect solution; even this minute quantity however tinges the liquid of a brown colour. Alcohol and Ether dissolve it more abundantly, forming dark-brown solutions. Iodine also dissolves freely in solutions of the alkaline Iodides, such as the Iodide of Potassium, of Sodium, and of Ammonium.

_Chemical Properties._--Iodine belongs to the Chlorine group of elements, characterized by forming acids with Hydrogen, and combining extensively with the metals (see Chlorine). They are however comparatively indifferent to Oxygen, and also to each other. The Iodides of the alkalies and alkaline earths are soluble in water; also those of Iron, Zinc, Cadmium, etc. The Iodides of Lead, Silver, and Mercury are nearly or quite insoluble.

Iodine possesses the property of forming a compound of a deep blue colour with Starch. In using this as a test, it is necessary first to liberate the Iodine (if in combination) by means of Chlorine, or Nitric Acids saturated with Peroxide of Nitrogen. The presence of Alcohol or Ether interferes to a certain extent with the result.

IODIDE OF AMMONIUM.

Symbol, NH{4}I. Atomic weight, 144.

The preparation and properties of this salt are described at page 198, to which the reader is referred.

IODIDE OF CADMIUM.

Symbol, CdI. Atomic weight, 182.

See page 199, for the preparation and properties of this salt.

IODIDE OF IRON.

Symbol, FeI. Atomic weight, 154.

Iodide of Iron is prepared by digesting an excess of Iron filings with solution of Iodine in Alcohol. It is very soluble in water and Alcohol, but the solution rapidly absorbs Oxygen and deposits Peroxide of Iron; hence the importance of preserving it in contact with metallic Iron, with which the separated Iodine may recombine. By very careful evaporation, hydrated crystals of Proto-iodide may be obtained, but the composition of the solid salt usually sold under that name cannot be depended on.

The _Periodide_ of Iron, corresponding to the _Perchloride_, has not been examined, and it is doubtful if any such compound exists.

IODIDE OF POTASSIUM.

Symbol, KI. Atomic weight, 166.

This salt is usually formed by dissolving Iodine in solution of Potash until it begins to acquire a brown colour; a mixture of Iodide of Potassium and _Iodate of Potash_ (KO IO{5}) is thus formed; but by evaporation and heating to redness, the latter salt parts with its Oxygen, and is converted into Iodide of Potassium.

_Properties._--It forms cubic and prismatic crystals, which should be hard, and _very slightly or not at all deliquescent_. Soluble in less than an equal weight of water at 6O°; it is also soluble in Alcohol, but not in Ether. The proportion of Iodide of Potassium contained in a saturated alcoholic solution, varies with the strength of the spirit:--with common Spirits of Wine, sp. gr. ·836, it would be about 8 grains to the drachm; with Alcohol rectified from Carbonate of Potash, sp. gr. ·823, 4 or 5 grains; with absolute Alcohol, 1 to 2 grains. The solution of Iodide of Potassium is instantly coloured brown by free Chlorine; also very rapidly by Peroxide of Nitrogen (page 86); ordinary acids however act less quickly, Hydriodic Acid being first formed, and subsequently decomposing spontaneously.

The impurities of commercial Iodide of Potassium, with the means to be adopted for their removal, are fully given at page 197.

IODIDE OF SILVER. _See_ Silver, Iodide of.

IODOFORM.

The composition of this substance is analogous to that of Chloroform, Iodine being substituted for Chlorine. It is obtained on boiling together Iodine, Carbonate of Potash, and Alcohol.

Iodoform occurs in yellow nacrous crystals, which have a saffron-like odour. It is insoluble in water, but soluble in spirit.

IRON, PROTOSULPHATE OF.

Symbol, FeO SO{3} + 7 HO. Atomic weight, 139.

The properties of this salt, and of the two salifiable Oxides of Iron, are described at page 29. It dissolves in rather more than an equal weight of cold water, or in less of boiling water.

Aqueous solution of Sulphate of Iron absorbs the Binoxide of Nitrogen, acquiring a deep olive-brown colour: as this gaseous Binoxide is itself a reducing agent, the liquid so formed has been proposed as a more energetic developer than the Sulphate of Iron alone (?).

IRON, PROTONITRATE OF.

Symbol, FeO NO{3} + 7 HO. Atomic weight, 153.

This salt, by careful evaporation _in vacuo_ over Sulphuric Acid, forms transparent crystals, of a light green colour, and containing 7 atoms of water, like the Protosulphate. It is exceedingly unstable, and soon becomes red from decomposition, unless preserved from contact with air. The preparation of solution of Protonitrate of Iron for developing Collodion Positives, is given at page 206.

IRON, PERCHLORIDE OF.

Symbol, Fe{2}Cl{3}. Atomic weight, 164.

There are two Chlorides of Iron, corresponding in composition to the Protoxide and the Sesquioxide respectively. The Protochloride is very soluble in water, forming a green solution, which precipitates a dirty white Protoxide on the addition of an alkali. The Perchloride, on the other hand, is dark brown, and gives a foxy-red precipitate with alkalies.

_Properties._--Perchloride of Iron may be obtained in the solid form by heating Iron wire in excess of Chlorine; it condenses in the shape of brilliant and iridescent brown crystals, which are volatile, and dissolve in water, the solution being acid to test-paper. It is also soluble in Alcohol, forming the Tinctura Ferri Sesquichloridi of the Pharmacopœia. Commercial Perchloride of Iron ordinarily contains an excess of Hydrochloric Acid.

LITMUS.

Litmus is a vegetable substance prepared from various _lichens_, which are principally collected on rocks adjoining the sea. The colouring matter is extracted by a peculiar process, and afterwards made up into a paste with chalk, plaster of Paris, etc.

Litmus occurs in commerce in the form of small cubes of a fine violet colour. In using it for the preparation of test-papers, it is digested in hot water, and sheets of porous paper are soaked in the blue liquid so formed. The red papers are prepared at first in the same manner, but afterwards placed in water which has been rendered faintly acid with Sulphuric or Hydrochloric Acid.

MERCURY, BICHLORIDE OF.

Symbol, HgCl{2}. Atomic weight, 274.

This salt, also called Corrosive Sublimate, and sometimes _Chloride of Mercury_ (the atomic weight of Mercury being halved), may be formed by heating Mercury in excess of Chlorine, or more economically, by subliming a mixture of Persulphate of Mercury and Chloride of Sodium.

_Properties._--A very corrosive and poisonous salt, usually sold in semi-transparent, crystalline masses, or in the state of powder. Soluble in 16 parts of cold, and in 3 of hot water; more abundantly so in Alcohol, and also in Ether. The solubility in water may be increased by the addition of free Hydrochloric Acid, or of Chloride of Ammonium.

The Protochloride of Mercury is an insoluble white powder, commonly known under the name of _Calomel_.

METHYLIC ALCOHOL.

This liquid, known also by the names of _wood naphtha_ and _pyroxylic spirit_, is one of the volatile products of the destructive distillation of wood. It is very volatile and limpid, with a pungent odour.

By a recent excise regulation, ordinary Spirit mixed with ten per cent, of wood naphtha is sold free of duty, under the name of "Methylated Spirit."

MILK.

The Milk of herbivorous animals contains three principal constituents--Fatty matter, Caseine, and Sugar; in addition to these, small quantities of the Chloride of Potassium, and of Phosphates of Lime and Magnesia, are present.

The fatty matter is contained in small cells, and forms the greater part of the cream which rises to the surface of the milk on standing; hence shimmed milk is to be preferred for Photographic use.

The second constituent, Caseine, is an organic principle somewhat analogous to Albumen in composition and properties. Its aqueous solution however does not, like Albumen, _coagulate_ on boiling, unless _an acid_ be present, which probably removes a small portion of alkali with which the Caseine was previously combined. The substance termed "rennet," which is the dried stomach of the calf, possesses the property of coagulating Caseine, but the exact mode of its action is unknown. Sherry-wine is also commonly employed to curdle Milk; but brandy and other spirituous liquids, when free from acid and astringent matter, have no effect.

In all these cases a portion of the Caseine usually remains in a soluble form in the _whey_; but when the Milk is coagulated by the addition of acids, the quantity so left is very small, and hence the use of the rennet is to be preferred, since the presence of Caseine facilitates the reduction of the sensitive Silver salts.

Caseine combines with Oxide of Silver in the same manner as Albumen, forming a white coagulum, which becomes _brick-red_ on exposure to light.

Sugar of Milk, the third principal constituent, differs from both cane and grape sugar; it may be obtained by evaporating _whey_ until crystallization begins to take place. It is hard and gritty, and only slightly sweet; slowly soluble, without forming a syrup, in about two and a half parts of boiling, and six of cold water. It does not ferment and form Alcohol on the addition of yeast, like grape sugar, but by the action of _decomposing animal matter_ is converted into Lactic Acid.

When skimmed Milk is exposed to the air for some hours, it gradually becomes _sour_, from Lactic Acid formed in this way; and if then heated to ebullition, the Caseine coagulates very perfectly.

NITRIC ACID.

Symbol, NO{5}. Atomic weight, 54.

Nitric Acid, or _Aqua-fortis_, is prepared by adding Sulphuric Acid to Nitrate of Potash, and distilling the mixture in a retort. Sulphate of Potash and free Nitric Acid are formed, the latter of which, being volatile, distils over in combination with one atom of water previously united with the Sulphuric Acid.

_Properties._--Anhydrous Nitric Acid is a solid substance, white and crystalline, but it cannot be prepared except by an expensive and complicated process.

