_Its colour is whitish or limpid; and it does not liquefy by the action of the air. It is usually observed in the form of fine capillary crystals, though it is sometimes found in a massive state. When pure, it crystallizes into six─sided prisms (Pl. II, Fig. 15) which have a rectangular base. It is denominated by chemists_ nitrat of potash. Nitre is found incrusted on the surface of the earth, in some parts of India, Africa, and Spain, and, in such abundance, as to admit of being swept off at certain seasons of the year, twice or three times a week. In our own country it not unfrequently occurs in a state of white efflorescence, on old plaster walls that are sheltered from rain. Nitre is also produced in stables and cart─houses, from the mixture of animal and vegetable substances in a state of putrescence. Many kinds of plants, which grow in soils favourable to the production of it, contain nitre: this is particularly the case with pellitory, borage, and the large sunflower. Immense quantities of nitre are annually required for the purposes of war. From its constituting one of the most important substances in the composition of gunpowder, it has been found necessary to adopt artificial modes of procuring it. In several districts of the East Indies there are places called saltpetre grounds. From these large quantities of the earth are dug, and put into cavities through which water is passed. This brings away with it the salt which the earth contains, and which is afterwards separated from the water by boiling. The East India Company, for more than a century past, has been under engagements to import into this country, and supply the board of ordnance, for his Majesty’s service, with 500 tons of nitre annually, at given rates and prices in times of peace and war. In France this article is obtained in what are called _nitrières_, or nitre beds. These consist of the refuse of animal and vegetable substances, which undergo putrefaction, mixed with calcareous and other earths; and the nitre is obtained from them by water, as above─mentioned.—The principal requisites for the formation of nitre are said to be lime, animal and vegetable matters, heat, and an open, but not too free communication with dry atmospheric air.