CHAPTER VI

PHOSPHORIC, BORIC, AND SILICIC ACIDS

The acids which are grouped in this chapter are not in themselves of much interest, though some of their salts are extremely important compounds.

Bone. Much of the refuse bone, sooner or later, reaches the marine store, and from that point starts on a career of usefulness in the industrial world.

“Green bone,” as it is then called, may have fat adhering to it or confined in its hollow interior as marrow. This is recovered by treatment with benzine, and after that the bone is subjected to the action of superheated steam in order to convert cartilage into glue. In some cases, the residue is then ground up to make bone meal, which is valuable as a manure because of the calcium phosphate which it contains. In this way, the phosphate returns again to the animal kingdom, for it supplies plants with the phosphates that they require, and from the vegetable kingdom it passes to animals and helps to build up bone again.

Calcium Phosphate and Bone Black. Instead of being ground up, bone may be heated in a retort in much the same way as coal is treated for the manufacture of coal gas; bone oil is distilled off, and a non-volatile residue, called bone black or animal charcoal, remains. This contains about 90 per cent. of calcium phosphate and 10 per cent. of finely divided carbon disseminated throughout the mass. It has the peculiar property of absorbing colouring matter, and is used for this purpose in the sugar industry and in the preparation of fine chemicals.

Phosphoric Acid. After being some time in use, bone black loses the property of absorbing colouring matter; and though it can be “revived” several times by heating it strongly in a closed retort, it ultimately becomes spent and of no further use to the sugar refiner. It is then heated again, this time in an open vessel, until all the carbon is burnt away. The residue is now a greyish solid consisting mainly of calcium phosphate. This, supplemented with native phosphate, which is probably fossilized bone, is used for the preparation of phosphoric acid.

The salt is decomposed by sulphuric acid in wooden vats; calcium sulphate is formed, and ultimately settles on the bottom of the vat, leaving a clear supernatant liquid, which is a dilute solution of phosphoric acid. This liquid is drawn off and evaporated to a syrup. This is “syrupy” phosphoric acid. On being still more strongly heated, the syrup loses still more water, and a semi-transparent glassy-looking substance, called metaphosphoric acid, remains.

Superphosphate. All fertile soils, especially those on which wheat is to be grown, must contain a certain amount of phosphate. With this, as with all other plant foods, the actual percentage weight required in the soil is very small indeed, but it is necessary that it should be disseminated throughout the soil. Even distribution is very difficult to secure in the case of a substance like calcium phosphate, which is practically insoluble in water.

To get over this difficulty, calcium phosphate is converted into a mixture known as “superphosphate” by the following process. Bone ash or the mineral phosphate is finely ground and thoroughly mixed by machinery with two-thirds its weight of sulphuric acid from the lead chambers. After a time, this mixture sets to a hard mass, containing principally gypsum and calcium tetrahydrogen phosphate. It is then ground up finely and is ready for use.

The special modification of calcium phosphate contained in superphosphate is soluble in water. It is, therefore, carried into the soil in solution, and in this way very evenly distributed. In the soil it reacts with the lime or chalk which is present, and is gradually reconverted into insoluble calcium phosphate.

The manufacture of superphosphate is a very important industry. The weight of the substance produced annually in Great Britain alone is not far below a million tons.

Basic Slag. In the Bessemer process for converting iron into steel, cast iron is melted up in a vessel called a converter and, by the aid of a powerful blast blown through the molten iron, most of the impurities are burnt off. If, however, phosphorus and sulphur are present, they are not removed if the converter has a silica (acid) lining. The original Bessemer process was, therefore, modified by Thomas and Gilchrist, and the converter for this kind of iron is lined with dolomite and lime (basic lining). Phosphorus is then converted into phosphate and retained by the lining, which is subsequently removed, ground up finely, and sold as “basic slag.”

Boric Acid, or boracic acid, is familiar because it is used in medicine as a mild antiseptic; it is also employed as a preservative for food. It is a white crystalline compound, sparingly soluble in water. It has no well-marked taste, and causes only a partial change in the colour of litmus solution; it is, therefore, one of the weak acids. It does not dissolve metals, but it displaces carbon dioxide from carbonates, forming salts.

Borax, the best known salt of boric acid, is used in laundry work and also for making some enamels, for when it is strongly heated it loses water, and ultimately melts down to a clear “glass” in which the oxides of metals will dissolve, yielding transparent substances which are beautifully coloured according to the nature of the oxide used. This property is often made use of in chemical analysis in what is known as the “borax-bead” test.

Boric acid is a natural product; the method by which it is obtained is of some interest, because it is so simple, and because it shows how mere traces can be gradually accumulated until a very fair total is ultimately obtained. Moreover, the method is copied directly from Nature.

