10. The manner of proceeding in the examination of various kinds of saltpetre in the magazines of the administration. This article relates to the different modes of examining saltpetre.
11. On the manufacture of potash and pearlash. This subject is important, as potash is an indispensable article in the preparation of saltpetre, and the formation of the alkali may be considered as of primary magnitude in establishments, conducted upon so large a scale as those of France.
It is thus, that a regular system is adopted, by the French government, for the production of saltpetre; and we may add also, for the manufacture of gunpowder, which we notice in that article.
It may be proper to mention some facts, respecting the formation of nitre-beds, and the means adopted, in this way, to obtain saltpetre, and to offer, at the same time, some observations on this mode of obtaining nitre.
The Mémoires de l'Académie des Sciences, 1720, contain the observations of M. Bouldoc, relative to the process of lixiviating saltpetre earths. Lacourt published a pamphlet some years after, entitled, Instruction concernant la Fabrication du Saltpetre. Various dissertations appeared on the same subject. In 1775, the French Academy of Sciences proposed a prize-question, which produced a more thorough investigation. The Memoirs of Thouvenal, of the Chevalier de Lorgna, and of MM. de Chevrand, and Ganivel, were highly approved, some of which took the prize. Chaptal, who has done more, perhaps, than any other person in France, to promote this all-important object, published, in 1794, an excellent dissertation, founded on experiment and observation. This Memoir was published in the Journal des Arts et Manufactures, t. iii, p. 12.
Kirwan (Geological Essays, p. 143,) remarks, that the saline crust, which is found on the walls of the houses of Malta, is owing to the walls being built of fine grained limestone. When wetted with sea-water, it never dries. The crust is nitrate of potassa, nitrate of lime, and muriate of soda, and is some tenths of an inch thick. Under this crust, the stone moulders into dust. When the first falls off, it is succeeded by a second, and so on, until the whole stone is destroyed. This particular effect, however, is attributed to the presence of marine salt.
Mr. Kirwan observes, that, "M. Dolomieu shows, at the end of his Tract on the Lipari Islands, that the atmosphere of Malta, in some seasons, when a south wind blows, is remarkably fouled with mephitic air; and, at other times, when a north wind blows, remarkably pure; and hence, of all others, most fit for the generation of nitrous acid." Mr. Kirwan remarks, "How the alkaline part of the nitre, which is one of the products resulting from the decomposition of this stone, is formed, is as yet mysterious: Is it not from the tartarin lately discovered in clays and many stones?" He adds, after speaking of animal and vegetable decomposition, "I should rather suppose, that the alkali is conveyed into these earths by the putrid air, than newly formed; and the reason is, that tartarin, (potash,) notwithstanding its fixity, is also found in soot; and, in the same manner, may be elevated in putrid exhalations."
Artificial nitre-beds consist of the refuse of animal and vegetable substances, undergoing putrefaction, mixed with calcareous earth; the refuse of old buildings, particularly plaster; earths from the vicinity of inhabited buildings; blood, urine, &c. They are covered, from the rain, by a shed, open at the sides. Cramer, an author of credit, informs us, that he made a little hut, with windows to admit the wind. In this, he put a mixture of garden mould, the rubbish of lime, and putrid animal and vegetable substances. He frequently moistened them with urine, and in a month or two found his composition very rich in saltpetre, yielding at least one-eighth part of its weight. The practice of obtaining nitre from nitre beds, was followed in France and Germany. It is extracted and refined by the process already given.
When oxygen gas is presented to azote at the moment of its liberation, nitric acid is formed. As ammonia is the result of animal putrefaction, or is formed in the process, hydrogen must unite also with azote. The azote is furnished by the animal substances. These facts being known, we are enabled to account for the generation of nitric acid, and, consequently, of the earthy and other nitrates, in artificial nitre beds.