The concentrated _liquid_ Nitric Acid contains 1 atom of water, and has a sp. gr. of about 1·5; if perfectly pure, it is colourless, but usually it has a slight yellow tint, from partial decomposition into Peroxide of Nitrogen: it fumes strongly in the air.

The strength of commercial Nitric Acid is subject to much variation. An acid of sp. gr. 1·42, containing about 4 atoms of water, is commonly met with. If the specific gravity is much lower than this (less than 1·36), it will scarcely be adapted for the preparation of Pyroxyline. The yellow _Nitrous Acid_, so called, is a strong Nitric Acid partially saturated with the brown vapours of Peroxide of Nitrogen; it has a high specific gravity, but this is somewhat deceptive, being caused in part by the presence of the Peroxide. On mixing with Sulphuric Acid, the colour disappears, a compound being formed which has been termed a _Sulphate of Nitrous Acid_.

In the Appendix a Table is given which exhibits the quantity of real anhydrous Nitric Acid contained in samples of different densities.

_Chemical Properties._--Nitric Acid is a powerful oxidizing agent (see page 13); it dissolves all the common metals, with the exception of Gold and Platinum. Animal substances, such as the cuticle, nails, etc., are tinged of a permanent yellow colour, and deeply corroded by a prolonged application. Nitric Acid forms a numerous class of salts, _all of which are soluble in water_. Hence its presence cannot be determined by any precipitating reagent, in the same manner as that of Hydrochloric and Sulphuric Acid.

_Impurities of Commercial Nitric Acid._--These are principally Chlorine and Sulphuric Acid; also Peroxide of Nitrogen, which tinges the acid yellow, as already described. Chlorine is detected by diluting the acid with an equal bulk of distilled water, and adding a few drops of Nitrate of Silver,--a _milkiness_, which, is Chloride of Silver in suspension, indicates the presence of Chlorine. In testing for Sulphuric Acid, dilute the Nitric Acid as before, and drop in _a single drop_ of solution of Chloride of Barium; if Sulphuric Acid be present, an insoluble precipitate of Sulphate of Baryta will be formed.

NITROUS ACID. _See_ Silver, Nitrite of.

NITRATE OF POTASH.

Symbol, KO NO{5}. Atomic weight, 102.

This salt, also termed _Nitre_, or _Saltpetre_, is an abundant natural product, found effloresced upon the soil in certain parts of the East Indies. It is also produced artificially in what are called Nitre-beds.

The properties of Nitrate of Potash are described as far as necessary at page 190.

NITRATE OF BARYTA.

Symbol, BaO NO{5}. Atomic weight, 131.

Nitrate of Baryta forms octahedral crystals, which are anhydrous. It is considerably less soluble than the Chloride of Barium, requiring 12 parts of cold and 4 of boiling water for solution. It may be substituted for the Nitrate of Lead in the preparation of Protonitrate of Iron.

NITRATE OF LEAD.

Symbol, PbO NO{5}. Atomic weight, 166.

Nitrate of Lead is obtained by dissolving the metal, or the Oxide of Lead, in _excess_ of Nitric Acid, diluted with 2 parts of water. It crystallizes on evaporation in white anhydrous tetrahedra and octahedra, which are hard, and decrepitate on being heated; they are soluble in 8 parts of water at 60°.

Nitrate of Lead forms with Sulphuric Acid, or soluble Sulphates, a white precipitate, which is the insoluble Sulphate of Lead. The _Iodide_ of Lead is also very sparingly soluble in water.

NITRATE OF SILVER, _See_ Silver, Nitrate or.

NITRO-GLUCOSE.

When 3 fluid ounces of cold Nitro-Sulphuric Acid, consisting of 2 ounces of Oil of Vitriol and 1 ounce of highly concentrated Nitric Acid, are mixed with 1 ounce of finely powdered Cane Sugar, there is formed at first a thin, transparent, pasty mass. If it is stirred with a glass rod for a few minutes without interruption, the paste coagulates as it were, and separates from the liquid as a thick tenacious mass, aggregating into lumps, which can easily be removed from the acid mixture.

This substance has a very acid and intensely bitter taste. Kneaded in warm water until the latter no longer reddens litmus-paper, it acquires a silver colour and a beautiful silky lustre. It may be used in Photography to confer intensity upon newly mixed Collodion; but is inferior to Glycyrrhizine employed for the same purpose.

NITRO-HYDROCHLORIC ACID.

Symbol, NO{4} + Cl.

This liquid is the Aqua-Regia of the old alchemists. It is produced by mixing Nitric and Hydrochloric Acids: the Oxygen contained in the former combines with the Hydrogen of the latter, forming water and liberating Chlorine, thus:--

NO{5} + HCl = NO{4} + HO + Cl.

The presence of free Chlorine confers on the mixture the power of dissolving Gold and Platinum, which neither of the two acids possesses separately. In preparing Aqua-Regia it is usual to mix one part, by measure, of Nitric Acid with four of Hydrochloric Acid, and to dilute with an equal bulk of water. The application of a gentle heat assists the solution of the metal; but if the temperature rises to the boiling point, a violent effervescence and escape of Chlorine takes place.

NITRO-SULPHURIC ACID.

For the chemistry of this acid liquid, see page 77.

OXYGEN.

Symbol, O. Atomic weight, 8.

Oxygen gas may be obtained by heating Nitrate of Potash to redness, but in this case it is contaminated with a portion of Nitrogen. The salt termed Chlorate of Potash (the composition of which is closely analogous to that of the Nitrate, Chlorine being substituted for Nitrogen) yields abundance of pure Oxygen gas on the application of heat, leaving behind Chloride of Potassium.

_Chemical Properties._--Oxygen combines eagerly with many of the chemical elements, forming Oxides. This chemical affinity however is not well seen when the elementary body is exposed to the action of _Oxygen in the gaseous form_. It is the _nascent_ Oxygen which acts most powerfully as an oxidizer. By nascent Oxygen is meant Oxygen on the point of separation from other elementary atoms with which it was previously associated; it may then be considered to be in the liquid form, and hence it comes more perfectly into contact with the particles of the body to be oxidized.

Illustrations of the superior chemical energy of nascent Oxygen are numerous, but none perhaps are more striking than the mild and gradual oxidizing influence exerted by atmospheric air, as compared with the violent action of Nitric Acid and bodies of that class which contain Oxygen loosely combined.

OXYMEL.

This syrup of Honey and Vinegar is prepared as follows. Take of

Honey 1 pound. Acid, Acetic, fortiss. (Beaufoy's Acid) 11 drachms. Water 13 drachms.

Stand the pot containing the Honey in boiling water until a scum rises to the surface, which is to be removed two or three times. Then add the Acetic Acid and water, and skim once more if required. Allow to cool, and it will be fit for use.

POTASH.

Symbol, KO + HO. Atomic weight, 57.

Potash is obtained by separating the Carbonic Acid from Carbonate of Potash by means of Caustic Lime. Lime is a more feeble base than Potash, but the Carbonate of Lime, being _insoluble_ in water, is at once formed on adding Milk of Lime to a solution of Carbonate of Potash (see page 314).

_Properties._--Usually met with in the form of solid lumps, or in cylindrical sticks, which are formed by melting the Potash and running it into a mould. It always contains one atom of water, which cannot be driven off by the application of heat.

Potash is soluble almost to any extent in water, much heat being evolved. The solution is powerfully alkaline (p. 308), and acts rapidly upon the skin; it dissolves fatty and resinous bodies, converting them into soaps. Solution of Potash absorbs Carbonic Acid quickly from the air, and should therefore be preserved in stoppered bottles; the glass stoppers must be wiped occasionally, in order to prevent them from becoming immovably fixed by the solvent action of the Potash upon the Silica of the glass.

The Liquor Potassæ of the London Pharmacopœia has a sp. gr. of 1·063, and contains about 5 per cent, of real Potash. It is usually contaminated with _Carbonate_ of Potash, which causes it to effervesce on the addition of acids; also, to a less extent, with Sulphate of Potash, Chloride of Potassium, Silica, etc.

POTASH, CARBONATE OF.

Symbol, KO CO{2}. Atomic weight, 70.

The impure Carbonate of Potash, termed _Pearlash_, is obtained from the ashes of wood and vegetable matter, in the same manner as Carbonate of Soda is prepared from the ashes of seaweeds. Salts of Potash and of Soda appear essential to vegetation, and are absorbed and approximated by the living tissues of the plant. They exist in the vegetable structure, combined with organic acids in the form of salts, like the Oxalate, Tartrate, etc., which, when burned are converted into Carbonates.

_Properties._--The Pearlash of commerce contains large and variable quantities of Chloride of Potassium, Sulphate of Potash, etc. A purer Carbonate is sold, which is free from Sulphates, and with only a trace of Chlorides. Carbonate of Potash is a strongly alkaline salt, deliquescent, and soluble in twice its weight of cold water; insoluble in Alcohol, and employed to deprive it of water (see page 196).

PYROGALLIC ACID.

Symbol, C{8}H{4}O{4} (Stenhouse). Atomic weight, 84.

The chemistry of Pyrogallic Acid has been described at page 28.

SEL D'OR. _See_ Gold, Hyposulphite of.

SILVER.

Symbol, Ag. Atomic weight, 108.

This metal, the _Luna_ or _Diana_ of the alchemists, is found native in Peru and Mexico; it occurs also in the form of Sulphuret of Silver.

