If the acid be in any quantity, and without mixture, there cannot exist any difficulty in demonstrating its presence. If added to copper filings, there will be a copious disengagement of orange-coloured fumes, and a nitrate of copper of a blue colour will remain, as the product. If it be saturated with potass, we shall at once obtain by due evaporation the well known substance, nitre; this salt will announce its nature by deflagrating with charcoal or sulphur. This latter test is the one we must employ for the detection of nitric acid, when mixed with vinegar, and other liquids. Where the acid has combined with the animal matters with which it may have come into contact, they must be boiled for an hour in a solution of pure potass, when the solution will assume a reddish appearance; this must be filtered, and evaporated in a capsule of porcelain, when the mass so obtained will leave a residuum of nitrate of potass. _Dr. Marcet_, in a paper just published in the _Philosophical Transactions_,[324] on the composition of sea water, employed a new mode of assaying the solution for nitric acid, and for which he acknowledges himself indebted to _Dr. Wollaston_. Having concentrated the _bittern_ in a glass vessel, until it began to deposit solid matter, he added sulphuric acid and gold leaf, and boiled the mixture; the gold leaf was not in the least acted upon, nor was any smell of nitric acid perceived; but on adding the smallest quantity of nitre to the same mixture, the gold was dissolved, and the smell of _aqua regia_ instantly perceived. The rationale of the experiment is obvious, gold, although insoluble in muriatic acid, is instantly dissolved on the addition of nitric acid, in consequence of the developement of chlorine. SPIRIT OF SALT. _Muriatic Acid._ The liquid acid, of which we are about to treat, is a solution of muriatic acid gas in water; when of the specific gravity 1·16, according to _Davy_, it contains 32·32 per cent. of the gas, which recent experiments have proved to be a compound of _Chlorine_ (oxy-muriatic acid) and _hydrogen_, in equal volumes. It has accordingly received a name expressive of its composition, and is called _Hydro-chloric acid_. Its odour is strong and peculiar; when exposed to the air it emits white fumes; its taste is intensely sour and caustic; it is, however, the weakest of the three mineral acids, and no remarkable elevation of temperature is produced by dilution. It readily combines with potass, soda, &c. and furnishes a class of salts which may be easily recognised by their characters. _Symptoms of Poisoning by Muriatic Acid._