For printing on calico, the mordant is thickened with gum arabic or other glutinous substance. The design is then stamped on the material with the thickened mordant liquor. The subsequent treatment consists of dipping the material in the dye and afterwards in water, when the colour comes away from those parts which have not received the impress of the mordant. Sodium Sulphate, or Glauber’s salt, is made from common salt by the action of concentrated sulphuric acid. It is one of the raw materials used in making glass. 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.