When pure it has a sp. gr. of 10·5, and is very malleable and ductile; melts at a bright red heat. Silver does not oxidize in the air, but when exposed to an impure atmosphere containing traces of Sulphuretted Hydrogen, it is slowly tarnished from formation of Sulphuret of Silver. It dissolves in Sulphuric Acid, but the best solvent is Nitric Acid.

The standard coin of the realm is an alloy of Silver and Copper, containing about one-eleventh of the latter metal.

To prepare pure Nitrate of Silver from it, dissolve in Nitric Acid and evaporate until crystals are obtained. Then wash the crystals with a little dilute Nitric Acid, redissolve them in water, and crystallize by evaporation a second time. Lastly, fuse the product at a moderate heat, in order to expel the last traces of Nitric and Nitrous Acids.

SILVER, AMMONIO-NITRATE OF.

Crystallized Nitrate of Silver absorbs Ammoniacal gas rapidly, with production of heat sufficient to fuse the resulting compound, which is white, and consists of 100 parts of the Nitrate + 29·5 of Ammonia. The compound however which Photographers employ under the name of Ammonio-Nitrate of Silver may be viewed more simply as a solution of the Oxide of Silver in Ammonia, without reference to the Nitrate of Ammonia necessarily produced in the reaction.

Very strong Ammonia, in acting upon Oxide of Silver, converts it into a black powder, termed _Fulminating Silver_, which possesses the most dangerous explosive properties. Its composition is uncertain. In preparing Ammonio-Nitrate of Silver by the common process, the Oxide first precipitated occasionally leaves a little black powder behind, on re-solution; this does not appear however, according to the observations of the Author, to be Fulminating Silver.

In sensitizing salted paper by the Ammonio-Nitrate of Silver, _free Ammonia_ is necessarily formed. Thus--

Chloride of Ammonium + Oxide of Silver in Ammonia = Chloride of Silver + Ammonia + Water.

SILVER, OXIDE OF.

Symbol, AgO. Atomic weight, 116.

This compound has already been described in Part I., page 17.

SILVER, CHLORIDE OF.

Symbol, AgCl. Atomic weight, 144.

The preparation and properties of Chloride of Silver are given in Part I. page 14.

SILVER, BROMIDE OF.

Symbol, AgBr. Atomic weight, 186.

See Part I. page 17.

SILVER, CITRATE OF. _See_ Citric Acid.

SILVER, IODIDE OF.

Symbol, AgI. Atomic weight, 234.

See Part I. page 16.

SILVER, FLUORIDE OF.

Symbol, AgF. Atomic weight, 127.

This compound differs from those last described in being soluble in water. The dry salt fuses on being heated, and is reduced by a higher temperature, or by exposure to light.

SILVER, SULPHURET OF.

Symbol, AgS. Atomic weight, 124.

This compound is formed by the action of Sulphur upon metallic Silver, or of Sulphuretted Hydrogen or Hydrosulphate of Ammonia upon the Silver salts; the decomposition of Hyposulphite of Silver also furnishes the black Sulphuret.

Sulphuret of Silver is insoluble in water, and nearly so in those substances which dissolve the Chloride, Bromide, and Iodide, such as Ammonia, Hyposulphites, Cyanides, etc.; but it dissolves in Nitric Acid, being converted into soluble Sulphate and Nitrate of Silver. (For a further account of the properties of the Sulphuret of Silver, see page 146.)

SILVER, NITRATE OF.

Symbol, AgO NO{5}. Atomic weight, 170.

The preparation and properties of this salt have been explained at pages 12 and 362.

SILVER, NITRITE OF.

Symbol, AgO NO{3}. Atomic weight, 154.

Nitrite of Silver is a compound of Nitrous Acid, or NO{3}, with Oxide of Silver. It is formed by heating Nitrate of Silver, so as to drive off a portion of its Oxygen, or more conveniently, by mixing Nitrate of Silver and Nitrite of Potash in equal parts, fusing strongly, and dissolving in a small quantity of boiling water: on cooling, the Nitrite crystallizes out, and may be purified by pressing in blotting-paper. Mr. Hadow describes an economical method of preparing Nitrite of Silver in quantity, viz. by heating 1 part of Starch in 8 of Nitric Acid of 1·25 specific gravity, and conducting the evolved gases into a solution of pure Carbonate of Soda until effervescence has ceased. The Nitrite of Soda thus formed is afterwards added to Nitrate of Silver in the usual way.

_Properties._--Nitrite of Silver is soluble in 120 parts of cold water; easily soluble in boiling water, and crystallizes, on cooling, in long slender needles. It has a certain degree of affinity for Oxygen, and tends to pass into the condition of Nitrate of Silver; but it is probable that its Photographic properties depend more upon a decomposition of the salt and liberation of Nitrous Acid.

_Properties of Nitrous Acid._--This substance possesses very feeble acid properties, its salts being decomposed even by Acetic Acid. It is an unstable body, and splits up, in contact with water, into Binoxide of Nitrogen and Nitric Acid. The Peroxide of Nitrogen, NO{4}, is also decomposed by water, and yields the same products.

SILVER, ACETATE OF.

Symbol, AgO (C{4}H{3}O{3}). Atomic weight, 167.

This is a difficultly soluble salt, deposited in lamellar crystals when an Acetate is added to a strong solution of Nitrate of Silver. If _Acetic Acid_ be used in place of an Acetate, the Acetate of Silver does not fall so readily, since the Nitric Acid which would then be liberated impedes the decomposition. Its properties have been sufficiently described at page 89.

SILVER, HYPOSULPHITE OF.

Symbol, AgO S{2}O{2}. Atomic weight, 164.

This salt is fully described in Part I. page 129. For the properties of the soluble double salt of Hyposulphite of Silver and Hyposulphite of Soda, see page 43.

SUGAR OF MILK. _See_ Milk.

SULPHURETTED HYDROGEN. _See_ Hydrosulphuric Acid.

SULPHURIC ACID.

Symbol, SO{3}. Atomic weight, 40.

Sulphuric Acid may be formed by oxidizing Sulphur with boiling Nitric Acid; but this plan would be too expensive to be adopted on a large scale. The commercial process for the manufacture of Sulphuric Acid is exceedingly ingenious and beautiful, but it involves reactions which are too complicated to admit of a superficial explanation. The Sulphur is first burnt into gaseous Sulphurous Acid (SO{2}), and then by the agency of Binoxide of Nitrogen gas, an additional atom of Oxygen is imparted from the atmosphere, so as to convert the SO{2} into SO{3}, or Sulphuric Acid.

_Properties._--Anhydrous Sulphuric Acid is a white crystalline solid. The strongest liquid acid always contains one atom of water, which is closely associated with it, and cannot be driven off by the application of heat.

This _mono-hydrated_ Sulphuric Acid, represented by the formula HO SO{3}, is a dense fluid, having a specific gravity of about 1·845; boils at 620°, and distils without decomposition. It is not volatile at common temperatures, and therefore does not _fume_ in the same manner as Nitric or Hydrochloric Acid. The concentrated acid maybe cooled down even to zero without solidifying; but a weaker compound, containing twice the quantity of water, and termed _glacial_ Sulphuric Acid, crystallizes at 40° Fahr. Sulphuric Acid is intensely acid and caustic, but it does not destroy the skin or dissolve metals so readily as Nitric Acid. It has an energetic attraction for water, and when the two are mixed, condensation ensues, and much heat is evolved; four parts of acid and one of water produce a temperature equal to that of boiling water. Mixed with aqueous Nitric Acid, it forms the compound know a as Nitro-Sulphuric Acid.

Sulphuric Acid possesses intense chemical powers, and displaces the greater number of ordinary acids from their salts. It _chars_ organic substances, by removing the elements of water, and converts Alcohol into Ether in a similar manner. The _strength_ of a given sample of Sulphuric Acid may be calculated, nearly, from its specific gravity, and a Table is given by Dr. Ure for that purpose. (See Appendix.)

_Impurities of Commercial Sulphuric Acid._--The liquid acid sold as Oil of Vitriol is tolerably constant in composition, and seems to be as well adapted for Photographic use as the _pure_ Sulphuric Acid, which is far more expensive. The specific gravity should be about 1·836 at 60°. If a drop, evaporated upon Platinum-foil, gives a fixed residue, probably Bisulphate of Potash is present. A milkiness, on dilution, indicates Sulphate of Lead (see page 186).

_Test for Sulphuric Acid._--If the presence of Sulphuric Acid, or a soluble Sulphate, be suspected in any liquid, it is tested for by adding a few drops of dilute solution of Chloride of Barium, or Nitrate of Baryta. A white precipitate, _insoluble in Nitric Acid_, indicates Sulphuric Acid. If the liquid to be tested is very acid, from Nitric or Hydrochloric Acid, it must be largely diluted before testing, or a crystalline precipitate will form, caused by the sparing solubility of the Chloride of Barium itself in acid solutions.

SULPHUROUS ACID.

Symbol, SO{2}. Atomic weight, 32.

This is a gaseous compound, formed by burning Sulphur in atmospheric air or Oxygen gas: also by heating Oil of Vitriol in contact with metallic Copper, or with Charcoal.

When an acid of any kind is added to Hyposulphite of Soda, Sulphurous Acid is formed as a product of the decomposition of Hyposulphurous Acid, but it afterwards disappears from the liquid by a secondary reaction, resulting in the production of Trithionate and Tetrathionate of Soda.

_Properties._--Sulphurous Acid possesses a peculiar and suffocating odour, familiar to all in the fumes of burning Sulphur. It is a feeble acid, and escapes with effervescence, like Carbonic Acid, when its salts are treated with Oil of Vitriol. It is soluble in water.

