CHAPTER I.

THE CHEMICAL ELEMENTS AND THEIR COMBINATIONS.

The limits of the present Work allow only of a simple sketch of the subjects which it is proposed to treat in this Chapter. Our attention therefore must be confined to an explanation of certain points which are alluded to in the First Part of the Work, and without a proper understanding of which it will be impossible for the reader to make progress.

The following division may be adopted:--The more important Elementary Bodies, with their symbols and atomic weights; the Compounds formed by their union; the class of Salts; illustrations of the nature of Chemical Affinity; Chemical Nomenclature; Symbolic Notation; the laws of Combination; the Atomic Theory; the Chemistry of Organic Bodies.

THE CHEMICAL ELEMENTS, WITH THEIR SYMBOLS AND ATOMIC WEIGHTS.

The class of elementary bodies embraces all those substances which cannot, in the present state of our knowledge, be resolved into simpler forms of matter.

The chemical elements are divided into "metallic" and "non-metallic," according to the possession of certain general characters.

The following are some of the principal non-metallic elements, with the symbols employed to designate them, and their atomic weights:[55]--

Symbol. Atomic Wt. Gases. { Oxygen O 8 { Hydrogen H 1 { Nitrogen N 14 { Chlorine Cl 36

Solids. { Iodine I 126 { Carbon C 6 { Sulphur S 16 { Phosphorus P 32

Liquid. Bromine Br 78 Unknown. Fluorine F 19

The metallic elements are more numerous. The following list includes only those which are commonly known:--

Symbol. Atomic Wt.

Metals of the { Potassium K 40 Alkalies. { Sodium Na 24

Metals of the { Barium Ba 69 Alkaline { Calcium Ca 20 Earths { Magnesium Mg 12

Metals { Iron Fe 28 Proper. { Zinc Zn 32 { Cadmium Cd 56 { Copper Cu 32 { Lead Pb 104 { Tin Sn 59 { Arsenic As 75 { Antimony Sb 129

Nobel { Mercury Hg 202 Metals. { Silver Ag 108 { Gold Au 197 { Platinum Pt 99

[55] The atomic weights, with the exception of that of Gold, are taken from the last edition of Brande's 'Manual of Chemistry.'

ON THE BINARY COMPOUNDS OF THE ELEMENTS.

Many of the elementary bodies exhibit a strong tendency to combine with each other, and to form compounds, which differ in properties from either of their constituent elements. This attraction, which is termed "Chemical Affinity," is exerted principally between bodies which are opposed to each other in their general characters. Thus, taking for example the elements Chlorine and Iodine--they are analogous in their reactions, and therefore there is but little attraction between them, whereas either of the two combines eagerly with Silver, which is an element of a different class. So, again. Sulphur unites with the metals, but two metallic elements are comparatively indifferent to each other.

Oxygen is by far the most important in the list of chemical elements. It combines with all the others, with the single exception, perhaps, of Fluorine. The attraction, or chemical affinity, however, which is exerted, varies much, in different cases. The metals, as a class, are easily oxidized; whilst many of the non-metallic elements, such as Chlorine, Iodine, Bromine, etc., exhibit but little affinity for Oxygen. Nitrogen is also a peculiarly negative element, showing little or no tendency to unite with the others.

Classification of binary compounds containing Oxygen.--When one simple element unites with another, the product is termed a "binary" compound.

There are three distinct classes of binary compounds of Oxygen:--Neutral Oxides, basic Oxides, and acid Oxides.

Neutral and basic Oxides.--Take as examples--the Oxide of Hydrogen, or Water, a neutral Oxide; the Oxide of Potassium, or Potash, a basic Oxide.

Water is termed a neutral oxide, because its affinities are low, and it is comparatively indifferent to other bodies. Potash and Oxide of Silver are examples of basic oxides; but there is a great difference between the two in chemical energy, the former belonging to a superior class of bases, viz. the alkaline.

By studying the properties of an alkali (such as Potash or Soda) which are familiar to all, we gain a correct notion of the whole class of basic oxides. An alkali is a substance readily soluble in water, and yielding a solution which has a slimy feel from its solvent action upon the skin. It immediately restores the blue colour of reddened litmus, and changes the blue infusion of cabbage to green. Lastly, it is neutralized and loses all its characteristic properties upon the addition of an acid.

