Whenever therefore such a source of fallacy can be suspected, the operator would do well to repeat his experiment on white paper, in the manner I have before proposed, and the results which are obtained in glasses should always be examined by day-light, and viewed by reflected and not by transmitted light. The presence of Peroxide of Iron in the Cupreous salt will also impart a green colour to the precipitate produced by an alkali. To obviate any fallacy which might arise from this circumstance, Mr. Phillips proposes to add some pure Potass to the sulphate of copper; if pure, a fine blue precipitate will be thus obtained; to this may be then added the suspected solution, and if Arsenious acid be present, it will then convert this blue precipitate to a green one.
(C.) Sulphuretted Hydrogen. This is a very delicate test for arsenic, producing with its solution a beautiful golden coloured liquor, which after a short time lets fall a precipitate. Mr. Phillips,[[420]] in reviewing the third edition of the present work, has stated, that no such precipitate occurs, but I find that in close, as well as in vessels exposed to the air, it takes place by repose. By this reagent, so small a quantity as 1
000,000 may be detected in solution; and it may be also stated in farther proof of the utility of this test, that it is less affected than any other by the presence of animal or vegetable matter. The method of preparing a solution of sulphuretted hydrogen gas is extremely simple. Put into an oil flask about two ounces of undiluted muriatic acid and an ounce and a half of powdered Sulphuret of Antimony; fit a cork to the flask and pass through it the short leg of a small glass tube twice bent at right angles; pass the longer leg of the tube into a phial containing distilled water, and then by the heat of a spirit lamp applied to the flask, sulphuretted hydrogen gas will be abundantly liberated, and though much of it will escape, yet a sufficient quantity will be dissolved by the water. The annexed sketch represents the apparatus proposed by Mr. Phillips, and which will require but little practical skill, either for its construction or use.
But it is not always necessary to prepare a watery solution of Sulphuretted Hydrogen, a stream of the gas, introduced into the suspected liquor by means of the above apparatus, will act with equal delicacy, and possesses, as Dr. Christison has observed, the advantage of not diluting it. Before applying this test, it is necessary to add an acid, if any alkali should exist along with the Arsenic, otherwise no precipitate will take place.
(D.) Alkaline Hydro-sulphurets. These bodies do not affect the arsenious solution, unless a few drops of acetic acid be added. To the Hydro-sulphuret, or perhaps more properly, Hydroguretted Sulphuret, of Ammonia there is an insuperable objection, since this fluid, when diluted, possesses the colour which we expect to produce by the action of Sulphuretted Hydrogen upon Arsenious Acid.
(E.) Charcoal Powder. This test was proposed by Mr. A. Thomson, (London Dispensatory, 2nd edition, p. 53.) Into the suspected solution stir a moderate quantity of charcoal powder, allow it to settle, then pour off the supernatant liquor, and when the powder which remains is dry, sprinkle some of it on a red hot poker, when, if the solution should contain Arsenic, the odour of Garlic will be rendered sensible. I have already offered some remarks upon the cause of this phenomenon (page 304.)
There are several other tests by which arsenic may be identified. The process described in the Dublin Pharmacopœia for the preparation of Arsenias Kali, the arseniate, or rather super-arseniate of potass, which has been long known under the name of “the arsenical salt of Macquer,” has been strongly advised as a collateral proof; it consists in decomposing the nitrate of potass[[421]] by the arsenious acid, but since this problem requires that the suspected poison should be in a solid and palpable form, it is impossible to examine its pretensions to our confidence, without being reminded of the story so often told to us in our infancy, of catching a bird by laying salt upon its tail.
It is necessary to observe in this place, that the arseniate, like the arsenite of potass, or that of ammonia, is obedient to the silver test, but that instead of the yellow precipitate which is produced by the latter salt, we obtain, by the former, a red or brick-coloured one.
If arsenious acid and quick-lime be heated together in a glass tube, a sudden ignition is occasioned at a certain temperature, when metallic arsenic will sublime, and an arseniate of lime be formed. In this case one portion of the arsenious acid is robbed of its oxygen to complete the acidification of the remainder.
In taking an impartial review of all the evidence which the investigation of this subject can furnish, it must appear to the most fastidious, that the silver and copper test above described are capable, under proper management, of furnishing striking and infallible indications, and that in most cases they will be equally conclusive, and in some even more satisfactory in their results, than the metallic reproduction upon which such stress has been laid, and for this obvious reason, that unless the quantity of metal be considerable,[[422]] its metallic splendour and appearance is often very ambiguous and questionable. It has to my knowledge happened to a medical person, by no means deficient in chemical address, to ascribe to the presence of arsenic that which was no other than a film of very finely divided charcoal: in this state of doubt the last resource was to ascertain whether it yielded, or not, upon being heated, an alliaceous odour. Surely an unprejudiced judge would prefer the evidence of sight, as furnished by the arsenical tests, to that of smell, as afforded in the last experiment. No one will attempt to deny that it is the duty of the medical practitioner who is called upon to decide so important a question as the presence of arsenic, to prosecute by experiment every point which admits the least doubt; he should also remember that in a criminal case, he has not only to satisfy his own conscience, but that he is bound, as far as he is able, to convince the public mind of the accuracy and truth of his researches; and he fails in his duty if he omits, through any false principle of humanity, to express the strong conviction which the success of his experiments must necessarily have produced in his mind. Let it however be remembered, that the application of chemical reagents on solutions suspected to contain arsenic, so far from throwing any obstacle in the way of the metallic reduction of that body, are the very steps which should be adopted as preparatory to the “experimentum crucis,” since the precipitates which are thus produced may be collected, and easily decomposed, as before stated. Those who for judicial purposes may require farther information upon these subjects are referred to the second volume of our work on “Medical Jurisprudence.” Tit: Poisons.