TETRATHIONIC ACID.

Symbol, S{4}O{5}. Atomic weight, 104.

The chemistry of the Polythionic Acids and their salts will be found described in the First Part of this Work, page 157.

WATER.

Symbol, HO. Atomic weight, 9.

Water is an Oxide of Hydrogen, containing single atoms of each of the gases.

_Distilled water_ is water which has been vaporized and again condensed; by this means it is freed from earthy and saline impurities, which, not being volatile, are left in the body of the retort. _Pure_ distilled water leaves no residue on evaporation, and should remain perfectly clear on the addition of Nitrate of Silver, _even when exposed to the light_; it should also be neutral to test-paper.

The condensed water of steam-boilers sold as distilled water is apt to be contaminated with oily and empyreumatic matter, which discolours Nitrate of Silver, and is therefore injurious.

_Rain-water_, having undergone a natural process of distillation, is free from inorganic salts, but it usually contains a minute portion of _Ammonia_, which gives it an alkaline reaction to test-paper. It is very good for Photographic purposes if collected in clean vessels, but when taken from a common rain-water tank should always be examined, and if much organic matter be present, tingeing it of a brown colour and imparting an unpleasant smell, it must be rejected.

_Spring_ or _River_ water, commonly known as "hard water," usually contains Sulphate of Lime, and Carbonate of Lime dissolved in Carbonic Acid; also Chloride of Sodium in greater or less quantity. On boiling the water, the Carbonic Acid gas is evolved, and the greater part of the Carbonate of Lime (if any is present) deposits, forming an earthy incrustation on the boiler.

In testing water for Sulphates and Chlorides, acidify a portion with a few drops of _pure_ Nitric Acid, free from Chlorine (if this is not at hand, use pure Acetic Acid); then divide it into two parts, and add to the first a _dilute_ solution of Chloride of Barium, and to the second, Nitrate of Silver,--a milkiness indicates the presence of Sulphates in the first case or of Chlorides in the second. The _Photographic Nitrate Bath_ cannot be used as a test, since the Iodide of Silver it contains is precipitated on dilution, giving a milkiness which might be mistaken for Chloride of Silver.

Common hard water can often be used for making a Nitrate Bath when nothing better is at hand. The Chlorides it contains are precipitated by the Nitrate of Silver, leaving soluble _Nitrates_ in solution, which are not injurious. The Carbonate of Lime, if any is present, neutralizes free Nitric Acid, rendering the Bath alkaline in the same manner as Carbonate of Soda. (See page 89.) Sulphate of Lime, usually present in well water, is said to exercise a retarding action upon the sensitive Silver Salts, but on this point the writer is unable to give certain information.

Hard water is not often sufficiently pure for the developing fluids. The Chloride of Sodium it contains decomposes the Nitrate of Silver upon the film, and the image cannot be brought out perfectly. The _New River water_, however, supplied to many parts of London, is almost free from Chlorides, and answers very well. In other cases a few drops of Nitrate of Silver solution may be added, to separate the Chlorine, taking care not to use a large excess.

APPENDIX.

QUANTITATIVE TESTING OF SOLUTIONS OF NITRATE OF SILVER.

The amount of Nitrate of Silver contained in solutions of that salt may be estimated with sufficient delicacy for ordinary Photographic operations by the following simple process.

Take the _pure_ crystallized Chloride of Sodium, and either dry it strongly or fuse it at a moderate heat, in order to drive off any water which may be retained between the interstices of the crystals; then dissolve in distilled water, in the proportion of 8-1/2 grains to 6 fluid ounces.

In this way, a standard solution of salt is formed, each drachm of which (containing slightly more than one-sixth of a grain of salt) will precipitate exactly half a grain of Nitrate of Silver.

In order to use it, measure out accurately one drachm of the Bath in a minim measure and place it in a two-ounce stoppered phial, taking care to rinse out the measure with a drachm of distilled water, which is to be added to the former; then pour in the salt solution, in the proportion of a drachm for every 4 grains of Nitrate _known to be present_ in an ounce of the Bath which is to be tested; shake the contents of the bottle briskly, until the white curds have perfectly separated, and the supernatant liquid is clear and colourless; then add fresh portions of the standard solution, by 30 minims at a time, with constant shaking. When the last addition causes no _milkiness_, read off the total number of drachms employed (the last half-drachm being subtracted), and multiply that number by 4 for the weight in grains of the Nitrate of Silver present in an ounce of the Bath.

In this manner the strength of the Bath is indicated within two grains to the ounce, or even to a single grain if the last additions of standard salt-solution be made in portions of 15, instead of 30 minims.

Supposing the Bath to be tested is thought to contain about 35 grains of Nitrate to the ounce, it will be convenient to begin by adding to the measured drachm, 7 _drachms_ of the standard solution; afterwards, as the milkiness and precipitation become less marked, the process must be carried on more cautiously, and the bottle shaken violently for several minutes, in order to obtain a clear solution. A few drops of Nitric Acid added to the Nitrate of Silver facilitate the deposition of the Chloride; but care must be taken that the sample of Nitric Acid employed is pure and free from Chlorine, the presence of which would cause an error.

RECOVERY OF SILVER FROM WASTE SOLUTIONS,--FROM THE BLACK DEPOSIT OF HYPO-BATHS, ETC.

The manner of separating metallic Silver from waste solutions varies according to the presence or absence of alkaline Hyposulphites and Cyanides.

a. _Separation of metallic Silver from old Nitrate Baths._--The Silver contained in solutions of the Nitrate, Acetate, etc. may easily be precipitated by suspending a strip of sheet Copper in the liquid; the action is completed in two or three days, the whole of the Nitric Acid and Oxygen passing to the Copper, and forming a blue solution of the Nitrate of Copper. The metallic Silver however, separated in this manner, always contains a portion of Copper, and gives a blue solution when dissolved in Nitric Acid.

A better process is to commence by precipitating the Silver entirely in the form of _Chloride of Silver_, by adding common Salt until no further milkiness can be produced. If the liquid is well stirred, the Chloride of Silver sinks to the bottom, and may be washed by repeatedly filling the vessel with common water, and pouring off the upper clear portion when the clots have again settled down. The Chloride of Silver thus formed may afterwards be reduced to metallic Silver by a process which will presently be described (p. 374).

b. _Separation of Silver from solutions containing alkaline Hyposulphites, Cyanides, or Iodides._--In this case the Silver cannot be precipitated by adding Chloride of Sodium, since the Chloride of Silver is soluble in such liquids. It is necessary therefore to use the Sulphuretted Hydrogen, or the Hydrosulphate of Ammonia, and to separate the Silver in the form of Sulphuret.

Sulphuretted Hydrogen gas is readily prepared, by fitting a cork and flexible tubing to the neck of a pint bottle, and having introduced Sulphuret of Iron (sold by operative chemists for the purpose), about as much as will stand in the palm of the hand, pouring upon it 1-1/2 fluid ounce of Oil of Vitriol diluted with 10 ounces of water. The gas is generated gradually without the application of heat, and must be allowed to bubble up through the liquid from which the Silver is to be separated. The smell of Sulphuretted Hydrogen being offensive, and highly poisonous if inhaled in a concentrated form, the operation must be carried on in the open air, or in a place where the fumes may escape without doing injury.

When the liquid begins to acquire a strong and persistent odour of Sulphuretted Hydrogen, the precipitation of Sulphuret is completed. The black mass must then be collected upon a filter, and washed by pouring water over it, until the liquid which runs through gives little or no precipitate with a drop of Nitrate of Silver.

The Silver may also be separated in the form of Sulphuret from old Hypo-Baths, by adding Oil of Vitriol in quantity sufficient to decompose the Hyposulphite of Soda; and burning off the free Sulphur from the brown deposit.

Conversion of Sulphuret of Silver into metallic Silver.--The black Sulphuret of Silver may be reduced to the state of metal by roasting and subsequent fusion with Carbonate of Soda; but it is more convenient, in operating on a small scale, to proceed in the following manner:--first convert the Sulphuret into Nitrate of Silver, by boiling with Nitric Acid diluted with two parts of water; when all evolution of red fumes has ceased, the liquid may be diluted, allowed to cool, and filtered from the insoluble portion, which consists principally of Sulphur, but also contains a mixture of Chloride and Sulphuret of Silver, unless the Nitric Acid employed was free from Chlorine; this precipitate may be heated, in order to volatilize the Sulphur, and then digested with Hyposulphite of Soda, or added to the Hypo-Bath.

The solution of Nitrate of Silver obtained by dissolving Sulphuret of Silver, is always strongly acid with Nitric Acid, and also contains _Sulphate_ of Silver. It may be crystallized by evaporation; but unless the quantity of material operated on is large, it will be better to precipitate the Silver in the form of Chloride, by adding common Salt, as already recommended.

REDUCTION OF CHLORIDE OF SILVER TO THE METALLIC STATE.

The Chloride of Silver is first to be carefully washed, by filling up the vessel which contains it, many times with water, and pouring off the liquid, or drawing it off close with a siphon. It may then be dried at a gentle heat, and fused with twice its weight of dry Carbonate of Potash, or better still, with a mixture of the Carbonates of Potash and Soda.

