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.” In connection with the different modes of identifying arsenic by metallization, we may relate a test lately proposed by _Mr. A. Thomson_, which, as a collateral proof, merits some attention. “Into any solution, in which arsenic may be suspected, stir a moderate quantity of charcoal powder; allow it to settle; then pour off the clear supernatant liquor, or filter the mixture; and when the powder which remains on the filter is dry, sprinkle some of it on a red hot poker; if the solution contain arsenic, the odour of garlic will be rendered sensible. This effect becomes more obvious if a few grains of dry sub-carbonate of potass be added to the dried charcoal powder.”[248] If, instead of _Black flux_, or charcoal, the arsenious acid be heated in a glass tube with quick-lime, a sudden ignition will take place, when one part of the white arsenic will be metallized, and the other farther acidified, so as to produce an _arseniate of lime_; in this case, therefore, a certain portion of the arsenious acid is robbed of its oxygen to complete the acidification of the rest. The habitudes of arsenious acid with the _nitrates_, as first observed by _Kunkel_, deserve also some attention. If they be heated together, the former will be oxygenated at the expense of the nitric acid, nitrous acid vapour will be disengaged, and an _arseniate of potass_ remain. The forensic chemist may avail himself of these facts, and obtain a very useful test, which may be applied in the following manner.[249] Take a grain or two of the suspected powder, and mix it with double the quantity of _Nitrate of Potass_; introduce this mixture in a small glass tube, and apply the flame of a spirit lamp under the powder; when, if it contain arsenic, the nitrate will be decomposed, nitric oxide and nitrous acid be evolved in a gaseous form, and an arseniate of potass remain.