In the early years of the nineteenth century, certain jets of natural steam, called _suffioni_, which issue from the earth in Tuscany, were found to contain the vapour of boric acid. These jets of steam are of volcanic origin. The quantity of boric acid in the vapour is very small indeed; nevertheless, by the method which is adopted, it can be profitably recovered, and more than a ton of the solid is daily produced.

In the same country there are many lagoons, the water of which contains boric acid. It was rightly conjectured that this boric acid came from jets of steam which issued from the earth in the bed of the lagoon. This suggested the idea of building up an artificial lagoon around a group of jets.

Series of about five of these collecting basins (Fig. 9) are formed, each one at a slightly lower level than the one which precedes it. The first basin is filled with water from an adjacent spring, and this is allowed to remain for twenty-four hours. A sluice is then opened and the liquid contained in the first basin flows down to the second, where it remains for another day, and so on until it reaches the last basin of the series. The liquid by this time is almost fully charged with boric acid, but it contains only about 2 per cent., because the acid is so sparingly soluble in water.

From the last basin (A), the liquid runs into large vats (B, D), where the suspended impurities settle down. The solution of boric acid is then concentrated by causing it to flow over a broad inclined plane made of corrugated lead or through a series of shallow vessels heated by jets of natural steam. The hot liquid flows into another vat (C), and, as it cools, boric acid crystallizes out and is removed by perforated ladles.

The mother liquor from which the crystals have been withdrawn is, of course, a cold saturated solution of the acid, and this is returned to the top of the incline to flow down again and lose more water. The boric acid is finally transferred to drying chambers, which are also heated by the natural steam.

Native borax or “tinkal” comes from Thibet and also from Ceylon. In California, a large quantity of borax is obtained from a borax lake, and also from the mud of marshes in its neighbourhood.

Silica. The element silicon does not occur in the free state in Nature, neither has any particular use been found for it, and therefore it is not often isolated except to provide a lecture specimen. The compounds of silicon, however, are both plentiful and important, especially silica, the oxide, and the silicates or salts of silicic acid.

The commonest forms of silica are sand, flint, and quartz. Silver sand is composed of small crystals of pure silica, while flint is the amorphous variety of the same substance. Quartz, or rock crystal, is a very hard and transparent mineral. It forms six-sided prisms ending in pyramids. It is distinguished from other common transparent minerals, such as calcspar, by the fact that it cannot be scratched even with a good knife or file, and that a drop of hydrochloric acid has no action on it. The melting point of silica is very high.

Sometimes silica is very delicately coloured with minute traces of metallic oxides and other substances, and these forms, because of their rarity and beauty, are more highly valued. Smoky quartz, cat’s-eye, and amethyst are some of the coloured varieties of quartz. Opal, agate, jasper, onyx, and chalcedony are, in the chemist’s classification, merely coloured flints.

In recent years, chemical apparatus has been made from pure fused silica. This can only be worked in the oxy-hydrogen blow-pipe flame or in the electric furnace; nevertheless, crucibles, flasks, beakers, and retorts can be made. Silica ware has several advantages over glass, notably, that water has no action upon it at all; moreover, its coefficient of expansion is so very small that a piece of apparatus made of silica can be suddenly heated or cooled without risk of fracture; indeed, it can be made red-hot and cooled immediately by plunging into cold water.

Quartz or silica fibres, used for suspending magnets and other bodies in very delicate physical apparatus, are made in the following way. Molten silica is attached to the bolt of a crossbow, which is then released above a carpet of black velvet. As the bolt flies forward, it draws out the silica into a filament, which is so fine that it would be difficult to find were it not for the velvet background.

Silicic Acid itself is only of theoretical interest. It is obtained by adding hydrochloric acid to a solution of potassium or sodium silicate. It is a gelatinous substance of somewhat indefinite composition. It has no effect on litmus, no taste, and no solvent action; in fact, it is only recognizable as an acid because it dissolves in alkalis, forming salts called silicates. It is one of the weakest acids known.

The natural silicates are very abundant and varied; orthoclase or potash felspar, and albite or soda felspar, are those which most commonly occur. The former is potassium aluminium silicate, and the latter, sodium aluminium silicate. Iron is generally present in both as an impurity. The weathering of the felspars, in conjunction with the action of water, has produced the clays. In this way, pure white China clay has been formed from felspars which contain little or no iron, and the coarser kinds of clay from others containing a greater proportion of foreign substances.

Mica, which is used for making lamp chimneys, is a potassium aluminium silicate. Asbestos, meerschaum, beryl, garnet, jade, and hornblende are all silicates of various metals.

Glass is a complex mixture of insoluble silicates with excess of silica. The varieties in common use are soda glass, Bohemian glass, and lead glass (which is also called flint glass). Soda glass is mainly a mixture of calcium and sodium silicates, and is distinguished by its low melting point, which makes it easy to work at moderate temperatures. It appears in commerce as plate glass, window glass, and common bottles. Bohemian glass contains calcium and potassium silicates, and has a high melting point. It is used for making chemical apparatus. Lead or flint glass contains the silicates of lead and potassium; this is a dense glass, but at the same time rather soft. It takes a high polish and is used for making ornamental or cut-glass ware.

