The mineral acids are also very interesting on scientific grounds. They afford the purest examples of true corrosive poisons, their poisonous effects depending entirely on the organic injury they occasion in the textures to which they are applied. It is of use to set out, in investigating the effects of poisons, by determining the phenomena presented under such circumstances. When made aware of the rapidity with which other irritating poisons prove fatal, and the slight signs they commonly leave of their operation, one cannot fail to be struck with discovering what the animal frame will sometimes endure from these the most violent of all irritants, and nevertheless recover.
In laying down the mode of determining by chemical evidence a case of supposed poisoning with any of the three mineral acids mentioned above, it will be unnecessary to notice any of their chemical properties, except those from which their medico-legal tests are derived.
The only common properties that require notice are, their power of reddening the vegetable blue colours, for showing which litmus-paper is commonly used, and is most convenient: and their power of staining and corroding all articles of dress, especially such as are made of wool, hair, and leather. This last property is specified, though a familiar one, because it always forms important evidence in criminal cases. In order to give precision to such evidence, it is necessary to remember, that if the article of dress is a coloured one, it is generally rendered red by the mineral acids; but that the vegetable acids also will redden most articles of dress, although they do not corrode them.
I.—Of Poisoning with Sulphuric Acid.
Sulphuric acid is extensively employed in very many trades, is used even for some domestic purposes, and is consequently familiar to every one. Hence it is the mineral acid which has been most commonly used as a poison, especially for committing suicide. Of 35 cases of poisoning with the mineral acids which occurred in England in the years 1837 and 1838, 32 were caused by this acid (p. [90]).
Section I.—Of the Tests for Sulphuric Acid.
Sulphuric acid is known as a poison chiefly in the form of the concentrated commercial acid. But a few cases of poisoning have also been produced by blue-liquor or the solution of indigo in strong sulphuric acid; and one instance[[233]] has been recorded of poisoning with the aromatic sulphuric acid of the Pharmacopœias, which is an infusion of aromatics in a mixture of sulphuric acid, ether and alcohol. In the following remarks on its tests, it will be sufficient to consider it first in the concentrated form,—secondly, in a state of simple dilution,—and thirdly, when mixed with various impurities, more especially with vegetable and animal matter. The acid solution of indigo may be known by the tests for the concentrated acid, and its blue colour, removable by a solution of chlorine; and the aromatic sulphuric acid may be distinguished by its odour and the tests for the diluted acid.
1. When concentrated it is oily-looking, colourless, or brownish from having acted on organic particles, without odour, much heavier than water, and capable of quickly corroding animal substances. If from these properties, and its effect in reddening litmus, its exact nature be not considered obvious, it may be heated with a few chips of copper; when sulphurous acid is disengaged and may be readily recognised by its odour.
2. When diluted, it may be distinguished from all ordinary acids by solution of nitrate of baryta occasioning a heavy white precipitate of sulphate of baryta, which is insoluble in nitric acid. Selenic and sulphurous acids, however, and also, as Mr. Alfred Taylor informs me he has lately found, the fluo-silicic acid, are similarly acted on in all respects. But selenic and fluo-silicic acids in all forms, and sulphurous acid in a state of solution, are so seldom met with, being known only in the laboratory of the scientific chemist, that they can scarcely be considered sources of fallacy. Sulphuric acid may at once be distinguished from sulphurous acid, by the latter possessing a peculiar pungent odour. From the two other acids it may be distinguished by collecting and drying the barytic precipitate, mixing this with charcoal, converting it into sulphuret of barium by heating it in a platinum spoon before the blowpipe, and then adding diluted muriatic acid to the sulphuret, so as to disengage sulphuretted hydrogen gas,—which again is easily known by its odour, or its property of blackening paper dipped in solution of acetate of lead. A much more important source of fallacy than these is the possible presence of a bisulphate in solution, or a neutral sulphate along with any other free acid; for these substances will present the same reactions with litmus and barytic salts as free sulphuric acid itself. Much has been published lately upon this point; but the difficulty has not yet been satisfactorily overcome. It may be got rid of indeed by proving, that no bisulphate or neutral sulphate is present. Their absence may be shown by no solid residuum being left on evaporating the suspected fluid, or at least no more than a mere haziness, owing to the sulphate of lead which commercial sulphuric acid always contains in small quantity. Or as Orfila suggests, we may establish their absence still better by concentrating the fluid, and finding that neither carbonate of soda, which would cause a precipitate with earthy or metallic bases, nor chloride of platinum, which would do so with potash or ammonia in combination, nor fluo-silicic acid, which precipitates soda salts, has any effect when applied to separate portions of the subject of inquiry. But suppose it appears in the course of these trials that one or more bases are actually present, how is it to be settled whether the sulphuric acid, indicated by litmus and a salt of baryta, is really free or not? To this question I must reply, that no method has yet been proposed, which is at once satisfactory and easily available. Mr. Alfred Taylor proposes to concentrate the fluid, and agitate it with alcohol, in the hope that the alcohol will remove sulphuric acid, and not a sulphate, from the water.[[234]] But it removes sulphuric acid from a bisulphate even when dry, and still more when a little water is present. Orfila[[235]] proposes, in the case of sulphuric acid in vinegar,—where there is both a vegetable acid and a neutral sulphate of lime,—to concentrate to a sixth, and agitate the residuum with four times its volume of sulphuric ether, in the expectation that this fluid will remove the free acid alone, and separate it from sulphates. But notwithstanding the authority of his name for the fact, pure ether will not remove sulphuric acid from a watery fluid; and etherized alcohol, which does remove it, takes it away also, like alcohol itself, from bisulphates. These results I have observed in some careful trials made along with Dr. Douglas Maclagan. I suspect, therefore, that where sulphates or bisulphates do exist, there is no absolutely satisfactory way of determining whether free sulphuric acid also co-exists, except by a quantitative analysis, for ascertaining whether the amount of acid and of bases corresponds with this supposition or not. And it is scarcely necessary to add, that so operose a method is scarcely applicable to ordinary medico-legal investigations.
3. It is seldom that the medical jurist is called on to search for sulphuric acid in either of the states already mentioned. Much more generally it has mingled with and acted on various organic substances. The circumstances in which it has usually to be sought for in the practice of medical jurisprudence are twofold,—on the one hand, in stains on clothes,—and on the other, in vomited matter, the contents of the stomach, or organic mixtures generally.