latter tube is quite long, and terminates in a jet at its free end, enclosed in tin-foil;[C] it passes through the sheet-iron furnace _R_, supported upon _G_. The screen _D_ protects the portion _D E_ of the tube _C_ from the heat. The gas disengaged is ignited at _E_ and the porcelain dish _P_ is held by the hand in contact with the flame. The apparatus being mounted, zinc, water and some sulphuric acid are placed in the generating flask,[D] and the solution containing arsenious acid added: the evolution of gas commences immediately. The tube _H_ serves to retain any liquids that may be held suspended. The gas then passes through the part _C D_ of the tube _C_, which is heated by placing a few live coals upon the furnace _R_. The greater portion of the arsenetted hydrogen is decomposed here, and is deposited on the cold part of the tube, in a mirror-like ring. The small quantity of gas that escapes decomposition, if ignited at _E_, produces a metallic spot on the dish _P_. In order to determine that the spots are due to the presence of arsenic, and not produced by antimony, the following tests should be applied: [C] The fusing of the point of the tube is also prevented by platinizing it. The tube is drawn out, its end roughened by filing, and then immersed in solution of bichloride of platinum, so that a drop or two of the fluid adheres. The point, upon heating, now acquires a fine metallic lustre, and by repeating the operation a few times a good coating of platinum is produced both on the exterior and interior of the tube.--_Trans._ [D] The addition of a few drops of solution of bichloride of platinum to the mixture of zinc, water and sulphuric acid is advisable.--_Trans._ 1. The color of the spots is distinctive: arsenical spots are brown and exhibit a metallic lustre, whereas those originating from antimony possess a black color, especially near their border. This difference is, however, not perceptible when the deposits have a large surface. 2. If the mirror be arsenical, it is readily volatilized from one part of the tube to another, when the latter is heated, and a current of hydrogen, or carbonic acid gas made to pass through it. Spots that are due to the presence of antimony are much less volatile. 3. If the tube is held in an inclined position so that a current of air traverses it, and the part containing the arsenical mirror heated, the arsenic oxidizes and arsenious acid is sublimed and deposited higher up in the tube in the form of a ring, which exhibits octahedral crystals when examined with a magnifying glass. This ring should be further tested as follows: _a._ If it is dissolved in a drop of hydrochloric acid and a solution of sulphuretted hydrogen added, a yellow precipitate of sulphide of arsenic is formed. This compound is soluble in ammonia and in alkaline sulphides, but insoluble in hydrochloric acid.