The process for reducing Chloride of Silver in the moist way, by metallic Zinc and Sulphuric Acid, is more economical and less troublesome than that just given; it is conducted as follows:--The Chloride, after having been well washed as before, is placed in a large flat dish, and a bar of metallic Zinc laid in contact with it. A small quantity of Oil of Vitriol, diluted with four parts of water, is then added, until a slight effervescence of Hydrogen gas is seen to take place. The vessel is set aside for two or three days, and is not to be disturbed, either by stirring or by moving the bar. The reduction begins with the Chloride immediately in contact with the Zinc, and radiates in all directions. When the whole mass has become of a grey colour, the bar is to be carefully removed and the adhering Silver washed off with a stream of water; the Zinc usually presents a honeycombed appearance, with irregularities upon the surface, which however are not metallic Silver;--they consist only of Zinc or of Oxide of Zinc.

In order to ensure the purity of the Silver, a fresh addition of Sulphuric Acid must be made, after the Zinc bar has been removed, and the digestion continued for several hours, in order to dissolve any fragments of metallic Zinc which may have been inadvertently detached. The grey powder must be repeatedly washed, first with Sulphuric Acid and water (this is necessary to dissolve a portion of an insoluble Salt of Zinc, probably an oxychloride) and then with water alone, until the liquid runs away _neutral_, and gives no precipitate with Carbonate of Soda; it may then be fused into a button, to burn off organic matter if present, and subsequently converted into Nitrate of Silver by boiling with Nitric Acid diluted with two parts of water.

In reducing Chloride of Silver precipitated from old Nitrate Baths _containing Iodide of Silver_, the grey metallic powder is sometimes contaminated with unreduced Iodide of Silver, which dissolves in the solution of Nitrate of Silver formed on treating the mass with Nitric Acid. To avoid this, wash the purified Silver with solution of Hyposulphite of Soda, and then again with water.

MODE OF TAKING THE SPECIFIC GRAVITY OF LIQUIDS.

Instruments are sold, termed "Hydrometers," which indicate specific gravity by the extent to which a glass bulb containing air, and properly balanced, rises or sinks, in the liquid; but a more exact process, and one equally simple, is by the use of the specific gravity bottle.

These bottles are made to contain exactly 1000 grains of distilled water, and with each is sold _a brass weight_, which counterbalances it when filled with pure water.

In taking the specific gravity of a liquid, fill the bottle quite full and insert the stopper, which being pierced through by a fine capillary tube allows the excess to escape. Then, having wiped the bottle quite dry, place it in the scale-pan, and ascertain the number of grains required to produce equilibrium; this number added to, or subtracted from, _unity_ (the assumed specific gravity of water), will give the density of the liquid.

Thus, to take examples, supposing the bottle filled with _rectified Ether_ to require 250 grains to enable it to counterbalance the brass weight,--then 1· _minus_ ·250, or ·750, is the specific gravity; but in the case of _Oil of Vitriol_ the bottle, when full, will be heavier than the counterpoise by perhaps 836 grains; therefore 1· _plus_ ·836, _id est_ 1·836, is the density of the sample examined.

Sometimes the bottle is made to hold only 500 grains of distilled water, in place of 1000; in this case the number of grains to be added or subtracted must be multiplied by 2.

In taking specific gravities, observe that the temperature be within a few degrees of 60° Fahrenheit (if higher or lower, immerse the bottle in warm or cold water); and wash out the bottle thoroughly with water each time after use.

ON FILTRATION AND WASHING PRECIPITATES.

In preparing filters, cut the paper into squares of a sufficient size, and fold each square neatly upon itself, first into a half-square, and then again, at right angles, into a quarter-square;--round off the corners with a pair of scissors, and open out the filter into a conical form, when it will be found to drop exactly into the funnel, and to be uniformly supported throughout.

Before pouring in the liquid, always moisten the filter with distilled water, in order to expand the fibres; if this precaution be neglected, the pores are apt to become choked in filtering liquids which contain finely divided matter in suspension. The solution to be filtered may be poured gently down a glass rod, held in the left hand (_a silver spoon_ may be used, in case of necessity, for Nitrate Baths, and all liquids not containing Nitric or Hydrochloric Acid), and directed against the side of the funnel, near to the upper part. If it does not immediately run clear, it will usually do so on returning it into the filter and allowing it to pass through a second time.

_Mode of Washing Precipitates._--Collect the precipitate upon a filter and drain off as much of the mother-liquor as possible; then pour in distilled water by small portions at a time, allowing each to percolate through the deposit before adding a fresh quantity. When the water passes through perfectly pure, the washing is complete; in testing it, a single drop may be laid upon a strip of glass and allowed to evaporate spontaneously in a warm place, or the proper chemical reagents may be applied, and the washing continued until no impurity can be detected. Thus, for example, in washing the Sulphuret of Silver precipitated from a Hypo-Bath by means of Hydrosulphate of Ammonia, the process will be completed when the water which runs through causes no deposit with a drop of Nitrate of Silver solution.

ON THE USE OF TEST-PAPERS.

The nature of the colouring matter which is employed in the preparation of litmus-paper has already been described at page 353.

In testing for the alkalies and basic oxides generally, the blue litmus-paper which has been reddened by an acid may be used, or, in place of it, the _turmeric_-paper. Turmeric is a yellow vegetable substance which possesses the property of becoming brown when treated with an alkali; it is however less sensitive than the reddened litmus, and is scarcely affected by the weaker bases, such as Oxide of Silver.

In using test-papers, observe the following precautions:--they should be kept in a dark place, and protected from the action of the air, or they soon become purple from Carbonic Acid, always present in the atmosphere in small quantity. By immersion in water containing about one drop of Liquor Potassæ or Ammoniæ, or a grain of Carbonate of Soda to four ounces, the blue colour is restored. As the quantities which are tested for in Photography are often infinitesimally small, it is essential that the litmus-paper should be in good condition; and test-papers prepared with porous paper will be found to show the colour better than those upon glazed or strongly-sized paper. The mode of employing the paper is as follows:--Place a small strip in the liquid to be examined: if it becomes at once _bright red_, a strong acid is present; but if it changes _slowly to a wine-red_ tint, a weak acid, such as Acetic or Carbonic, is indicated. In the case of the Photographic Nitrate Bath faintly acidified with Acetic Acid, a purple colour only may be expected, and a decided red colour would suggest the presence of Nitric Acid. In the Hypo fixing and toning Bath which has acquired acidity, the litmus-paper will perhaps redden in about three or four minutes.

Blue litmus-papers may be changed to the red papers used for alkalies by soaking in water acidified with Sulphuric Acid, one drop to half a pint; or by holding for an instant near the mouth of a bottle containing Glacial Acetic Acid. In examining a Nitrate Bath for alkalinity by means of the reddened litmus-paper, at least five or ten minutes should be allowed for the action, since the change of colour from red to blue takes place very slowly.

REMOVAL OF SILVER STAINS FROM THE HANDS, LINEN, ETC.

The black stains upon the hands caused by Nitrate of Silver, may readily be removed by moistening them and rubbing with a lump of Cyanide of Potassium. As this salt however is highly poisonous, many may prefer the following plan:--Wet the spot with a saturated solution of Iodide of Potassium, and afterwards with Nitric Acid (the strong Nitric Acid acts upon the skin and turns it yellow, it must therefore be diluted with two parts of water before use); then wash with solution of Hyposulphite of Soda.

Stains upon white linen may be easily removed by brushing them with a solution of Iodine in Iodide of Potassium, and afterwards washing with water and soaking in Hyposulphite of Soda, or Cyanide of Potassium, until the yellow Iodide of Silver is dissolved out; the Bichloride of Mercury (neutral solution) also answers well in many cases, changing the dark spot to white (p. 151).

A TABLE SHOWING THE QUANTITY OF ANHYDROUS ACID IN DILUTE SULPHURIC ACID OF DIFFERENT SPECIFIC GRAVITIES. (URE.)

+---------+------------++---------+------------++---------+------------+ | |Real Acid || |Real Acid || |Real Acid | |Specific | in 100 ||Specific | in 100 ||Specific | in 100 | |Gravity. |parts of the||Gravity. |parts of the||Gravity. |parts of the| | | Liquid. || | Liquid. || | Liquid. | +---------+------------++---------+------------++---------+------------+ | 1·8485 | 81·54 || 1·8115 | 73·39 || 1·7120 | 65·23 | | 1·8475 | 80·72 || 1·8043 | 72·57 || 1·6993 | 64·42 | | 1·8460 | 79·90 || 1·7962 | 71·75 || 1·6870 | 63·60 | | 1·8439 | 79·09 || 1·7870 | 70·94 || 1·6750 | 62·78 | | 1·8410 | 78·28 || 1·7774 | 70·12 || 1·6630 | 61·97 | | 1·8376 | 77·46 || 1·7673 | 69·31 || 1·6520 | 61·15 | | 1·8336 | 76·65 || 1·7570 | 68·49 || 1·6415 | 60·34 | | 1·8290 | 75·83 || 1·7465 | 67·68 || 1·6321 | 59·52 | | 1·8233 | 75·02 || 1·7360 | 66·86 || 1·6204 | 58·71 | | 1·8179 | 74·20 || 1·7245 | 66·05 || 1·6090 | 57·89 | +---------+------------++---------+------------++---------+------------+

A TABLE SHOWING THE QUANTITY OF ANHYDROUS ACID IN THE LIQUID NITRIC ACID OF DIFFERENT SPECIFIC GRAVITIES. (URE.)

