The strongest acid that Mr. Kirwan could procure at 60° was 1.5543, which by his calculation contained .7354 of real acid ; but Itouelle professes to have ob tained it of 1.583. It is observable, that, on comparing the tables of Kirwan and Davy, the aeriform acid appears to contain a considerable portion of water more than that which is combined with soda to form the nitrate.
Nitric acid should be of the specific gravity of 1.5, or a little more, and co lourless. It boils at 248°, and may be dis tilled without any essential alteration. Exposed to the air it absorbs moisture. If two parts be suddenly diluted with one of water, the temperature will rise to about 112° ; but the addition of more wa ter to this diluted acid will lower its tem perature. It retains its oxygen with little force, so that it is decomposed by all com bustible bodies. Brought into contact with hrydogen gas at a high temperature, a violent detonation ensues, so that this must not be done without great caution. It inflames volatile oils, such as those of turpentine and cloves, when suddenly poured on them : but, to perform this ex periment with safety, the acid must be poured out of a bottle tied to the end of a long stick, otherwise the operator's face and eyes will be greatly endangered. If it be poured on perfectly dry charcoal powder, it excites combustion, with the emission of copious fumes. By boiling it with sulphur it is decomposed, and its oxygen, uniting with the sulphur, forms sulphuric acid. Chemistsin general agree, that it acts very powerfully on almost all the metals ; but Ilanm4 has asserted, that it will not dissolve tin ; and Dr. Wood house of Pennsylvania affirms, that in a highly concentrated and pure state it acts noa at all on silver, copper, or tin, though with the addition of a little water its ac tion on them is very powerful. He does not mention the specific gravity of tlii4 acid : he only says, that it was prepared by first expelling the water of crystalliza tion from nitre by heat, and then decom posing this nitre by means of strong sul phuric acid.
The nitric acid is of considerable use in the arts. It is employed for etching on copper ; as a solvent of tin to form with that metal a mordant tor some of the finest dyes ; in metallurgy and assaying ; in various chemical processes, on account of the facility with which it parts with oxygen and dissolves metals ; in medicine as a tonic, and as a substitute for mercurial preparations in siphytis and affections of the liver ; as also in the form of vapour, to destroy contagion. For the purposes
of the arts it is commonly used in a di luted state, and contaminated with the sulphuric and muriatic acids, by the name of aqua fortis. This is generally prepared by mixing common nitre with an equal weight of sulphate of iron, and half its weight of the same sulphate calcined, and distilling the mixture : or by mixing nitre with twice its weight of dry powdered clay, and distilling in a reverberatory fur nace. Two kinds are found in the shops, one called double aqua fortis, which is about half the strength of nitric acid ; the other simply aqua fortis, which is half the strength of the double.
A compound made by mixing two parts of the nitric acid with one of muriatic, known formerly by the name of aqua re gia, and now by that of nitro-muriatic acid, has the property of dissolving gold and platina. On mixing the two acids; heat is given out, an effervescence takes place, oxygenated muriatic acid gas is evolved, and the mixture acquires an orange colour. This is likewise made by adding gradually to an ounce of powder ed muriate of ammonia, four ounces of double aqua fortis, and keeping the mix ture in a sand-heat till the salt is dissolved; taking care to avoid the fumes, as the vessel must be left open : or by distilling nitric acid with an equal weight, or rather more, of common salt.
With the different bases the nitric acid forms nitrates.
The nitrate of barytes, when perfectly pure, is in regular octaedral crystals, though it is sometimes obtained in small shining scales. It may be prepared by uniting barytes directly with nitric acid, or by decomposing the carbonate of sul phuret of barytes with this acid. Ex posed to heat it decrepitates, and at length gives out its acid, which is decomposed;, but if the heat be urged too far, the ba rytes is apt to vitrify with the earth of crucible. It is soluble in 12 parts ofcold, and 3 or 4 of boiling water. It is said to exist in some mineral waters.