The _weaker bases_ are, as a rule, sparingly or not at all soluble in water, neither have they the same caustic and solvent action upon the skin; but they restore the colour of reddened litmus, and neutralize acids in the same manner as the more powerful bases, or alkalies.

_The_ Acid _Oxides._--This class, taking the stronger acids as the type, may be described as follows:--very soluble in water, the solution possessing an intensely sour taste, and a _corroding_ rather than a solvent action upon the skin; changes the blue colour of litmus and other vegetable substances to red, and neutralizes the alkalies and basic oxides generally.

Observe however that these properties are possessed in very various degrees by different acids. Prussic Acid and Carbonic Acid, for instance, are not sour to the taste, and being feeble in their reactions, redden litmus scarcely or not at all. All acids however, without any exception, tend to combine with bases and to neutralize themselves; so that this may be said to be the most characteristic property of the class.

_Chemical composition of Acid and Basic Oxides contrasted._--It is a law commonly observed, although with many exceptions, that bases are formed by the union of Oxygen with _metals_; and acids, by Oxygen uniting with _non-metallic elements_. Thus, Sulphuric Acid is a compound of Sulphur and Oxygen; Nitric Acid, of Nitrogen and Oxygen. But the alkali, Potash, is an oxide of the _metal_ Potassium; and the oxides of Iron, Silver, Zinc, etc. are bases, and not acids.

Again, the composition of acids and bases is different in another respect; the former invariably contain more Oxygen in proportion to the other element than the latter. Taking the same examples as before, the two classes may be represented thus:--

Acids { Oil of Vitriol, Sulphur 1 atom, Oxygen 3 atoms. { Aqua-fortis, Nitrogen 1 " Oxygen 5 " Bases { Oxide of Silver, Silver 1 atom. Oxygen 1 atom. { Oxide of Iron, Iron 1 " Oxygen 1 "

The class of Hydrogen Acids.--Oxygen is so essentially the element which forms the acidifying principle of acids, that its very name is derived from that fact (οξυς, acid, and γενναω, to generate). Still there are exceptions to this rule, and in some acids _Hydrogen_ appears to play the same part; the _Hydracids_, as they are termed, are formed principally by Hydrogen uniting with elements like Chlorine, Bromine, Iodine, Fluorine, etc. Thus, Muriatic or Hydrochloric Acid contains Chlorine and Hydrogen; Hydriodic Acid contains Iodine and Hydrogen.

Observe, however, that the position held by the Hydrogen in these compounds, is different from that of the Oxygen in the "Oxyacids," as regards the number of atoms usually present; thus--

Aqua-fortis = Nitrogen 1 atom, Oxygen 5 atoms, Muriatic Acid = Chlorine 1 " Hydrogen 1 atom;

so that the composition of the Hydracids is analogous to the _basic_ oxides, in containing a single atom of each constituent.

THE TERNARY COMPOUNDS OF THE ELEMENTS.

As the various elementary substances unite with each other to form Binary Compounds, so these binary compounds again unite and form _Ternary_ Compounds.

Compound bodies however do not, as a rule, unite with simple elements. In illustration, take the action of Nitric Acid upon Silver, described at page 12. No effect is produced upon the metal until _Oxygen_ is imparted; then the Oxide of Silver so formed dissolves in the Nitric Acid. In other words, it is necessary that a binary compound should be first formed, before the solution can take place. The mutual attraction or chemical affinity exhibited by compound bodies is, as in the case of elements, most strongly marked when the two substances are opposed to each other in their general properties.

Thus, _acids_ do not unite with other acids, but they combine instantly with _alkalies_; the two mutually neutralizing each other and forming "a salt."

_Salts_ therefore are ternary compounds produced by the union of acids and bases; common Salt, formed by neutralizing Muriatic Acid with Soda, being taken as the type of the whole class.

_General characters of the Salts._--An aqueous solution of Chloride of Sodium, or common Salt, possesses those characters which are usually termed saline; it is neither sour nor corrosive, but, on the other hand, has a cooling agreeable taste. It produces no effect upon litmus and other vegetable colours, and is wanting in those energetic reactions which are characteristic of both acids and alkalies; hence, although formed by the union of two binary compounds, it differs essentially in properties from both.

