[F] Owing to the impurities often occurring in zinc, the use of distilled magnesium in Marsh's apparatus has also been suggested. This metal is now to be obtained in a state of great purity; it is, however, sometimes contaminated with silicium, which body likewise gives rise to a metallic deposit, but one that is readily distinguished from arsenical spots by its insolubility in nitric acid, _aqua regia_, and in hypochlorite of soda. The presence of magnesium causes the precipitation of the non-volatile metals possibly contained in the fluid tested for arsenic.--_Trans._ It should be borne in mind that the solution introduced into Marsh's apparatus must not contain organic substances, and that, in case their destruction has been accomplished by means of nitric acid all traces of this compound are to be removed. The sulphuric acid used should also be completely freed from nitrous vapors. According to _M. Blondeau_, nascent hydrogen in the presence of nitrous compounds converts the acids of arsenic not into arsenetted hydrogen (As H{3}), but into the _solid_ arsenide of hydrogen (As{4} H{2}). This latter compound, upon which pure nascent hydrogen has no effect, is transformed into gaseous arsenetted hydrogen by the simultaneous action of nascent hydrogen and organic substances. These facts are of the greatest importance, for they might possibly cause a loss of arsenic when it is present, as well as determine its discovery when it is absent. The first case is supposable: should traces of nitric acid remain in the solution, the arsenic would be transformed into solid arsenide of hydrogen and its detection rendered impossible. The second case may also occur: if the zinc placed in the apparatus contains arsenic, and the sulphuric acid used contains nitrous compounds, the evolved gas will fail to exhibit any evidence of the presence of arsenic, owing to the formation of the solid arsenide of hydrogen. Upon adding the suspected solution, which, perchance, may still contain organic substances, this arsenide is converted into arsenetted hydrogen, and the presence of arsenic will be detected, although the solution under examination was originally free from this metal. RASPAIL'S METHOD.