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_.”