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 When they were withdrawn from the fire, the metal was found to have lost its former appearance, and to have acquired the dull white colour of lead or zinc; the insides of the disks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