TOXICOLOGY. 353 



silver till the brown oxide of silver at first precipitated is 

 nearly redissolved) throws down with arsenious acid a rich 

 yellow deposit of arsenite of silver. 



(2.) The ammonio-sulphate of copper (formed by adding 

 liquor ammonise to a solution of sulphate of copper till the 

 bluish-white oxide of copper is almost redissolved) pro- 

 duces a pale-green precipitate (arsenite of copper). 



Care must be taken not to add too much of the test in 

 the first instance, otherwise its blue may overpower the 

 green of the precipitate. 



(3.) Sulphuretted hydrogen water throws down a yellow 

 deposit of sulphide of arsenic. 



It is better, however, to use pure and well-washed sul- 

 phuretted hydrogen, generated in the usual maimer. 



In performing this test care must be taken that the 

 liquid to be examined is not alkaline, or no precipitate will 

 be produced, even though arsenic be present. 



For this reason yellow sulphide of ammonium will not 

 precipitate arsenic until acidified with a mineral acid. 



There are several other processes for the detection of 

 arsenic which are of great value, especially when mixed 

 with foreign substances, viz. : (1) Marsh's process ; (2) 

 Reinsch's process ; (3) the method for detecting the poison 

 in an organic mixture. 



Marsh's Test. — This test is founded on the fact that 

 arsenious acid is decomposed by nascent hydrogen, form- 

 ing arseniuretted hydrogen gas. 



This vapour has the following properties : 



It burns with a bluish-white flame, and gives off a white 

 smoke (arsenious acid) which has an odour of garlic. 



If a piece of glass or porcelain be held in the flame, a 

 blackish metallic stain, consisting of metallic arsenic, will be 

 deposited upon it. This stain might be confounded with 

 one produced by antimony under similar circumstances. 



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