2. The solution having been obtained in a condition suitable for examination, the air is completely expelled from the apparatus by allowing the gas to evolve for some time, and the suspected fluid then introduced into the generating flask. Danger of explosion would be incurred were the gas ignited when mixed with air.[E] [E] The effervescence of the mixture is prevented by _slowly_ adding the arsenical solution to the generating flask. In order to avoid loss of arsenetted hydrogen, the cold dish should be directly applied to the flame even before the introduction of the suspected solution, and its position changed at short intervals, so as to allow the deposit to be formed on different parts.-_Trans._ 3. It is indispensable, in applying this test, to have a second apparatus in which only the reagents necessary to generate hydrogen are placed: in this way, if no spots are now produced by the use of the second apparatus, it is certain that those obtained when the first apparatus is employed do not originate from impurities present in the reagents used. It has come under the author's observation, however, that a sheet of zinc sometimes contains arsenic in one part and not in another; in fact, the shavings of this metal, as purchased for laboratory use, are often taken from lots previously collected, and may therefore have been prepared from several different sheets. If this be the case, it is supposable that the zinc used in the second apparatus may be free from arsenic, whereas the metal with which the suspected solution is brought in contact may contain this poison; serious danger would then exist of finding indications of the presence of arsenic in materials that did not originally contain a trace of the metal. In order to obviate this important objection, which might possibly place a human life in jeopardy, we propose the following modifications: Pure mercury is distilled and its absolute purity established. As the metal is a fluid and is therefore homogeneous, it is evident if one portion be found pure, the entire mass is so. Sodium is then fused under oil of naphtha, in order to cause the complete admixture of its particles, and the purity of the fused metal in regard to arsenic tested. An amalgam is next prepared by uniting the mercury and sodium. This is eminently adapted to toxicological investigations: in order to generate a supply of very pure hydrogen, it is only necessary to place the amalgam in water kept slightly acid by the addition of a few drops of sulphuric acid, by means of which the disengagement of gas is rendered more energetic.[F]