All salts however do not correspond to this description of the properties of Chloride of Sodium. The Carbonate of Potash, for instance, is an acrid and alkaline salt, and the Nitrate of Iron reddens litmus-paper. A perfectly neutral salt is formed when a strong acid unites with an energetic base; but if, of the two constituents, one is more powerful than the other, the properties of that one are often seen in the resulting salt. Thus the Carbonate of Potash is _alkaline_ to test-paper, because the Carbonic Acid is feeble in its reactions; but if _Nitric Acid_ and _Potash_ are brought together, then a Nitrate of Potash is produced, which is neutral in every sense of the term.

The Chloride of Sodium and salts of a similar kind are freely soluble in water, but all salts are not so. Some dissolve only sparingly, and others not at all. The Chloride and Iodide of Silver are examples of the latter class; they are not bitter and caustic like the Nitrate of Silver, but are perfectly tasteless from being insoluble in the fluids of the mouth.

It is seen therefore from these examples, and many others which might be adduced, that the popular notion of a saline body is far from being correct, and that, in the language of strict definition, any substance is a salt which is produced by the union of an acid with an alkali, independent of the properties it may possess.

Thus, _Cyanide of Potassium_ is a true salt, although highly poisonous; Nitrate of Silver is a salt; the green Sulphate of Iron is a salt; so also is Chalk or Carbonate of Lime, which has neither taste, colour, nor smell.

_On the "Hydracid" class of Salts._--The distinction between Oxyacids and Hydracids has already been pointed out (p. 309), the latter having been shown to consist of Hydrogen united with elements analogous in their reactions to Chlorine, Iodine, Bromine, etc.

In a salt formed by an Oxygen Acid, both the basic and acid elements appear. Thus the common Nitre, which is a Nitrate of Potash, is found by analysis to contain Oxide of Potassium as a base, in a state of combination with Nitric Acid. But if a salt be formed by neutralizing an alkali with a _Hydrogen Acid_, the product in that case does not contain all the elements. This is seen from the following example:--

Hydrochloric Acid + Soda = Chloride of Sodium + Water;

or, stated more at length,--

(Chlorine Hydrogen) + (Oxygen Sodium) = (Chlorine Sodium) + (Oxygen Hydrogen).

Observe that the Hydrogen and Oxygen, being present in the correct proportions, unite to form Water, which is an Oxide of Hydrogen. This water passes off when the solution is evaporated, and leaves the dry crystals of salt. On the other hand, with the Oxyacid Salts, the elementary Hydrogen being absent, no water is formed, and the Oxygen remains.

It must therefore be borne in mind that salts like the Chlorides, Bromides, Iodides, etc. contain only _two_ elements; but that in the Oxyacid Salts, such as Sulphates, Nitrates, Acetates, _three_ are present. Thus, Nitrate of Silver consists of Nitrogen, Oxygen, and Silver, but Chloride of Silver contains simply Chlorine and metallic Silver united, without Oxygen.

The Hydracid salts however, when decomposed, yield products similar to the Oxyacid salts. For instance, if Iodide of Potassium be dissolved in water, and dilute Sulphuric Acid added, this acid, being powerful in its chemical affinities, tends to appropriate to itself the alkali; but it does not remove _Potassium_ and liberate _Iodine_, but takes the _Oxide_ of Potassium and sets free _Hydriodic Acid_, In other words, as an atom of water is produced during the _formation_ of a Hydracid Salt, so is an atom destroyed and made to yield up its elements in the _decomposition_ of a Hydracid Salt.

The reaction of dilute Sulphuric Acid upon Iodide of Potassium may be stated thus:--

Sulphuric Acid _plus_ (Iodine Potassium) _plus_ (Hydrogen Oxygen) _equals_ (Sulphuric Acid, Oxygen Potassium) or Sulphate of Potash, _and_ (Hydrogen Iodine) or Hydriodic Acid.

THE NATURE OF CHEMICAL AFFINITY FURTHER ILLUSTRATED.

_Illustration from the Non-metallic Elements._--If a stream of Chlorine gas be passed into a solution containing the same salt as before mentioned, viz. the Iodide of Potassium, the result is to liberate a certain portion of Iodine, which dissolves in the liquid, and tinges it of a brown colour. The element Chlorine, possessing a degree of chemical energy superior to that of Iodine, prevails over it, and removes the Potassium with which the Iodine was previously combined.

Chlorine + Iodide of Potassium = Iodine + Chloride of Potassium.

