(>32 ARSENIC. 



precipitating in combination with the metal. This gas, when 

 pure, is completely absorbed by a solution of sulphate of cop- 

 per, and As Cu 3 precipitated. 



TESTING FOR ARSENIC. 



Poisoning from arsenious acid is greatly more frequent than 

 from any other substance. Hence, a more than usual degree of 

 importance is attached to the modes of detecting the presence of 

 arsenic in minute quantity. Of the different preparations of the 

 metal, arsenic acid, and after it arsenious acid, are the most 

 poisonous; the salts and sulphurets are so to a much less 

 degree. Arsenious acid in the solid form and unmixed with 

 foreign matters, is easily recognised as a white heavy powder, 

 which is tasteless or nearly so, is entirely volatile by heat, and 

 diffuses a garlic odour in the reducing flame of a lamp. When 

 in solution in water, arsenious acid may be detected by fluid 

 tests, of which the three following are the most important. 



1 . Sulphuretted hydrogen gas, made to a stream through a 

 solution of arsenious acid, produces a precipitate of orpiment, 

 or a golden yellow solution if the quantity of arsenic be very 

 small. In this experiment, the liquid should always be slightly 

 acidulated with hydrochloric or nitric acid, and also be boiled, 

 afterwards, to complete the precipitation of the sulphuret. 



2. Ammonio-nitrate of silver, is an exceedingly delicate test 

 of arsenious acid, whether free, or in combination with an alkali. 

 This reagent is prepared by adding diluted ammonia to a solu- 

 tion of nitrate of silver, till the oxide of silver, which is first 

 thrown down, is again redissolved. When the ammonia has 

 been added in proper quantity and not in excess, the odour of 

 that substance is scarcely perceptible, and the liquid contains 

 in solution the crystallizable ammonio-nitrate of silver, 

 Ag O,NO 5 + 2NH 3 . This test liquid throws down from arse- 

 nious acid, the yellow arsenite of silver, which is redissolved 

 both by acids and ammonia. A solution of nitrate of silver 

 gives the same indication, as the prepared ammonia-nitrate, in 

 an alkaline, but not in an acid solution of arsenious acid. 

 Nitrate of silver produces a yellow precipitate of phosphate of 

 silver, in phosphate of soda or any other soluble phosphate, of 

 the same colour as the arsenite of silver, and which might, 

 therefore, be mistaken for the latter. But the action of the am- 



