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. In noticing this subject, it is unnecessary to quote the opinion of Stahl, who believed that there was but one acid in nature, the sulphuric; and that nitric acid was the sulphuric acid, combined with phlogiston, which he affirmed was produced by putrefaction; nor is it necessary to mention the opinion of Lemery, who believed that nitre exists ready formed in animals and vegetables by the processes of vegetation and animalization. The experiments of the French philosophers have put these opinions at rest. Thouvenal discovered, that nothing more was necessary for the production of nitre than a basis of lime, heat, and open air; so that nitre beds, formed of putrefying animal and vegetable substances, with the conditions thus stated, must produce saltpetre; a fact which experience abundantly justifies. The process for the formation of nitre, is called _nitrification_.