_The same Law illustrated by the Metals._--A strip of Iron dipped in solution of Nitrate of Silver becomes immediately coated with metallic Silver; but a piece of Silver-foil may be left for any length of time in Sulphate of Iron without undergoing change: the difference depends upon the fact, that metallic Iron has a greater attraction for Oxygen than Silver, and hence it displaces it from its solution.

Iron + Nitrate of Silver = Silver + Nitrate of Iron.

_Illustrations amongst Binary Compounds._--If a few drops of solution of Potash be added to solution of Nitrate of Silver, a brown deposit is formed, which is the Oxide of Silver, sparingly soluble in water. That is to say, as a stronger metal displaces _metallic Silver_, so does an oxide of the same metal displace _Oxide of Silver_. Therefore bases like the alkalies, alkaline earths, etc. cannot exist in a free state in solutions of the salts of weaker bases,--a liquid containing Nitrate of Silver could not also contain free Potash or Ammonia.

In the list given at page 306, the metallic elements are arranged principally in the order of their chemical affinities; those of Potassium, Sodium, Barium, etc. being the most marked.

As the alkalies displace the weaker bases from their combination with acids, so the strong _acids_ displace weak acids from their combination with bases. Thus, as

Oxide of Potassium + Acetate of Silver = Oxide of Silver + Acetate of Potash;

So

Nitric Acid + Acetate of Silver = Acetic Acid + Nitrate of Silver.

In the list of acids. Sulphuric Acid is usually placed first as being the strongest, and Carbonic Acid, which is a gaseous substance, last. The vegetable acids, such as Acetic, Tartaric, etc., are _intermediate_, being weaker than the mineral acids, but stronger than Carbonic, or Hydrocyanic Acid.

_The order of decompositions affected by the insolubility or the volatility of the products which may be formed._--It might be inferred from remarks already made, that on mixing saline solutions, a gradual interchange of elements would take place, until the strongest acids were associated with the strongest bases, and _vice versâ_. There are many causes however which interfere to prevent this; one of which is _volatility_.---

The violent effervescence which takes place on treating a _Carbonate_ of any kind with an acid is due to the _gaseous_ nature of Carbonic Acid and its escape in that form, which greatly facilitates the decomposition.

_Insolubility_ is also a cause which exercises a great influence on the result which will follow in mixing solutions. If the formation of an insoluble substance is possible by any interchange of elements, it will take place. A solution of Chloride of Sodium added to Nitrate of Silver invariably produces Chloride of Silver; the _insolubility_ of Chloride of Silver being the cause which determines its formation.

So again, Sulphate of Lead and Protonitrate of Iron are produced by mixing Nitrate of Lead with Sulphate of Iron; but if Nitrate of _Potash_ be substituted for Nitrate of Lead, the result is uncertain, because there are no elements present which can, by interchanging, form an insoluble salt; Sulphate of Potash, although _sparingly_ soluble in water, not being _insoluble_, like the Sulphate of Lead or the Sulphate of Baryta.

ON CHEMICAL NOMENCLATURE.

The nomenclature of the chemical _elements_ is mostly independent of any rule; but an attempt has been made to obviate this in the case of those of later discovery. Thus the names of the newly-found _metals_ usually end in _um_, as Potassium, Sodium, Barium, Calcium, etc.; and those elements which possess analogous characters have corresponding terminations assigned to them, as Chlorine, Bromine, Iodine, Fluorine, etc.

_Nomenclature of Binary Compounds._--These are often named by attaching the termination _ide_ to the more important element of the two; as, the Ox_ide_ of Hydrogen, or Water; the Chlor_ide_ of Silver; the Sulph_ide_ of Silver. Binary compounds of Sulphur however are sometimes termed Sulphurets, as the _Sulphuret_ or the _Sulphide_ of Silver indifferently.

When the same body combines with Oxygen, or the corresponding element, in more than one proportion, the prefix _proto_ is applied to that containing the least Oxygen; _sesqui_ to that with once and a half as much as the _proto_; _bi_ or _bin_ to that with twice as much; and _per_ to the one containing the most Oxygen of all. As examples, take the following:--The Protoxide of Iron; the Sesquioxide of Iron: the Protochloride of Mercury; the Bichloride of Mercury. In these examples the Sesquioxide of Iron is also a _Per_oxide, because no higher simple oxide is known, and the Bichloride of Mercury is a _Per_chloride for a similar reason.