+---------+------------++---------+------------++---------+------------+ | |Real Acid || |Real Acid || |Real Acid | |Specific | in 100 ||Specific | in 100 ||Specific | in 100 | |Gravity. |parts of the||Gravity. |parts of the||Gravity. |parts of the| | | Liquid. || | Liquid. || | Liquid. | +---------+------------++---------+------------++---------+------------+ | 1·5000 | 79·700 || 1·4640 | 69·339 || 1·4147 | 58·978 | | 1·4980 | 78·903 || 1·4600 | 68·542 || 1·4107 | 58·181 | | 1·4960 | 78·106 || 1·4570 | 67·745 || 1·4065 | 57·384 | | 1·4940 | 77·309 || 1·4530 | 66·948 || 1·4023 | 56·587 | | 1·4910 | 76·512 || 1·4500 | 66·155 || 1·3978 | 55·790 | | 1·4880 | 75·715 || 1·4460 | 65·354 || 1·3945 | 54·993 | | 1·4850 | 74·918 || 1·4424 | 64·557 || 1·3882 | 54·196 | | 1·4820 | 74·121 || 1·4385 | 63·760 || 1·3833 | 53·399 | | 1·4790 | 73·324 || 1·4346 | 62·963 || 1·3783 | 52·602 | | 1·4760 | 72·527 || 1·4306 | 62·166 || 1·3732 | 51·805 | | 1·4730 | 71·730 || 1·4269 | 61·369 || 1·3681 | 51·068 | | 1·4700 | 70·933 || 1·4228 | 60·572 || 1·3630 | 50·211 | | 1·4670 | 70·136 || 1·4189 | 59·775 || 1·3579 | 49·414 | +---------+------------++---------+------------++---------+------------+

WEIGHTS AND MEASURES.

_Troy, or Apothecaries' Weight._

1 Pound = 12 Ounces. 1 Ounce = 8 Drachms. 1 Drachm = 3 Scruples. 1 Scruple = 20 Grains. (1 Ounce Troy = 480 Grains, or 1 Ounce Avoirdupois _plus_ 42·5 grains.)

_Avoirdupois Weight._

1 Pound = 16 Ounces. 1 Ounce = 16 Drachms. 1 Drachm = 27·343 grains. (1 Ounce Avoirdupois = 437·5 grains.) (1 Pound Avoirdupois = 7000 Grains, or 1 Pound Troy _plus_ 2-1/2 Troy Ounces _plus_ 40 grains.)

_Imperial Measure._

1 Gallon = 8 Pints. 1 Pint = 20 Ounces. 1 Ounce = 8 Drachms. 1 Drachm = 60 Minims. (A Wine Pint of water measures 16 Ounces, and _weighs_ a Pound.)

An Imperial Gallon of water _weighs_ 10 Pounds Avoirdupois, or 70,000 Grains. An Imperial Pint of water _weighs_ 1-1/4 Pound Avoirdupois. A fluid Ounce of water _weighs_ 1 Ounce Avoirdupois, or 437·5 Grains. A Drachm of water _weighs_ 54·7 Grains.

_French Measures of Weight._

1 Kilogramme = 1000 Grammes = something less than 2-1/4 Pounds Avoirdupois.

1 Gramme = 10 Decigrammes--100 Centigrammes = 1000 Milligrammes = 15·433 English Grains.

A Gramme of water _measures_ 17 English Minims, nearly. 1000 Grammes of water _measure_ 35-1/4 English fluid Ounces.

_French Measures of Volume._

1 Litre = 13 Decilitres = 100 Centilitres = 1000 Millilitres = 35-1/4 English fluid Ounces.

1 Litre = 1 Cubic Decimetre = 1000 Cubic Centimetres.

1 Cubic Centimetre = 17 English Minims.

A Litre of water _weighs_ a Kilogramme, or something less than 2-1/4 Pounds Avoirdupois. A Cubic Centimetre of water _weighs_ a Gramme.

INDEX.[56]

[56] The preparation and properties of the Chemicals used in Photography will be found in the Alphabetical List commencing at page 327.

Aberration, chromatic, 54; spherical, 56. Accelerating agents, their mode of action in Collodion explained, 95. Acetate of Silver, its preparation and formula, 365; its formation in Nitrate Bath explained, 89; ensures absence of free Nitric Acid, 116; tends slightly to favour fogging and spots, 104; contra-indicated for glass Positives, 111. Acetic Acid, properties and mode of testing purity of, 327; useful in preventing fogging, 104; in rendering the development slow and even, 99; does not coagulate Albumen, 329; a good commercial form of acid, 212; Acetic Acid essential in Calotype, waxed paper, and Albumen processes, 177; also in printing paper Positives by development, 260. Aceto-Nitrate of Silver, term explained, 177. Achromatic Lenses, their construction explained, 55; the visual and chemical foci often coincident in, 60. Acids, nature of, 308. Actinism, explained, 61; importance of distinguishing Actinic from visual rays, 62; mode of finding Actinic focus, 229. Affinity, chemical, 312. Albumen, its chemistry, 328; forms a compound with oxide of Silver, 20; used in Positive printing to produce a fine surface layer, 122; to increase sensitiveness, 125; affects the colour of the prints, 127; protects the image from oxidation, 150; putrifies when exposed to moisture, 155; discolours the Nitrate Bath, 245. Albumen negative process, its invention, 10; theory of, 180; Collodio-Albumen process of M. Taupenot, 294. Albuminized paper, formula for, 241; slow in fixing, 131; not well adapted for toning by Sel d'or, 269; good for stereoscopic subjects and small portraits, 249; cannot be sensitized with Ammonio-Nitrate of Silver, 246. Alcohol, its chemistry, 330; sometimes too dilute for making Collodion, 84; mode of rectifying, 196; must not contain impurities, 96; effects of adding to Collodion, 84, 96; to developer, 205. Alkalies, nature of, 308. Alkalinity of Nitrate Bath, explained, 88; the evils it produces, 104; how to test for it, 377; how to remove it, 277. Amber varnish, 226. Ammonia, preparation and properties, 331; its use in fixing, 42; Mr. Shadbolt's formula for, 271; its action upon Chloride of Gold, 343; effect of concentrated Ammonia upon Oxide of Silver, 362. Ammonio-Nitrate of Silver, its chemistry, 262; used in Positive printing to increase sensitiveness, 125; to give black tones, 127; cannot be used with Albumen, 246; increases permanency of print, 169; old Nitrate Baths not easily convertible into Ammonio-Nitrate, 248; mode of preparing, 247; best applied to the paper by brush or rod, 248; Oxide of Silver in Nitrate of Ammonia, a useful substitute for it, 249. Ammonio-Nitrate paper, formula for, 246; a more simple formula, but less sensitive than the last, 258. Atomic theory explained, 322.

Bath for fixing and toning Positives. _See_ Fixing and toning Bath. Bichloride of Mercury, whitening action on glass Positives explained, 113; solution for, 207; used to intensify Negatives, 118; bleaches paper prints, 151; should not be added to paste used in mounting prints, 164; removes Silver stains, 377. Binocular vision, phenomena of, explained, 66. Blackening Negatives, 37, 117. Black tones, mode of obtaining, in paper Positives, 168, 246. Bromide of Silver, its preparation and properties, 17; its superior sensibility to coloured light, 63; less acted on by white light than Chloride, 19; less sensitive to invisible image than Iodide, 25; employment in Collodion, 101; found useful in Photographing by artificial light, 66; diagram of chemical spectrum on, 64. Bromo-Iodide of Silver, 173. Brushes, mode of applying Silver solutions by, 248.

Calotype process, theory of, 176. Camera, its first invention, 7; theory of its construction, 54; mode of testing accuracy of, 229; cause of the image being inverted, 53; the term "flatness of field" explained, 54; best position of the Camera for portraits, 220; for architectural subjects, 231; a funnel-shaped tube placed in front of the lens, 229; stereoscopic Camera, 234; microscope Camera, 236. Causes of failure in Collodion process, 276. Chemical affinity, illustrations of, 312. Chemical elements, 306. Chemical focus, directions for finding, 229; shorter than visual in non achromatic lenses, 60; longer than visual in microscopic objectives, 237; varies slightly with the nature of the light, 238. Chemical spectrum, 61. Chemicals, Photographic, Vocabulary of, 327. Chloride of Silver, its preparation and properties, 14; more sensitive to white light than Bromide or Iodide, 19; less sensitive to invisible image, 24; its blackening by light explained, 20, 141; accelerated by excess of Nitrate, 19; by organic matter, 20, 142; experiments illustrating darkening of papers prepared with, 21; simple explanation of the mode of preparing sensitive papers with, 22; agents which dissolve it, 42; mode of reducing it to metallic state. 374. Chloride of Gold, its preparation and properties, 342; action of Ammonia upon it, 343; use of an alkaline solution of, for toning, 132, 271; compounds formed on adding it to Hyposulphite of Soda, 133; mode of preparing the fixing and toning Bath with, 250; the Sel d'or Bath with, 267. Chromatic aberration, 54. Citric Acid, forms a red compound with Suboxide of Silver, 21, 338; used in printing to give purple tones, 128; formula for preparing paper with, 246. Cleaning glass plates, theory of, 39; details of, 213. Collodion, its discovery, 10; chemistry of Pyroxyline, 75; physical effect of Ether and Alcohol in, 83; of water in, 85; glutinosity of, 83; coloration of iodized, explained, 85; sensitiveness and intensity affected by the change, 97, 99; details of manufacture of Collodion, 185; Positive Collodion, theory of, 108; formula for, 201; Negative Collodion, theory of, 113; formula for, 208; Collodion for copying engravings, 231; for keeping processes, 298; for hot climates, 210; for working by artificial light, 238; to remove the brown colour from Collodion, 86. Collodion film, the proper time for immersing it in the Bath, 219; a thin film often good for direct Positives, 109; a thicker film for Negatives, 113; cause of the film falling away from the glass, 83, 293; spots and markings on, 281; conditions which affect its sensitiveness to light, 92; causes influencing its behaviour with the developer, 98; mode of preserving sensitiveness of film, 289. Collodio-Albumen process, theory of, 181; practical details of, 294. Colours, their nature explained, 47; their chemical action on sensitive film, 64; their photographic action assisted by reflection of white light, 66. Combination, laws of, 307. Conjugate foci, explained, 52, 272. Crookes, Mr., remarks upon chemical spectrum, 63; upon waxed paper process, 180; preservative process for Collodion films, 289. Curvature of luminous image formed by lens, explained, 53. Cyanide of Potassium, its fixing action explained, 44; preparation of solution of, 207; used to remove stains, 377. Daguerreotype, its invention, 8; theory of the process, 171. Development of invisible image, explanation of, 34-40; second, or intensifying stage explained, 37; details of developing glass Positives and Negatives, 221-223; development of paper Positives, 259; conditions which increase or diminish rapidity of development, 98; irregularities of development, 103. Developers, their preparation and properties, 26; comparative strength of, 98; theory of, for Positives, 111; for Negatives, 117; formulæ for Positive developers, 205; for Negative, 211. Diagrams, mode of copying, 232. Diaphragms for lenses. _See_ Stops. Double decomposition, illustrated, 14; explained, 314. Dry Collodion process, 298.

