NITRIC acid. The two principal con stituent parts of our atmosphere, when in certain proportions, are capable, under particular circumstances, of combining chemically, into one of the most powerful acids, the nitric, which consists, according to Mr. Davy, of 70.5 of oxygen, and 29.5 of azote, or nitrogen. If these gases be mixed in this proportion in a glass tube, about a line in diameter, over mercury, and a series of electric shocks be passed through them for some hours, they will form nitric acid ; or, if a solution of pot ash be present with them, nitrate of pot ash will be obtained. The constitution of this acid may be further proved, analyti cally, by driving it through a red-hot por celain tube, as thus it will be decomposed into oxygen and nitrogen gases. For all practical purposes, however, the nitric Acid is obtained from nitrate of potash, from which it is expelled by acid.
Four parts of pure nitrate of potash, coarsely powdered, are to be put into a glass retort, and three parts of conceit. trated sulphuric acid cautiously added, taking care to avoid the fumes that arise, which is best done by standing in a cur. rent of air, to convey them up the aim. ney. Join to the retort a tubulated re• ceiver of large capacity, with an adopter interposed, and lute the junctures with a mixture of pipe-clay, sifted sand, and cut tow. In the tubulure fix with fat lute a glass tube terminating in another large receiver, in which is a small quantity of water ; and, if you wish to collect the gaseous products, let a bent glass tube from this receiver communicate with a pneumatic trough. Apply heat to the re. ceiver by means of a sand bath. The first product that passes into the receiver iS generally red and fuming ; but the Bp• pearances gradually diminish, till the acid comes over pale, and even colourless, if the materials used were clean. After this it again becomes more and more red and fuming, till the end of the operation ; and the whole mingled together will be of a yellow or orange colour.
In the large way, and for the purposes of the arts, extremely thick cast iron or earthen retorts are usually employed, to which an earthen head is adapted, and connected with a range of proper con densers. The strength of the acid too is varied, by putting more or less water in the receivers. Thenitrie acid thus made generally contains sulphuric acid, and also mUriatic, from the impurity of the nitrate employed. If the former, a solution of nitrate of barytes will occasion n white precipitate : if the latter, nitrate of silver will render it milky. The sulphuric acid may be separated by a second distillation from very pure nitre, equal in weight to an eighth of that originally employed ; or by precipitating with nitrate of barytes, decanting the clear liquid, and distilling it. The muriatic acid may be separated by proceeding in the same way with ni• trate of silver, or with litharge, decanting the clear liquor, and redistilling it, leav ing an eighth or tenth part in the retort, The acid for the last process should be condensed as much as possible, and the redistillation conducted very slowly; and if it be stopped when half is come over, beautiful crystals of muriate of lead will be obtained on cooling the remainder, if litharge be used, as M. Steinacher informs tts ; who also adds, that the vessels should be made to fit tight by grinding, as any lute is liable to contaminate the product.
As this acid still holds in solution more or less nitrous gas, it is not, in fact, nitric acid, but a kind of nitrous : it is therefore necessary to put it into a retort, to which a receiver is added, the two vessels not being luted, but merely joined by paper ; and to apply a very gentle heat for several hours, changing the receiver as soon as it is filled with red vapours. The nitrous gas will thus be expelled, and the nitric acid will remain in the retort, as limpid and colourless as water. It should he kept in a bottle secluded from the light, otherwise it will lose part of its oxygen.