In all medico-legal analyses for nitric acid, care must be taken that the different reagents used are free of this acid, and also of nitrates. Sulphuric acid often contains a little nitric, or rather nitrous acid; which may be discovered by the sulphuric acid becoming brown or dark-red when a solution of proto-sulphate of iron is gently poured over it in a test-tube; and which may be removed either by boiling the acid with a few grains of sugar, according to the formula of the Edinburgh Pharmacopœia, or, as Orfila directs, by boiling it with sulphate of ammonia. SECTIONS II. III. IV.—_Of the Action, Symptoms, Morbid Appearances, and Treatment of Poisoning with Nitric Acid._ All the observations made on these topics under the head of sulphuric acid apply, with few exceptions, to the nitric acid also. A few statements therefore on the peculiarities ascertained to exist in the latter case are all that will be required in the present sections. Nitric acid is not less powerful as a corrosive and irritant than sulphuric acid. It will act with energy as an irritant even when considerably diluted, for example with six or eight parts of water or even more.—The lips which are rendered at first whitish by all the acids, and eventually brownish by sulphuric acid, becomes soon yellow with nitric acid. The tongue too sometimes acquires a yellow colour instead of a white glazed appearance; but this character is not invariable.—All spots caused by it on the skin become speedily yellow, and long retain this hue; or if the tint become dull, which generally happens in a few days, it is enlivened and the yellow colour restored for a time, by ammonia, potash, soda, or soap.—An important fact, for which toxicology is indebted to Professor Orfila, is that the acid may be often found in the urine, both when it had been swallowed, and when it had been introduced through the medium of the cellular tissue.[311] It is to be discovered by the process for compound mixtures. Orfila adds that he has hitherto been unable to find it in the liver or spleen. A difference of tint in the lining membrane of the mouth and gullet is the only difference observed in the morbid appearances caused by nitric and sulphuric acid. The former sometimes renders these parts yellow; but this appearance is far from being invariable. The treatment in both instances is the same in every respect. III.—OF POISONING WITH HYDROCHLORIC ACID. This acid occurs more rarely than any of the other mineral acids in medico-legal cases; a fact which appears singular enough on considering, that it is a powerful corrosive, and more perhaps in the hands of the working-classes than any other. SECTION I.—_Of the Tests for Hydrochloric Acid._ Like the other acids, hydrochloric acid occurs in the concentrated shape, in a state of simple dilution, and mixed with various matters, especially from organic kingdoms.