This poison may either be submitted to the judicial physician for examination, in its solid form, or in that of solution; and in this latter state it may be mixed with various alimentary substances, whose presence will necessarily embarrass the inexperienced operator, and multiply the apparent difficulties of his task. It becomes our duty, therefore, upon this occasion, to enter very fully and minutely into the history of the various processes, which have been proposed for the solution of the important problem under consideration; to appreciate the relative value of each, and to point out the sources of fallacy and failure, to which they are severally exposed.
Such a review of the subject would, moreover, appear to be essentially necessary at the present period, since the evidence, lately delivered on an extraordinary trial,[[240]] has, to a certain extent, very unjustly shaken the public confidence in the tests of chemistry. We shall therefore proceed to consider the processes which are calculated to lead to the detection of Arsenic, in relation to the different circumstances under which it may be presented for investigation, viz. 1, In a solid form; 2, In the simple state of solution; and 3, In the state of combination with various alimentary substances.
1. The Arsenic is in a solid form. This is the most simple case which can occur, and the experiments by which its presence is to be demonstrated, will constitute the basis of the inquiry, which we shall be hereafter called upon to institute, for the detection of the same substance under other circumstances of mixture and combination.
The order of succession to be observed in the different experiments which we are about to describe, must, in a great measure, be regulated by the quantity of the material to be submitted to examination. Should it be small, it will be prudent to reserve the process of metallization, by which a considerable loss must necessarily arise, until we have submitted it to the various re-agents which are calculated to afford indications of its nature. If, on the contrary, the quantity of the substance exceed two or three grains, it will be adviseable to proceed in its examination by the following processes, reserving a portion for future analysis.
A. By its reduction to a metallic state. Mix a portion of the suspected substance in powder, with three times its weight of black flux[[241]]; put the mixture into a thin glass tube, about eight inches in length, and a quarter of an inch in diameter, and which is hermetically sealed[[242]] at one end. Should any of the powder adhere to the sides of the tube it must be carefully brushed off with a feather, so that the inner surface of its upper part may be perfectly clean and dry. The closed end of the tube, by way of security, may be thinly coated with a mixture of pipe-clay and sand[[243]]; but this operation is not absolutely necessary. The open extremity of the tube is to be loosely plugged with a piece of paper. The coated end must now be submitted to the action of heat, by placing it in a chaffing dish of red hot coals, for ten minutes, or a quarter of an hour; when, if our supposition respecting the nature of the substance has been correct, metallic arsenic will sublime, and be found lining the upper part of the tube with a brilliant metallic crust. The glass tube, when cold, may be separated from its sealed end by the action of a file, which will enable us to collect and examine the metallic sublimate. If a portion of this brilliant matter be laid on heated iron, it will indicate its nature by exhaling in dense fumes, having a powerful smell of garlic. Another portion should be reserved for future experiments.
This method of detecting the presence of Arsenious acid has been considered the most decisive, and indeed the only unexceptionable one, but of this we shall speak hereafter; at present we have only to observe, that it is very far from being a minute test, for Dr. Bostock[[244]] confesses that where less than three-fourths of a grain were used, he could not say that the metallic crust was clearly perceptible; and Dr. Black[[245]] appears to have considered that one grain was the smallest quantity which could be distinctly recognised by such a process.
Chemists were formerly[[246]] in the habit of at once projecting any substance, supposed to be Arsenic, on some burning body, in order to develope the alliaceous odour; we have accidentally stumbled upon an instance of this kind, in the fourth volume of the London Medical and Physical Journal, which may serve as an illustration; it is a case communicated by F. Thackeray, Esq. of a child poisoned by arsenic, in which the author says, “the inner surface of the stomach was very red, and was studded throughout with a white powder, which when exposed to the flame of a candle, yielded fumes, and a garlic odour was emitted, proving it was arsenic; of which there can be no doubt, as the girl afterwards confessed that she had given arsenic to the infant.”
After the facts we have offered with respect to the alliaceous odour of arsenical fumes, it is only necessary to state, in this place, that such a test, when conducted in the manner just related, must be considered as extremely equivocal.
Another method of identifying “White Arsenic,” by metallization, is to form at the moment of its reduction, an alloy with copper, which may be easily effected in the following manner: Mix the suspected powder with black flux, as in the former experiment, and place the mixture between two polished plates of copper; bind them tightly together by iron wire, and expose them to a low red heat; if the included substance contain arsenic, a silvery white stain will be left on the surface of the copper, which is an alloy of the two metals. In this, as in the former experiment, the presence of an alkali in the flux is essential, since it forms immediately an arsenite of potass, and thereby fixes the arsenious acid, and prevents it from being volatilized before the temperature is sufficiently high to enable the charcoal to decompose it; we therefore differ with Dr. Bostock, when he states that powdered charcoal may be substituted for the black flux.
The property of whitening copper is regarded as a very satisfactory test of the presence of arsenic; but Dr. Bostock has pointed out some circumstances attending it, which we shall here enumerate for the instruction and satisfaction of the less experienced operator. “It may be necessary,” says he, “in the first place, to describe the phenomena that take place when copper is heated according to the process that is described above, but without the addition of the arsenic. Two copper disks, of nearly an inch and a half in diameter, scoured bright with sand, had one grain of powdered charcoal, made into a paste with oil, placed between them; they were bound together with an iron wire, and then kept red hot for ten minutes. When they were withdrawn from the fire, the metal was found to have lost its former appearance, and to have acquired the dull white colour of lead or zinc; the insides of the disks were found to present the same whitish appearance, except on the spot where the charcoal was placed, a small part of which still remained unconsumed. As the disks cooled the whitish matter which covered them began to separate, and fly off with some force, in the form of small scales, leaving a clean surface of the proper copper colour. The charcoal was rubbed off, and the surface below it was found smooth and polished; it had acquired a light colour, resembling that of brass; and, near the centre, there was a small spot, which approached to a steel grey. This appearance still continued, after it had been rubbed with fine sand. The above description,” concludes Dr. Bostock, “will probably impress the Society[[247]] with the same idea, that, I confess, it gave to myself, that if I had performed this experiment upon a substance, which had been suspected to contain arsenic, and I had not been aware of the appearance that I was to meet with, I should have conceived that I had detected its presence. Upon repeating the process, in precisely a similar manner, except that one grain of arsenic was added to the charcoal, the oxidation of the copper took place as before, and a small part of the charcoal remained unconsumed; but upon rubbing it, the white stain was perfectly visible. However, when these disks were compared with those in which the former experiment had been made, the difference between them seemed more in degree than in kind; so that I should not choose to decide upon the presence of arsenic, as indicated by this test, unless the result were more obvious than we can ever expect to find it, where the quantity of arsenic is so small. It may be proper to observe, that copper, whitened in this manner by arsenic, is very subject to tarnish; in three days I could with difficulty distinguish which of the disks had been employed in these two experiments.”