When an inferior compound is discovered, it is often termed _sub_; as the Suboxide of Silver, the Subchloride of Silver. These bodies contain the least known quantity of Oxygen and Chlorine respectively, and are hence entitled to the prefix _proto_; but being of minor importance, they are excepted from the general rule.

The combinations of metallic elements with each other are termed "alloys;" or if containing Mercury, "amalgams."

_Nomenclature of binary Compounds possessing acid properties._--These are named on a different principle. The termination _ic_ is applied to one element. Thus, taking as an illustration the liquid known as "Oil of Vitriol," it is truly an _Oxide_ of Sulphur, but as it possesses strong acid properties it is termed Sulphur_ic_ Acid. So Nitric Acid is an Oxide of Nitrogen; Carbonic Acid is an Oxide of Carbon, etc. When there are two oxides of the same element, both possessing acid properties, the most important has the termination _ic_, and the other _ous_; as Sulphuric Acid, Sulphur_ous_ Acid; Nitric Acid, Nitr_ous_ Acid.

_Nomenclature of the Hydracids._--The Hydrogen Acids are distinguished from Oxyacids by retaining the names of both constituents, the termination _ic_ being annexed as usual. Thus, _Hydro_chloric Acid, or the Chloride of Hydrogen; _Hydr_iodic Acid, or the Iodide of Hydrogen.

_Further illustrations of the nomenclature of Binary Compounds._--The Oxides of Nitrogen, and also of Sulphur, afford an interesting illustration of the principles of nomenclature. The former are as follows:--

Nitrogen. Oxygen.

Protoxide of Nitrogen 1 atom. 1 atom. Binoxide of Nitrogen 1 " 2 " Nitrous Acid 1 " 3 " Peroxide of Nitrogen 1 " 4 " Nitric Acid 1 " 5 "

Observe, that two only out of the five possess acid properties, the others being simple oxides. Nitric Acid is, strictly speaking, the "Peroxide," but as it belongs to the class of acids, that term naturally falls to the compound below.

The binary compounds of Sulphur with Oxygen all possess acid properties; they may be represented (in part) as follows:--

Sulphur. Oxygen.

Hyposulphurous Acid 2 atoms. 2 atoms. Sulphurous Acid 1 " 2 " Hyposulphuric Acid 2 " 5 " Sulphuric Acid 1 " 3 "

In this case the Sulphuric and Sulphurous Acids had become familiarly known before the others, intermediate in composition, were discovered. Hence, to avoid the confusion which would result from changing the nomenclature, the new bodies are termed _Hypo_sulphuric and _Hypo_sulphurous (from ὑπο, _under_).

_Nomenclature of Salts._--Salts are named according to the acid they contain; the termination _ic_ being changed into _ate_, and _ous_ into _ite_. Thus, Sulphuric Acid forms Sulph_ates_; Nitric Acid, Nitr_ates_; but Sulphur_ous_ Acid forms Sulph_ites_, and Nitrous Acid, Nitr_ites_.

In naming a salt, the base is always placed _after_ the acid, the term _oxide_ being omitted; thus. _Nitrate of Oxide of Silver_ is more shortly known as "Nitrate of Silver," the presence of Oxygen being understood.

When there are two oxides of the same base, both of which are _salifiable_,--in naming the salts, the term _proto_ is prefixed to the acid of the salt formed by the lowest, and per to that of the higher oxide; as, the _Proto_sulphate of Iron, or Sulphate of the Protoxide; the _Per_sulphate of Iron, or Sulphate of the Peroxide.

Many salts contain more than one atom of acid to each atom of base. In that case, the usual prefixes expressive of quantity are adopted: thus, the _Bi_sulphate of Potash contains twice as much Sulphuric Acid as the neutral Sulphate, etc.

On the other hand, there are salts in which the base is in excess with regard to the acid, and which are usually known as "basic salts;" thus, the red powder which deposits from solution of Sulphate of Iron, is a _basic_ Persulphate of Iron, or a Sulphate of the Peroxide of Iron with more than the normal proportion of oxide.

_Nomenclature of the Hydracid Salts._--The composition of these salts being different from those formed by Oxygen Acids, the nomenclature varies also. Thus, in neutralizing Hydrochloric Acid with Soda, the product formed is not known as Hydrochlorate of Soda, but as _Chloride of Sodium_; this salt, and others of a similar constitution, being _binary_, and not _ternary_, compounds. The salt produced by Hydrochloric Acid and _Ammonia_ however is often called "Muriate or Hydrochlorate of Ammonia," although more strictly it should be the _Chloride of Ammonium_.

