Contributions to Experimental Chemistry. 377 



into a test tube, containing a few drops of a solution of the 

 perchloride of mercury. The arseniuretted hydrogen produces 

 first a yellow precipitate, which, after passing through various 

 shades of brown, at last becomes black, and decomposes the 

 perchloride gradually into calomel*. This test is so nice, that 

 ■^Ijjy even -j^J^, of a grain of metallic arsenic dissolved (either 

 in the form of arsenious acid or of arseniate of potash, though 

 the latter requires a longer period for its decomposition) in 

 100 parts of a diluted muriatic acid, was still indicated, 

 although the greater part of the arsenic was precipitated as a 

 black powder. This effect is not prevented if the arsenical 

 liquid contains, at the same time, much organic matter in 

 solution, even though the latter surpass by 2000 times the 

 weight of the arsenic present. Nitric and nitromuriatic acid 

 prevent the disengaging of arseniuretted hydrogen, almost 

 entirely, even if ^\) of a grain of arsenic be present. When a 

 copper wire is made use of, arsenic with metallic lustre is 

 precipitated on that part of the wire which is above the surface 

 of the liquid ; and if the surface of the wire is augmented by 

 being bent into spiral windings, the gas thus disengaged, 

 precipitates the mercurial salt less copiously. The arsenic is 

 not carried up mechanically, for a disc of thin pasteboard 

 sealed to the wire, at some little distance from the surface of 

 the liquid, did not impede the effects. The arsenic adhering 

 to that part of the wire which dips into the liquid, appears in 

 the form of a black powder. With platinum wire 1 could not 

 observe this phenomenon. In a similar galvanic circuit, the 

 interior of the tube being filled with diluted muriatic acid, the 

 exterior one with a solution of muriate of ammonia or soda, 

 the copper wire becomes tarnished, and that part of it which is 

 in the liquid, is covered after some time with a film of a black 

 substance. This change, however, cannot be at all confounded 

 with the appearance which the wire assumes from the presence 

 of arsenic ; nor is the solution of the perchloride of mercury 

 changed in the least, as was to be anticipated by the hydrogen 

 gas. 



♦ The phenomena atteudtng this decomposition will vary in some measure if an 

 excess of the arseniuretted hydrogen should happen to be present. Berzelius has 

 examined ihem, and he states the perchloride of mercury to be such a delicate test 

 for the arseaiurelted hydrogen as to indicate even j^;i(ru ^^ ^^ ^^ ^ gaseous mixture. 



