The nitrate of potash is the salt well known by the name of nitre, or salt petre. It is fbund ready formed in the East Indies, in Spain, in the kingdom of Naples, and elsewhere, in considerable quantities ; but nitrate of lime is still more abundant. Far the greater part of the nitrate made use of is produced by a combination of circumstances which tend to compose and condense nitric acid. This acid appears to be produced in all situa tions, where animal matters are com pletely decomposed with access of air, and of proper substances with which it can readily combine. Grounds frequent ly trodden by cattle, and impregnated with their excrements, or the walls of in habited places where putrid animal va pours abound, such as slaughter-houses, drains, or the like, afford nitre by long exposure to the air. Artificial nitre beds are made by an attention to the circum stances in which this salt is produced by nature. Dry ditches are dug, and cover ed with sheds, open at the sides, to keep off the rain : these are filled with animal substances, such as dung, or other excre ments, with the remains of vegetables, and old mortar, or other loose calcareous earth ; this substance being found to be the best and most convenient receptacle for the acid to combine with. Occasional watering, and turning up from time to time, are necessary, to accelerate the process, and increase the surfaces to which the air may apply ; but too much moisture is hurtfUl. When a certain por tion of nitrate is formed, the process ap pears to go on more quickly : but a cer tain quantity stops it altogether, and after this cessation the materials will go on to furnish more, if what is formed be ex tracted by lixiviation. After a succes sion of many months, more or less, ac cording to the management of the opera tion, in which the action of a regular current of fresh air is of the greatest importance, nitre is found in the mass. If the beds contained much vegetable matter, a considerable portion of the nitrous salt will be common saltpetre ; but, if otherwise, the acid will, foe the most part, be combined with the calcare ous earth.
To extract the saltpetre from the mass of earthy matter, a number of large casks are prepared, with a cock at the bottom of each, and a quantity of straw .within, to prevent its being stopped lip. Into these the matter is put, together with wood-ashes, either strewed at top, or add ed during the filling. Boiling water is then poured on, and suffered to stand for some time ; after which it is drawn off, and other water added in the same man ner, as long as any saline matter can be thus extracted. The weak brine is heat ed, and passed through other tubs, until it becomes of considerable strength. It is then carried to the boiler, and contains nitre and other salts ; the chief of which is common culinary salt, and sometimes muriate of magnesia.
It is the property of nitre to be much more soluble in hot than cold water ; but common salt is very nearly as soluble in cold as in hot water. Whenever, there fore, the evaporation is carried by boiling to a certain point, much of the common salt will fall to the bottom, for want of water to hold it in solution, though the nitre will remain suspended by virtue of the heat. The common salt thus sepa
rated is taken out with a perforated la dle, and a small quantity of the fluid is cooled, from time to time, that its con centration may he known by the nitre which crystallizes in it. When the fluid is sufficiently evaporated, it is taken out and cooled, and great part of the nitre separates in 'crystal ; while the remaining common salt dissolved, be cause equally soluble in cold and in hot water. Subsequent evaporation of the residue will separate more nitre in the same manlier.
This nitre, which is called nitre of the first boiling, contains some common salt ; from which it may be purified by solution in a small quantity of water, and subse quent evaporation : for the crystals thus obtained are much less contaminated with common salt than before ; because the proportion of water is so much larger with respect to• the small quantity con tained by the nitre, that very little of it will crystallize. For nice purposes, the solution and crystalliiation of nitre are re peated four times. The crystals of nitre are usually of the form of six-sided flat tened prisms, with diedral summits. Its taste is penetrating ; but the cold pro duced, by placing the salt to dissolve in the mouth, is such as to predominate over the real taste at first. Seven parts of water dissolve two of nitre, at the tem perature of sixty degrees: but boiling water dissolves its own weight. One hundred parts of alcohol, at a heat of one hundred and seventy-six degrees, dissolve only 2.9.
On being exposed to a gentle heat, nitre fuses ; and in this state being pour ed into moulds, so as to form little round cakes, or balls, it is called sal prunella, or crystal mineral. This at least is the way in which this salt is now usually prepar ed, conformably to the directions of Boer heave ; though in most dispensatories a twenty-fourth part of sulphur was direct ed to be deflagrated on the nitre, before it was poured out. This salt should not be left on the fire after it has entered in to fusion, otherwise it will be converted into a nitrite of potash. If the heat be in creased to redness, the acid itself is de composed, and a considerable quantity of tolerably pure oxygen gas is evolved, succeeded by nitrogen.
This salt powerfully promotes the com bustion of inflammable substances. Two or three parts mixed with one of charcoal, and set on fire, burn rapidly ; azote and carbonic acid gas are given out, and a small portion of the latter is retained by the alkaline residuum, which was former ly called clyssus of nitre. Three parts of nitre, two of subcarbonate of potash, and one of sulphur, mixed together in a warm mortar, form the fulminating pow der ; a small quantity of which, laid on a fire shovel, and held over the fire till it begins to melt, explodes with a loud sharp noise. Mixed with sulphur and char coal, it forms gunpowder. See Gerxrow PER.