Nitrate of potassa crystallises in six-sided prisms, terminated by six-sided pyramids. Its specific gravity is 1.933. Its taste is sharp and cooling. One part is soluble in seven parts of water, at the temperature of 60 degrees, and in rather less than its own weight of boiling water.
It melts in a strong heat, and by cooling congeals into an opaque mass, called crystal mineral, or sal prunelle.
Exposed to a red heat, it disengages oxygen gas, and passes to the state of a nitrate; at a higher temperature, this is decomposed, and oxygen, azote, and a portion of nitrous acid, which has not been decomposed, are evolved. What remains is potassa. When projected on ignited coals, it burns brilliantly. Detonation also ensues by mixing nitre and charcoal, and throwing the mixture into a red-hot crucible. The residuum is carbonate of potassa. Fourcroy (Système des Connoissances Chimiques, Tome iii, p. 124.) observes, that metals, with nitrate of potassa, will decompose this salt, and produce different coloured flame, extremely brilliant, on which account such substances are used in fire-works.
The alchymists believed, they could obtain, from nitre, a liquor, which would constitute, with other substances, the philosopher's stone. The clyssus of nitre, they imagined, possessed wonderful properties. The decomposition of nitre by charcoal, they effected in two ways, viz. by submitting the mixture to the action of heat in a crucible, or, otherwise in an earthen or iron retort. In the latter case, they collected a fluid, principally water, containing some carbonic acid, and the aeriform product they suffered to escape. The residue they named nitre fixed by charcoal, or, the extemporaneous alkali of nitre. When, in the place of charcoal, a mixture of sulphur and nitre was projected into a red-hot crucible, they obtained a saline substance, to which they gave the name of sal polychrest. This is the same as vitriolated tartar, or sulphate of potassa, and is that salt which is formed in the distillation of nitric acid from nitre, and sulphuric acid. The crystal mineral, of some of the old pharmacopœias, was nothing more than nitrate of potassa fused with a portion of sulphur, and, therefore, a mixed salt, consisting of nitrate and sulphate of potassa.
Nitrate of potassa, distilled with half its weight of sulphuric acid, furnishes nitric acid, or concentrated spirit of nitre. This, diluted with about an equal weight of water, forms the aqua fortis of the shops.
A mixture of nitre and phosphorus, if struck with a hammer, produces a violent detonation. Nitre oxidizes all the metals at a red heat, even gold and platinum.
Nitre and sulphur, thrown into a red-hot crucible, produces an instantaneous combustion, accompanied with a great disengagement of light and heat. Sulphurous acid gas, with sulphuric acid, is produced.
Equal parts of cream of tartar, (supertartrate of potassa,) and nitre, deflagrated in a crucible, form white flux. Two parts of tartar, and one of nitre, treated in the same manner, produce black flux.
Three parts of nitre, one part of sulphur, and one part of sawdust, mixed together, form the powder of fusion.
When three parts of nitre, two parts of potash, and one of sulphur, all previously well dried, are mixed together, the compound is called pulvis fulminans, or, fulminating powder. A small portion of this powder, or as much as will lay on a shilling-piece, put on a shovel, and exposed to heat, will first melt, become liver-coloured, and then explode with great noise. The theory of this explosion is, that a part of the sulphur, and the potassa unite, and form a sulphuret; the sulphuret then decomposes water, and produces sulphuretted hydrogen gas, which appears to be decomposed by the nitric acid; and there results sulphurous acid gas, water, and, as Thenard observes, protoxide of azote, azotic gas, and sulphate of potassa. The loudness of the report depends on the combustion of the whole powder at the same instant, which is secured by the previous fusion it undergoes. Gunpowder, on the contrary, burns in succession, although apparently instantaneous. In using common potash, there is also, as the alkali contains it, carbonic acid, given out in the state of gas. In fact carbonic acid appears to assist the explosive effect of this powder, for when it is prepared with potash, containing little carbonic acid, its detonating power is considerably less.