Ammonium Sulphate. (See [p. 99].)
Calcium Sulphate, or gypsum, occurs in large quantities in Nature. The salt contains 20·9 per cent. of combined water, and when carefully heated to 120° C, it loses about two-thirds of this water, yielding a white powder known as plaster of Paris. This substance, when made into a paste with water, gradually sets to a hard mass, because the partially dehydrated gypsum re-combines with the water.
Lead Sulphate, the chief impurity of commercial oil of vitriol, is a white powder which is very often used for making white paint in place of lead carbonate (white lead). The sulphate has the advantage over the carbonate in not being so readily discoloured; its disadvantage is that it lacks “body.”
Copper Sulphate, or blue vitriol, is frequently found in the drainage of copper mines, where it is formed by the oxidation of copper pyrites. It is made on a large scale by roasting sulphide ores of copper in a current of air. Oxygen combines with copper sulphide, forming copper sulphate, which is extracted with water and crystallized. It forms large blue crystals containing 36 per cent. of water. This salt is put to many different uses. Very large quantities are used for dyeing and calico printing; some of the green pigments, such as Schweinfurt green, are made from it.
CHAPTER III
NITRIC ACID AND NITRATES
Nitric acid, the aqua fortis of the alchemists, must be placed next to sulphuric acid in the scale of relative importance, because of the variety of its uses. It is indispensable for making explosives, and is used for the preparation of drugs and fine chemicals, including the coal-tar dyes. The acid also dissolves many metals, forming nitrates, which are put to several uses. Silver nitrate is the basis of marking ink, and it is also the substance from which the light-sensitive silver compounds required for the photographic industry are made. The important pigments, chrome yellow and chrome red, are prepared from lead nitrate. The solvent action of nitric acid on copper is made use of in etching designs on copper plates. Over and above all this, it must be mentioned that an adequate supply of “nitrate” is required for artificial manure. Thus it can be said that with the uses of this acid and its salts are associated our supply of daily bread, our freedom from foreign oppression, and many of the refinements and conveniences of life.
We shall begin the study of nitric acid by taking stock, as it were, of the natural sources of supply. The free acid is not found in Nature except for very small traces in the air after thunderstorms. We have, therefore, to rely entirely on that which can be obtained artificially. Until quite recently, it could be said that there was only one method of making the acid, namely, by the distillation of a mixture of potassium or sodium nitrates and concentrated sulphuric acid. Now, however, nitric acid is being made from the air, though as yet only in small quantity, notwithstanding the great development of this method owing to war requirements; hence, we are still mainly dependent on the naturally occurring nitrates just mentioned.
Potassium Nitrate (nitre, saltpetre, sal prunella) is found in the soil of hot countries, especially in the neighbourhood of towns and villages where the sanitary arrangements are primitive. In very favourable circumstances, it may even appear as a whitish, mealy efflorescence on the surface of the ground. To obtain the salt, it is only necessary to agitate the surface soil with water and, after the insoluble matter has settled down, to evaporate the clear solution.
Potassium nitrate is required for making gunpowder, which, until quite recent times, was the only explosive used in warfare. Continental countries that could not afford to rely entirely on sea-borne nitre had to make their own. The refuse of the farmyard, mixed with lime and ashes, was made up into a heap of loose texture, which was periodically moistened with the drainage from the stables. In the course of years, saltpetre and calcium nitrate were formed in the surface layers, from which they were extracted from time to time. The farmer was then allowed to pay part of his taxes in nitrates.
Sodium Nitrate, also called caliche, Chili-saltpetre, or Chili-nitrate, comes mainly from South America. The beds extend for a distance of about 220 miles in Chili, Peru, and Bolivia, between the Andes mountains and the sea. The deposit is about 5 ft. thick, and its average breadth 5 miles. The crude material is treated with water in steam-heated wooden vats. The clear solution is evaporated, and the residue obtained is washed with the mother liquor and dried. This product may contain as much as 98 per cent. of the nitrate.