Remembering that glass is composed of the salts of silicic acid, the reader will readily understand that the mixture from which it is made must contain acidic and basic constituents. The acidic or acid-forming material is in every case silica or sand. This must be pure, and for all but the commonest kind of bottle or window glass, it must be free from iron, otherwise the glass will have a more or less pronounced greenish colour. It must also be fine and even grained. Formerly, the glass sands used in this country came from Holland and Belgium, but now supplies from several British sources are being successfully used.

The basic portion of the glass mixture differs according to the kind of glass required. An average mixture for soda glass contains sand, 20 parts; salt cake (sodium sulphate), 10 parts; quicklime, 5 parts; charcoal, 1 part. For Bohemian glass, pearl ash (potassium carbonate) takes the place of salt cake, and no charcoal is necessary because the materials used are finer. For lead glass, the mixture is still further modified by the use of litharge, or more often red lead, in place of lime.

The ingredients are well mixed and thoroughly dried. Waste glass from a previous batch is also added. The mixture is heated to about 1200° C. in large pots made of Stourbridge clay, and the heating is continued for as much as sixteen hours, and until the whole of the material in the pot is molten and fairly mobile. Scum or glass-gall is removed, and when gas bubbles have disappeared, the temperature is allowed to fall to 700°-800°, when the glass becomes sufficiently viscous for subsequent working. The semi-fluid mass is then blown, moulded, or drawn, according to the kind of article that is required.

The physical properties of glass will now be considered in order that we may be able to account for its extended use. Such an inquiry as this, especially in the case of materials in common use, is often interesting, because it frequently happens that the special property upon which we set so much value is an abnormal one and, consequently, the feature which we take for granted is precisely the one into which we should inquire most closely.

The most striking feature of glass is its transparency. This property is abnormal, if glass is a solid. Consider what happens in most cases. A substance like nitre melts easily and in the molten state is perfectly transparent; when it cools, crystals form and, though these individually may be transparent, yet the solid mass is opaque. The reason for this is that the solid is not optically homogeneous, and therefore a ray of light cannot pass through it in a straight line. At each facet of a crystal light is deviated and reflected, and in the end is almost wholly scattered. Consequently, an object, even if it can be seen at all, can be discerned only in a blurred and indistinct fashion through such a medium.

There are very good reasons, however, for supposing that glass is not a true solid but an extremely viscous liquid. If glass is heated, it softens and begins to flow very sluggishly at first, but afterwards more readily. There is no abrupt change, as there generally is in passing from the solid to the liquid state. Similarly in cooling, there is no point at which it is possible to say that the glass is solidifying. The view that this substance is really a liquid is perhaps a little startling at first, but it becomes less so when we observe that a long glass rod supported at its ends in a horizontal position sags in the middle and is permanently deformed.

To avoid that change which would be technically called solidification by a scientist, the article which has been fashioned in glass is cooled down very slowly and gradually. This part of the process is called annealing; it may occupy some days in extreme cases, and it points to the fact that experience has shown that it is necessary to guard against some change which would normally take place if this precaution were neglected.

The change in glass which annealing is intended to prevent is known as devitrification. In spite of all precautions, this does occur sometimes, and specimens of old window glass are often seen to have lost their transparency completely and to have an opalescent sheen. In these cases, the silicates have crystallized.

An extreme case of badly annealed glass is illustrated by Rupert’s drops, a scientific curiosity of very old standing. These are “tears” of glass made by dropping the molten substance into water. When the tail of the drop is nipped off, the whole thing is shattered to powder with something like explosive violence. Clearly there is a very great internal strain, due to the fact that the outer parts have solidified and contracted, while the inner part is still warm and dilated.

Another remarkable feature of glass is the ease and simplicity with which it can be fashioned into articles of various shapes. As a plastic material, molten glass almost ranks with clay. This again is due to the property of passing through a viscous state, that is, one which is intermediate between a solid and a liquid.

Water Glass, or soluble glass, is mainly sodium silicate. It is made by fusing sand or powdered flint with caustic or with mild soda; sometimes, by digesting crushed flint or chert with caustic soda solution under considerable pressure in autoclaves or specially constructed boilers. In the latter case, no extraction is necessary; but in the former, the residue is treated with water and the solution evaporated until it becomes a viscous transparent liquid.

This liquid is used in various ways in industry. It is added to the cheaper varieties of yellow soap, and is employed as a mordant in dyeing and printing calico. An artificial sandstone is made by mixing sand, calcium chloride, and sodium silicate; the two last-named substances interact to form calcium silicate, which is insoluble in water. For domestic purposes, water glass is best known in connection with the preserving of eggs. When the film of water glass dries on the surface of the egg shell, the latter becomes impervious to air.