_a_, is an apparatus producing a constant supply of carbonic acid. Upon opening Mohr's clamp, _g_, the gas passes into the flask _h_, which contains sulphuric acid; it is then conducted, by means of the tube _i_, into the reduction tube _k_, which has an interior diameter of 8 mm. This tube is represented, in half size, in Fig 4. [B] The sulphur, usually accompanying the precipitate of sulphide of arsenic, is insoluble in ammonia.--_Trans._ [Illustration: Fig. 3.] [Illustration: Fig. 4.] The reduction is performed as follows: The sulphide of arsenic is ground in a small mortar, previously warmed, together with 12 parts of a mixture consisting of 3 parts of carbonate of soda and 1 part of cyanide of potassium, both salts being perfectly dry. The powder thus obtained is placed upon a piece of paper rolled in the form of a gutter, and introduced into the reduction tube. The latter is then turned half round its axis, so as to cause the mixture to fall in _de_ without soiling the other parts of the tube. The paper is now withdrawn and the apparatus mounted. Upon opening the clamp _g_, and strongly heating the mixture by either the flame of a gas or an alcohol lamp, a mirror-like ring of metallic arsenic is deposited at _h_, if this poison be present in the substances under examination. When the coating is too minute to permit of perfect identification, it should be driven by heat to a thinner part of the tube; in this way it is rendered easily visible, being condensed upon a smaller space. The above process possesses the advantage of not allowing arsenic to be confounded with any other body; it also permits of a quantitative estimation of the poison present. For this purpose, it is only necessary to previously weigh the watch-crystal, upon which the ammoniacal solution of sulphide of arsenic was evaporated, and to determine its increased weight after the evaporation; the difference of the two weighings multiplied by 0.8049, gives the corresponding weight of arsenious acid, and by 0.6098, the weight of the corresponding amount of metallic arsenic. MARSH'S TEST.