DETECTION OF ARSENIC. 123 



is destroyed, and nothing is apparently left but a black car- 

 bonaceous mass. The effect of this operation is to deprive 

 the animal or vegetable matter of its nitrogen, oxygen and 

 hydrogen, and to leave the carbon, which is indestructible 

 by these agents, with the arsenic, if any be present, diffused 

 through it. It is perhaps necessary to suggest as a caution 

 that the evaporation be conducted at a moderate heat, the 

 arsenic may otherwise be volatilized and escape a subsequent 

 examination. This metal if it have been introduced in the 

 metallic form or fly-powder, or white oxide, rats-bane^ is by this 

 action of the nitric acid, if any have been employed, con- 

 verted into arsenic acid, a higher degree of oxidation than it 

 has been supposed to possess in the first hypothetic case. 

 In cases when the destruction of the organic matter can 

 be accomplished without the use of nitric acid, I consider it 

 decidedly preferable to omit it, for this reason, that its effect 

 must invariably be to bring the arsenic into its highest state 

 of oxidation, arsenic acid, a precipitate from which by sul- 

 phuretted hydrogen is obtained with far less facility than 

 from arsenious acid, or one of its salts. When brought into 

 this condition, the next process is to lixiviate the carbonaceous 

 mass, by triturating it well with boiling distilled water, which 

 dissolves out the arsenic, or arsenious acid. The solution being 

 now filtered, there exists for the purposes of experiment, the 

 arsenical compound, in a simple almost colourless fluid, and 

 the mode of rendering its presence apparent, is almost precisely 

 the same as in the former case. To the solution thus obtained 

 lime water should be added. The addition of which occa- 

 sions a white precipitate of insoluble arsenite of lime. This 

 arsenite of lime is to be dried, mixed with black flux, and the 

 mixture submitted to the process for reduction and sublima- 

 tion as before explained. It may be proper to mention here 

 that if this process be conducted in a tube, open at both ends, 

 we may have beside the deposite of crystals and the lus- 

 trous ring, an aUiaceous odour, with fumes proceeding from 

 the upper orifice. Another very satisfactory process, if we 

 have obtained tlie solution of arsenic acid, in distilled water, 

 is to test a portion of it with the ammoniacal nitrate of silver, 

 when instead of a lemon yellow coloured precipitate, which 

 followed its addition in the former case, we have one of a brick 



