TESTING FOR ARSENIC. 633 



monio -nitrate is not liable to that ambiguity, as it does not 

 produce a yellow precipitate in an alkaline solution of phos- 

 phoric acid ; the phosphate of silver being then retained in so- 

 lution by the ammonia of the reagent, although arsenite of 

 silver is precipitated in the same circumstances. Both phos- 

 phate and arseniate of silver are indeed considerably more soluble 

 in ammonia, than the arsenite of the same metal. 



3. Ammonio-sulphate of copper, gives a beautiful green pre- 

 cipitate, the arsenite of copper, in both alkaline and acid solu- 

 tions of arsenious acid ; sulphate of copper gives the same pre- 

 cipitate in the former, but not in the latter. 



But in solutions containing organic matter, the indications of 

 these tests are sometimes delusive, and often doubtful, parti- 

 cularly the indications of the latter two. Recourse is then had 

 to the proper means of obtaining arsenic in the metallic form, 

 in which it cannot be mistaken, from the liquid suspected to 

 contain arsenious acid. Indeed, even where the indications 

 of the fluid tests are clear, the reduction test should never be 

 omitted, the evidence which it affords being of a superior and 

 completely demonstrative character. The reduction test of 

 arsenic is practised in two different ways : (1) by the reduction of 

 the sulphuret of arsenic by means of charcoal and carbonate of 

 potash, and (2) by the production, and subsequent decomposition 

 of the gaseous compound of arsenic and hydrogen. The fol- 

 lowing operations are necessary in the practice of the first 

 method : 



REDUCTION TEST OF ARSENIC. 



I. Preparation of the fluid : 



1 . Boil the matters with water and a few drops of nitric 

 acid. 



2. Strain through calico. 



3. Precipitate animal matter by an excess of nitrate of 

 silver, and subsequent addition of common salt. 



4. Filter through paper. 



II. Precipitation of the sulphuret of arsenic: 



1. Transmit a stream of sulphuretted hydrogen through 

 the liquid for half an hour. 



2. Heat the liquid in an open vessel for a few minutes, 

 to cause the precipitate to separate. 



