128 DETECTION OF ARSENIC. 



The ammo7iiacal nitrate of silver produces in fluids contain- 

 ing arsenious acid, a lively lemon yellow precipitate, changing 

 to dark brown when exposed to the light, this change is com- 

 mon to all the precipitates of silver. The chemical action is 

 analogous to that described above, the resulting salts being 

 arsenite of silver, which is insoluble, and nitrate of ammonia 

 retained in the solution. This test is also liable to some falla- 

 cies. Its action is prevented by the presence of nitric, acetic, 

 citric or tartaric acid in excess, particularly the first and the 

 last, says Christison, an excess of ammonia is also inconsistent 

 with its use. But the salt which is most likely to embarrass 

 its results, is the muriate of soda, which is more likely to 

 occur than any other, and the presence of which may be un- 

 suspected in a simple colourless fluid. The best way to over- 

 come this difiiculty, should it exist, is that proposed by Dr. 

 Forbes, professor of chemistry at Aberdeen, which is to 

 employ simple nitrate of silver, until all the muriatic acid be 

 precipitated, adding a slight excess, when the addition of a 

 few drops of caustic liquor of ammonia instantly produces the 

 lemon yellow coloured precipitate, so characteristic in its 

 colour as not readily to be mistaken. Another objection to 

 this test is the presence of any soluble phosphate in the liquid 

 under examination — as the precipitate of phosphate of sil- 

 ver bears a striking resemblance in colour to that of the arse- 

 nite, but here we have two safeguards against a false conclu- 

 sion ; one of which is that if but a small quantity of the phos- 

 phate be present it will be immediately redissolved in the 

 excess of ammonia present in the test, and the other, is the 

 marked difi'erence in the density of the precipitate, thrown 

 down by arsenious and phosphoric acids. The former being 

 heavy and subsiding promptly, while those of the latter are 

 very bulky, absorbing nearly all the fluid, when first precipi- 

 tated from which they subside slowly. This test cannot be 

 implicitly relied on, when organic fluids are the subjects of 

 examination, unless the quantity of arsenious acid be rather 

 large. We have before stated, that arsenic acid, or its salts 

 produced with this test, a precipitate of a brick red colour, the 

 arsenite of silver — this we believe is liable to no fallacy. 



Sulphuretted hydrogen gas, or water impregnated with that 

 gas is a test, to which most implicit confidence may be given, 



