SALTPETRE, the commercial name given to three naturally occurring nitrates, distinguished as (I) ordinary saltpetre, nitre or potassium nitrate, (2) Chile saltpetre, cubic nitre or sodium nitrate (3) wall saltpetre or calcium nitrate. These nitrates generally occur as efflorescences caused by the oxidation of nitroge nous matter in the presence of the alkalis and alkaline earths.
I. Ordinary Saltpetre or Potassium Nitrate, occurs, mingled with other nitrates, on the surface and in the superficial layers of the soil in many countries, especially in certain parts of India, Persia, Arabia and Spain. The deposits in the great limestone caves of Kentucky, Virginia and Indiana have been probably derived from the overlying soil and accumulated by percolating water ; they are of no commercial value. The actual formation of this salt is not quite clear; but it is certainly con ditioned by the simultaneous contact of decaying nitrogenous matter, alkalis, air and moisture. The demand for saltpetre as an ingredient of gunpowder led to the formation of saltpetre plantations or nitriaries, which at one time were common in France, Germany and other countries; the natural conditions were simulated by exposing heaps of decaying organic matter mixed with alkalis (lime, etc.) to atmospheric action. The salt is obtained from the soil in which it occurs naturally, or from the heaps in which it is formed artificially, by extracting with water, and adding to the solution wood-ashes or potassium carbon ate. The liquid is filtered and then crystallized. Since potassium nitrate is generally more serviceable than the sodium salt, whose deliquescent properties inhibit its use for gunpowder manufacture, the latter salt, of which immense natural deposits occur (see below), is converted into ordinary saltpetre in immense quan tities. This is generally effected by adding the calculated amount of potassium chloride (of which immense quantities are obtained as a by-product in the Stassfurt salt industry) dissolved in hot water to a saturated boiling solution of sodium nitrate; the com mon salt, which separates on evaporating the solution, is removed from the hot solution, and on cooling, the potassium nitrate crystallizes out and is separated and dried.
As found in nature, saltpetre generally forms aggregates of delicate acicular crystals, and sometimes silky tufts, but not as distinctly developed crystals. When crystallized from water, crystals belonging to the orthorhombic system, and having a prism angle of 61° o', are obtained; by the evaporation of hot solutions, unstable rhombohedra may be obtained, but these rapidly revert to the more stable form on cooling. The hardness is 2, and the specific gravity 2.1. It is fairly soluble in water; ioo parts at o° C dissolving 13.3 parts of the salt, and about 3o parts at 20° ; the most saturated solution contains 327-4 parts of the salt in 1 oo of water; this solution boils at 114.1°. It fuses at to a colourless liquid, which solidifies on cooling to a white fibrous mass, known in pharmacy as sal prunella. It is an energetic oxidizing agent, and on this property its most important applica tions depend. At a red heat it evolves oxygen with the formation of potassium nitrite, which, in turn, decomposes at a higher temperature. Heated with many metals it converts them into oxides, and with combustible substances, such as charcoal, sul phur, etc., a most intense conflagration occurs. Its chief uses are in glass-making to promote fluidity, in metallurgy to oxidize impurities, as a constituent of gunpowder and in pyrotechny; it is also largely used in the manufacture of nitric acid.
Potassium nitrate was used at one time in many different diseased conditions, but it is now never administered internally, as its extremely depressant action upon the heart is not com pensated for by any useful properties which are not possessed by many other drugs. One most valuable use it has, however, is the treatment of asthma. All nitrites (e.g., sodium nitrite, ethyl nitrite, amyl nitrite) cause relaxation of involuntary muscular fibre and therefore relieve the asthmatic attacks, which depend upon spasm of the involuntary muscles in the bronchial tubes.