Elementary bodies, table of, 306; combination of, 307. Engravings, mode of copying, 231; often yield dark-coloured prints, 255. Equivalent proportions, 320. Ether, properties of, 339; purification of, for Photography, 195; must be kept in a dark place, 196; should not be distilled from residues of old Collodion, 96. Experiments, illustrating action of Light upon Chloride of Silver, 21; illustrating formation and development of invisible images, 25; illustrating photographic action of coloured light, 62. Exposure in the Camera, rules for Positives, 221; for Negatives, 225; for preserved Collodion plates, 292; for microscopic photographs, 238; effects of under and over-exposing, 35; exposure required in Calotype process, 177; in waxed paper, 180; in Albumen negative process, 181; in dry Collodion process, 301; in Taupenot's process, 297.

Fading of Positives, explained at length, 160; Author's researches on, 153. Film, sensitive. _See_ Collodion film. Filters, mode of cutting, 376. Fixing, theory of, 41; of paper prints explained, 128; solution for fixing glass Positives and Negatives, 212; manipulatory details of fixing, 225; fixing paper Positives with Ammonia, 271. Fixing and toning Bath, its preparation, 250; conditions which favour or retard its action, 135; certain states of the Bath injurious to the proofs, 136; importance of keeping it in an active condition, 168; must not be employed immediately after mixing, 251; must not be allowed to become acid by constant use, 168; theory of the gradual change of properties it undergoes, 156. Foci, actinic and luminous, 60; actinic, mode of finding, 229; variation between them in microscopic objectives, 237. Focussing the object, 220. Fogging, theory of, 103; mode of detecting causes of, 276. Formulæ for solutions required in Collodion process, 201; for papers used in Positive printing, 241; want of correspondence between, 257.

Gallic Acid, its preparation and properties, 27; used in paper processes, 178; becomes mouldy by keeping, 261. formula for developing paper Positives with, 261. Gallo-Nitrate of Silver, 177; discolours rapidly when developing dishes are not clean, 179. Gelatine, its properties, 341; forms a compound with an Oxide of Silver, 21; employed in dry Collodion process, 299; modified form of, 302; affects the colour in printing processes, 128; used in Positive printing to form an even surface layer, 126; as a cement to mount Photographs, 257. Glass plates, rules for cleaning, 39; details of cleaning, 213; mode of coating with Collodion, 215; with Albumen, 180. Glutinous Collodion, explained, 83. Glycyrrhizine, its nature, 342; its action in Collodion, 114; formula for solution of, 209. Gold, Chloride of. _See_ Chloride of Gold. Gold salts, their use in Photographic printing explained, 131; in the Daguerreotype process, 175. Gradation of tone, in Collodion Photographs, affected by the density of the film, 109, 113; by use of Glycyrrhizine, 115. Gradation of tone, in paper Positives, conditions affecting it in prints obtained by direct exposure, 123; in Positives printed by development, 266.

Hadow, Mr., researches on Collodion, 77; formula for making Pyroxyline, 187. Heliography, invented by M. Niépce, 7. Historical sketch of Photography, 6. Honey keeping process, 289. Hunt, Mr., introduces Protosalts of Iron in developing, 111. Hypo Bath. _See_ Fixing and Toning Bath. Hyposulphite of Silver, its peculiar changes in colour, 129; the sweet compound which it forms with Hyposulphite of Soda, 44. Hyposulphite of Soda, preparation and properties, 43; theory of its fixing action, 43; blackens Nitrate of Silver, 129; causes a milkiness with acids, 137; its decomposition by constant use in fixing, 138; the salts it forms with Chloride of Gold, 133; its conversion into sulphuretting Tetrathionate by Iodine and Perchloride of Iron, 139; test for presence of, 169.

Iceland moss, its use in Positive printing, 128; formula for preparing paper with, 245. Imperfections in Collodion Negatives, 282; in Positives, 284; in paper Positives, 285. Intensity, explanation of term, 92; mode of increasing in Negatives, 99, 114; effect of Acetate of Silver upon, 116; of Nitrite of Silver upon, 102; mode of diminishing, in glass Positives, 109, 110; conditions affecting intensity in paper Positives, 123; in developed paper Positives, 266. Invisible images, theory of formation of, 34; development of, 36; experiments illustrating, 25. Iodate, how formed in Collodion film, 94; produces insensitiveness, 198. Iodide of Ammonium, preparation of, 198; not fitted for iodizing Collodion required to be kept long, 210. Iodide of Iron, an accelerator to Collodion, 116. Iodide of Potassium and Silver, properties of, 42; mode of iodizing Calotype papers by, 177. Iodide of Potassium, tests of purity of, 197; extent of solubility in Alcohol, 351; dissolves Iodide of Silver, 42. Iodide of Silver, its preparation and properties, 16; unaffected by direct action of light, 19; highly sensitive to invisible image, 24; hypothesis of formation of latent image on, 34; possibility of its reduction by Pyrogallic Acid shown, 33; excess of Nitrate of Silver essential to its blackening by developer, 36; diagrams of chemical spectrum on, 61, 64; fixing agents for, 42; its solubility in the Nitrate Bath, 86; retards the action of Hypo fixing and toning Bath, 136; superior permanency of developed prints on, 167; details of Negative printing process on, 263. Iodine, in Collodion, diminishes sensitiveness, 94; forms Nitric Acid and Iodate in the Bath, 94; often useful in Positive Collodion, 110; in Negative Collodion, if fogging occurs, 105; mode of removing from Collodion, 86. Iodized Collodion. See Collodion.

Kaolin, properties of, 335; used to decolorize Nitrate Baths, 91; importance of purifying it before use, 245.

Landscape Photography, with preserved Collodion plates, 288. Latent image. See Invisible image. Laws of substitution explained, 78. Le Grey, M,, his toning process with Chloride of Gold, 132; his waxed paper Negative process, 178. Lenses, various forms of, 51; foci of, 52; formation of images by, 53; use of stops, 58; portrait, 59; chemical foci of, 60; chromatic aberration of, 54; spherical aberration of, 56; simple directions for using lenses, 227; for finding chemical focus, 229. Light; its action upon Silver Salts, 19; experiments illustrating, 21; formation of invisible images by, 24; its alternating action upon Daguerreotype plate, 39; its compound nature, 46; photographic action of coloured light, 60; refraction of light, 49. Llewellyn, Mr., his Oxymel process, 291.

Manipulations of Collodion process, 213; of Photographic printing, 251; of toning by Sel d'or, 267. Manuscripts, mode of copying, 231. Markings on Collodion Pictures, 281. Measures and Weights, 379. Microscopic Photography, 235. Moser, M. Ludwig, his researches on the development of invisible images, 37. Mounting Positive Prints, substances which should be avoided in, 155, 164; details of, 257.