ON SYMBOLIC NOTATION.

The list of symbols employed to represent the various elementary bodies is given at page 306.--Commonly the initial letter of the Latin name is used, a second or smaller letter being added when two elements correspond in their initials: thus C stands for Carbon, Cl for Chlorine, Cd for Cadmium, and Cu for Copper.

The chemical symbol however does not simply represent a particular element; it denotes also a definite weight, or equivalent proportion, of that element. This will be explained more fully in the succeeding pages, when speaking of the Laws of Combination.

_Formulæ of Compounds._--In the _nomenclature_ of compounds it is usual to place the Oxygen or analogous element _first_ in the case of binary compounds, and the acid before the base in the ternary compounds, or salts; but in representing them _symbolically_ this order is reversed: thus, Oxide of Silver is written AgO, and never as OAg; Nitrate of Silver as AgO NO{5}, not NO{5}AgO.

The juxtaposition of symbols expresses combination; thus, FeO is a compound of one proportion of Iron with one of Oxygen, or the "Protoxide of Iron," If more than one equivalent be present, small figures are placed below the symbols: thus, Fe{2}O{3} represents two equivalents of Iron united with three of Oxygen, or the "Peroxide of Iron;" SO{3}, one equivalent of Sulphur with three of Oxygen, or Sulphuric Acid.

Larger figures placed before and in the same line with the symbols, affect the _whole compound_ which the symbols express: thus, 2 SO{3} means two equivalents of Sulphuric Acid; 3 NO{5}, three equivalents of Nitric Acid. The interposition of a comma prevents the influence of the large figure from extending further. Thus, the double Hyposulphite of Soda and Silver is represented as follows:--

2 NaO S{2}O{2}, AgO S{2}O{2},

or _two_ equivalents of Hyposulphite of Soda with one of Hyposulphite of Silver; the large figure referring only to the first half of the formula. Sometimes brackets, etc. are employed, in order to render a complicated formula more plain. For example, the formula for the double Hyposulphite of Gold and Soda, or "Sel d'or," may be written thus;--

3 (NaO S{2}O{2}) AuO S{2}O{2} + 4 HO.

In this formula, the _plus sign_ (+) denotes that the four atoms of water which follow, are less intimately united with the framework of the salt than the other constituents.

The use of a plus sign is commonly adopted in representing salts which contain water of crystallization. Thus, the formula for the crystallized Protosulphate of Iron is written as follows:--

FeO SO{3} + 7 HO.

These atoms of water are driven off by the application of heat, leaving a white substance, which is the Anhydrous salt, and would be written simply as FeO SO{3}.

The _plus_ sign however is often employed in token of simple _addition_, no combination of any kind being intended. Thus the decomposition which follows on mixing Chloride of Sodium with Nitrate of Silver may be written as follows:--

NaCl + AgO NO{5} = AgCl + NaO NO{5};

that is,--

Chloride of Sodium _added to_ Nitrate of Silver. = Chloride of Silver _and_ Nitrate of Soda.

ON EQUIVALENT PROPORTIONS.

When elementary or compound bodies enter into chemical union with each other, they do not combine in indefinite proportions, as in the case of a mixture of two liquids, or the solution of a saline body in water. On the other hand, a certain definite weight of the one unites with an equally definite weight of the other; and if an excess of either be present, it remains free and uncombined.

Thus, if we take a _single grain_ of the element Hydrogen--to convert that grain into Water there will be required exactly 8 grains of Oxygen; and if a larger quantity than this were added, as for instance _ten grains_, then two grains would be over and above. So, to form Hydrochloric Acid, 1 grain of Hydrogen takes 36 grains of Chlorine:--for the _Hydriodic Acid_, 1 grain of Hydrogen unites with 126 grains of Iodine.

Again, if separate portions of metallic Silver, of 108 grains each, are weighed out,--in order to convert them into Oxide, Chloride, and Iodide of Silver respectively, there would be required

Oxygen 8 grains. Chlorine 36 " Iodine 126 "

Therefore it appears that 8 grains of Oxygen are _equivalent_ to 36 grains of Chlorine and to 126 grains of Iodine, seeing that these quantities all play the same part in combining; and so it is with regard to the other elements,--to every one of them a figure can be assigned which represents the number of parts by weight in which that element unites with others. These figures are the "equivalents" or "combining proportions," and they are denoted by the _symbol_ of the element. A symbol does not stand as a simple representative of an element, but as a representative of _one equivalent_ of an element. Thus "O" indicates 8 parts by weight of Oxygen; "Cl" one equivalent, or 36 parts by weight, of Chlorine; and so with the rest.

Observe however that these figures, termed "equivalents," do not refer to the _actual number_ of parts by weight, but only to the _ratio_ which exists between them: if Oxygen is 8, then Chlorine is 36; but if we term Oxygen 100, as some have proposed, then Chlorine would be 442·65.

In the scale of equivalents now usually adopted, Hydrogen, as being the lowest of all, is taken as unity, and the others are related to it.

_Equivalents of Compounds._--The law of equivalent proportions applies to compounds as well as to simple bodies, the combining proportion of a compound being always the sum of the equivalents of its constituents. Thus Sulphur is 16, and Oxygen 8, therefore Sulphuric Acid, or SO{3}, equals 40. The equivalent of Nitrogen is 14, that of Nitric Acid, or NO{5}, is 54.

The same rule applies with regard to salts. Take for instance the Nitrate of Silver: it contains

Equivalent. Nitrogen 14 6 Oxygen 48 Silver 108 --- Total of equivalents, or equivalent } 170 of the Nitrate of Silver }

_Practical application of the Laws of Combination ._--The utility of being acquainted with the law of combining proportions is obvious when their nature is understood. As bodies both unite with and replace each other in equivalents, a simple calculation shows at once how much of each element or compound will be required in a given reaction. Thus, supposing it be desired to convert 100 grains of Nitrate of Silver into _Chloride_ of Silver, the weight of Chloride of Sodium which will be necessary is deduced thus:--one equivalent, or 170 parts, of Nitrate of Silver, is decomposed by an equivalent, or 60 parts, of Chloride of Sodium. Therefore

as 170 : 60 :: 100 : 35·2;

that is, 35·2 grains of Salt will precipitate, in the state of Chloride, the whole of the Silver contained in 100 grains of Nitrate.

So again, in order to form the Iodide of Silver, the proportions in which the two salts should be mixed is thus shown. The equivalent of Iodide of Potassium is 166, and that of Nitrate of Silver is 170. These numbers so nearly correspond, that it is common to direct that equal weights of the two salts should be taken.

One more illustration will suffice. Supposing it be required to form 20 grains of Iodide of Silver--how much Iodide of Potassium and Nitrate of Silver must be used? One equivalent, or 166 parts, of Iodide of Potassium, will yield an equivalent, or 234 parts, of Iodide of Silver; therefore

as 234 : 166 :: 20 : 14·2.

Hence, if 14·2 grains of the Iodide of Potassium be dissolved in water, and an equivalent quantity, viz. 14·5 grains, of the Nitrate of Silver added, the yellow precipitate, when washed and dried, will weigh precisely 20 grains.

ON THE ATOMIC THEORY.

The atomic theory, originally proposed by Dalton, so much facilitates the comprehension of chemical reactions generally, that it may be useful to give a short sketch of it.

It is supposed that all matter is made up of an infinite number of minute atoms, which are elementary, and do not admit of further division. Each of these atoms possesses an actual weight, although inappreciable by our present methods of investigation. Simple atoms, by uniting with each other, form _compound atoms_; and when these compounds are broken up, the elementary constituent atoms are not destroyed, but separate from each other, in possession of all their original properties.

In representing the simple atomic structure of bodies, _circles_ may be used, as in the following diagram.

Fig. 1 is a compound atom of Sulphuric Acid, consisting of an atom of Sulphur united intimately with three of Oxygen; fig. 2 is an atom of Peroxide of Nitrogen, NO{4}; and fig. 3, an atom of Nitric Acid, composed of Nitrogen 1 atom. Oxygen 5 atoms, or in symbols NO{5}.