Negative processes for printing Positives, 259, 263. Negatives, definition of, 106; Collodion Negatives, theory of production of, 113; Calotype, 176; waxed paper, 178; Albumen, 180; Collodio-Albumen (Taupenot), 181; mode of developing Collodion Negatives, 37, 117, 223; of converting Positives into, 117; formula for solutions for Negatives, 208; the Collodion best adapted for Negatives, 114; spots and markings upon Negatives, 282; decomposition of Pyroxyline a cause of fading of, 166. Nitrate of Silver, preparation and properties of, 12; preparation of from standard coin of realm, 362; often contains free Nitric Acid, 13; when very strongly fused, contains Nitrite (_see_ Nitrite of Silver), 14; not acted on by light, 18; its reduction by Pyrogallic Acid explained, 31; the melted Nitrate more certain in its action, 13, 101; its presence essential in developing the image, 36, 98; increases sensitiveness of Collodion plate, 92; dissolves Iodide of Silver, 86; discoloured by Albumen, 245, 329; forms a compound with Honey, 289; with various other organic bodies, 21; very little acted on by Glycerine, 342; mode of recovering the Silver from, 372. Nitrate Bath, mode of preparing for glass Positives, 110, 203; for Collodion Negatives, 116, 211; its power of dissolving Iodide of Silver, its occasional acidity and alkalinity explained, 86; the mode in which Acetate of Silver may be formed in it, 89; a list of the substances by which it is decomposed, 90; changes by use, 91; effect of these changes on sensitiveness, 97; on intensity, 102; care required to prevent it from yielding foggy pictures, 104; a caution against the too frequent addition of alkali, 204; quantitative testing of the Bath, 371. Nitrite of Silver, adds to rapidity of development, 102; tends slightly to produce fogging, 104; solarizes the high lights, 111. Nitric Acid, its preparation and properties, 355; its oxidizing powers, 12; impairs sensitiveness of Collodion film, 93; lessens rapidity of development, 98; tends to prevent fogging, 104; sometimes usefully employed for glass Positives, 110; contra-indicated for Negatives, 116; its accumulation in the Nitrate Bath explained, 94; mode of removing it, 90; cannot exist in contact with Acetate of Silver, 116; produces stains on cloth, 215; mode of determining the strength of Commercial Nitric Acid, 186; table of strength of Nitric Acid of different densities, 378. Nitro-Sulphuric Acid, explained, 77; process for making by mixed acids, 186; by Oil of Vitriol and Nitre, 190; should not be used cold, 83. Nomenclature, chemical, 315. Norris, Dr., his dry Collodion process, 298. Notation, chemical, 318.

Organic bodies, chemistry of, 324. Oxide of Silver, preparation and properties, 17; dissolves in the Nitrate Bath, rendering it alkaline, 88; properties of its solution in Ammonia, 362; preparation of ditto, 247; its solution in Nitrate of Ammonia used in Photography, 249. Oxymel, keeping process, 291; preparation of Oxymel, 360. Paper, Photographic, selection of, 240; peculiarity of English papers, 241. Paper, sensitive, for printing. _See_ Sensitive Paper. Perchloride of Iron, preparation of toning Bath with, 160. Permanence of Positives, mode of testing, 169. Photographic image, chemical composition of, 140; action of destructive tests on, 145. Photographic properties of Salts of Silver, 18; of Iodide of Silver upon Collodion, 74. Photographic researches by the Author, 140. Photography, historical sketch of, 6; the term explained, 61. Portrait lenses, theory of their construction, 59; rules for their use, 227; mode of finding chemical focus, 229. Portraits, drapery for, 66; directions for taking, 220; the position of the Camera, and other points of importance, 228; the time of exposure, 221. Positive printing, on Albuminized paper, formulæ for, 241; on plain paper, formulæ for, 245; on Ammonio-Nitrate paper, formula for, 246, 258; by development, formulæ for, 259; manipulatory details of printing, fixing, toning, washing, and mounting, 251; process of toning by Sel d'or, theory of, 134; practice of, 267; reasons for the want of correspondence between different formulæ, 257; use of Chloride of Gold in toning, 132, 271; theory of the preparation of the sensitive paper for Positives, 122; theory of the process of fixing, 129; of toning by Gold, 132; the Author's researches, 140; rationale of the printing process, 120; composition of the image, 140; fading of Positive prints, 160; destructive action of Sulphur on, 145; of oxidizing agents on, 148; of Chlorine, acids, boiling water, etc., on, 151; of combustion of coal-gas on, 153; effect of damp air on, 153; theory of mode of washing Positives, 162; comparative permanency of prints, 166; mode of testing permanency, 169. Positives, definition of, 106; Collodion Positives, theory of production of, 108; formulæ for solutions for, 201; development of, 111, 221; Collodion and Nitrate Bath best adapted for, 109; mode of whitening by Bichloride of Mercury, 112; solution for whitening, 207; mode of backing up, 226; spots and markings on, 284; mode of printing Positives on Collodion, 272. Positives, enlarged, mode of printing, 272. Practice of Collodion process, 183. Preservative processes for Collodion plates, 289. Printing, Photographic, theory of, 120; practical details of, 240. Prism, refraction of light by, 51; diagram of formation of spectrum by, 47; explained, 54. Prismatic spectrum, 47, 61. Protonitrate of Iron, preparation of, 206; a feeble developer when free from excess of Sulphate of Iron, 98; theory of its mode of action, and rules for its use, 112; cannot be prepared in quantity by adding Nitrate of Potash to Sulphate of Iron, 314; sometimes requires the addition of Nitrate of Silver, 206. Protosulphate of Iron, its preparation and properties, 29; its characteristics as a developer for Collodion Positives, 111; not well adapted for developing Collodion Negatives, 117; preparation of the solution for Positives, 205; mode of applying it to the plate, 221; to remove iron stains on glass, 215. Pyrogallic Acid, its preparation and properties, 28; solution for developing glass Positives, 205; for Negatives, 211; cannot be used without Acetic Acid, 105; less adapted for developing paper pictures, 178; requires addition of Nitric Acid when used for Positives, 111; superior to Sulphate of Iron for developing Negatives, 117, 144; mode of obviating the brown discoloration of developing solutions, 212. Pyroxyline, its nature and properties, 75; preparation of, by Mr. Hadow's formula, 186; by a rule-of-thumb mixture of the acids, 188; by the Oil of Vitriol and Nitre process, 190; details of immersing, washing, and drying, 191; the glutinous variety produced by cold acids, 83; recapitulation of the effects of varying the strength of the acid mixture, 193; spontaneous decomposition of Pyroxyline, 166.

Reduction of metallic oxides by developers, theory of, 26; of Silver salts by developers, theory of, 30; practical details of reducing Silver compounds to metallic state, 372.

Salts nature of, 310. Salts of Silver, their preparation and properties, 12; their Photographic action, 18; theory of their reduction by developer, 30; directions for obtaining metal from, 372. Sel d'or, toning process by, its theory, 134; its practical details, 267; its advantages, 271; gives permanent prints, 167. Sensitiveness, term explained, 92; conditions favourable to, 97. Sensitiveness of Collodion film, causes influencing, 92; superior sensitiveness partially explained, 74; preservation of sensitive film, 288. Sensitive paper, theory of preparation of, 22, 122; its darkening by light described, 123; preparation of Albuminized paper, 241; of plain paper, 245; of Ammonio-Nitrate paper, 246, 258; of paper for Negative processes, 259; causes which affect the sensitiveness of Positive paper, 123; which alter the colour of the image, 126; spots and markings on, 285; a large excess of Nitrate of Silver essential, 124; the paper should not be kept too long, 130, 286. Serum of Milk, preparation of, 262, 355; used in Negative printing process, 262. Shadbolt, Mr., his Honey keeping process, 289; employs artificial light in Micro-Photography, 237. Silver, properties of, 362; estimation of, in Nitrate Baths, 371; recovery from waste solutions, 372; reduction from Chloride, 374; stains, removal of, 377. Size, mode of removing, from paper Positives, 255. Solar spectrum, 47, 61. Soluble paper. _See_ Pyroxyline. Specific gravity of liquids, mode of finding, 375. Spherical aberration, 56. Spirits of Wine, preparation and properties, 330; not always sufficiently strong for Collodion, 84; mode of rectifying, 196; sometimes contaminated with, fusel oil, 96. Spots on Collodion plates, 279; on paper Positives, 285; on prints obtained by development, 266; on preserved Collodion plates, 293. Stains, Silver, removal of, 377. Stereoscope, invention of, 67; theory of, 68; Wheatstone's, 69; Brewster's, 70. Stereoscopic Photographs, rules for taking, 71; practical details of, 232. Stops, theory of use of, 57; simple mode of making, 228; position of the stop often important, 230. Strength of acids, tables of, 378. Subchloride of Silver, its preparation and properties, 15; decomposed by fixing agents, 141. Suboxide of Silver, its properties, 18; forms compounds with organic matters. Citric Acid, Albumen, etc., 21. Substitution, laws of, explained, 78. Sulphate of Iron. _See_ Protosulphate of Iron. Sulphate of Quinine, absorption of chemical rays by, 65. Sulphuric Acid, table of strength of, 378. Sutton, Mr., theory of Sel d'or toning process, 134; practical details of, 267; Negative printing process, 262; preparation of Serum of Milk for, 355. Symbols, use of, 318. Syruped Collodion film, 289.

Talbot, Mr., his discoveries, 9; theory of Calotype process, 176. Taupenot, M., his Collodio-Albumen process, 181; practical details of, 294. Temperature, its effect upon development of Collodion film, 102; upon fogging, 105; upon keeping Collodion, 210; upon action of fixing Bath for paper Positives, 130; upon Hypo toning Bath, 136. Test-papers, use of, 376. Toning Bath for Positives, with Sel d'or, 134, 267; with Hyposulphite and Gold, _see_ Fixing and Toning Bath; with Chloride of Iron and Hyposulphite, 160. Toning of Positives, term defined, 121; may injure the stability of the proof, 154; points to be kept in view to avoid fading, 167; manipulatory details of, 253; by Sel d'or, 267. Transparencies, mode of printing, 273.

Varnishes for Collodion Photographs, 226. View Lenses, directions for using, 230. Vocabulary of Photographic chemicals, 327.

Washing Positive prints, rules for, 162; details of, 255. Waxed paper process, theory of, 178. Weights and Measures, table of, 379.

PRINTED BY JOHN EDWARD TAYLOR, LITTLE QUEEN STREET, LINCOLN'S INN FIELDS.

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Transcriber Note

Minor typos have been corrected. Images moved to prevent splitting paragraphs.