_The term "atomic weight" substituted for equivalent proportion._--If we suppose that the simple atoms of different kinds of matter _differ in weight_, and that this difference is expressed by their equivalent numbers, the whole laws of combination follow by the simplest reasoning. It is easy to understand that an atom of one element, or compound, would displace, or be substituted for, a single atom of another; therefore, taking as the illustration the decomposition of Iodide of Potassium by Chlorine,--the weight of the latter element required to liberate 126 grains of Iodine is 36 grains, because _the weights of the atoms of those two elementary bodies are as 36 to 126_. So again, in the reaction between Chloride of Sodium and Nitrate of Silver, a compound atom of the former, represented by the weight 60, reacts upon a compound atom of the latter, which equals 170.

Therefore in place of the term "equivalent" or "combining proportion," it is more usual to employ that of "atomic weight." Thus the atomic weight of Oxygen is 8, represented by the symbol O; that of Sulphur is 16; hence the atomic weight of the compound atom of Sulphuric Acid, or SO{3}, is necessarily equal to the combined weights of the four simple atoms; _id est_, 16 + 24 = 40.

ON THE CHEMISTRY OF ORGANIC SUBSTANCES.

By "organic" substances are meant those which have possessed _life_, with definite organs and tissues, in contra-distinction to the various forms of dead inorganic matter, in which no structural organization of that kind is found.

The term organic however is also applied to substances which are obtained by chemical processes from the vegetable and animal kingdoms, although they cannot themselves be said to be living bodies; thus Acetic Acid, procured by the distillation of woody fibre, and Alcohol, by fermentation from sugar, are strictly organic substances.

The class of organic bodies embraces a great variety of products; which, like inorganic Oxides, may be divided into neutral, acid, and basic.

The organic _acids_ are numerous, including Acetic Acid, Tartaric, Citric, and a variety of others.

The _neutral substances_ cannot easily be assimilated to any class of inorganic compounds; as examples, take Starch, Sugar, Lignine, etc.

The _bases_ are also a large class. They are mostly rare substances, not familiarly known: Morphia, obtained from Opium; Quinia, from Quinine; Nicotine, from Tobacco, are illustrations.

_Composition of organic and inorganic bodies contrasted._--There are more than fifty elementary substances found in the inorganic kingdom, but only _four_, commonly speaking, in the organic: these four are Carbon, Hydrogen, Nitrogen, and Oxygen.

Some organic bodies,--oil of turpentine, naphtha, etc., contain only Carbon and Hydrogen; many others, such as sugar, gum, alcohol, fats, vegetable acids--Carbon, Hydrogen, and Oxygen. The _Nitrogenous bodies_, so called, containing Nitrogen in addition to the other elements, are principally substances derived from animal and vegetable tissues, such as Albumen, Caseine, Gelatine, etc.; Sulphur and Phosphorus are also present in many of the Nitrogenous bodies, but only to a small extent.

Organic substances, although simple as regards the _number_ of elements involved in their formation, are often highly complex in the arrangement of the atoms; this may be illustrated by the following formulæ:--

Starch C{24}H{20}O{20} Lignine C{24}H{20}O{20} Cane Sugar C{24}H{22}O{22} Grape Sugar C{24}H{28}O{28}

Inorganic bodies, as already shown, unite _in pairs_,--two elements join to form a binary compound; two binary compounds produce a salt; two salts associated together form a double salt. With organic bodies however the arrangement is different,--the elementary atoms are all grouped equally in one compound atom, which is highly complex in structure, and cannot be split up into binary products.

Observe also, as characteristic of Organic Chemistry, the apparent similarity in composition between bodies which differ widely in properties. As examples take _Lignine_, or cotton fibre, and Starch,--each of which contains the three elements united as C{24}H{20}O{20}.

_Mode of distinguishing between Organic and Inorganic matter._--A simple means of doing this is as follows:-- place the suspected substance upon a piece of Platinum-foil, and heat it to redness with a spirit-lamp: if it first _blackens_, and then burns completely away, it is probably of organic origin. This test depends upon the fact, that the constituent elements of organic bodies are all either themselves volatile, or capable of forming volatile combinations with Oxygen. Inorganic substances, on the other hand, are often unaffected by heat, or, if volatile, are dissipated without previous charring.

The action of heat upon organic matter may further be illustrated by the combustion of coal or wood in an ordinary furnace;--first, an escape of Carbon and Hydrogen, united in the form of volatile gaseous matter, takes place, leaving behind a black cinder, which consists of Carbon and inorganic matter combined; afterwards this Carbon burns away into Carbonic Acid, and a grey ash is left which is composed of inorganic salts, and is indestructible by heat.