as a Test of Arsenic. I2S 



decompose nitrat of silver ; but in Fowler's solution, ia 

 uhich the .irsenic is already combined with ati alkali, the 

 decomposition takes place at once, without any addition 

 ot ammonia. The fixed alkalies, therefore, can answer a 

 similar purpose ; but ammonia has this advantage, that it 

 does not, when added singly, decompose nitrat ot silver, — a 

 circumstance which, in using the fixed alkalies, might oc- 

 casion some confusion*. 



With regard to Mr. Sylvester's objection, T shall, pre- 

 viously to my ofTermg any remarks upon it, stare it in his 

 own words : " Tf ever muriatic acid be |)resent," savs this 

 gentleman, *' the test is then wholly useless, as a muriat 

 of silver will be immediately formed, and the yellow com- 

 pound, said to be so unequivocal in its indication of arsenic, 

 of course be prevented from appearing." 



This danger of ambiguity, however, though applyingf in 

 some degree to the process in question, and well deserving 

 to be noticed, will be found to have been greatly overrated ; 

 and there are such easv and obvious means by which this 

 ambiguity can be entirelv removed, that it can make no 

 solid objection to the utility of the test. 



There cannot be the least doubt, as Mr. S. observes, but 

 that whenever nitrat of silver is added to a solution con- 

 taining muriatic acid, a precipitate of muriat of silver must 

 be the consequence. But if the nitrat of silver be added 

 in excess, tiie arsenite of silver is also thrown down bv the 

 intervention of ammonia, and a mixed precipitate of luna 

 cornea and arsenite of silver is obtained, which partakes 

 more or loss of the yellow colour of the latter, according 

 to the proportion of the two salts. 



If to this dubious precipitate a few drops of dilute nitric 

 acid be added, the arsenite of silver is instantly dissolved, 

 and the muriat of silver, which is insoluble, immediately 

 resumes its peculiar density and whiteness. If a little am- 

 monia be now added to the clear fluid, the yellow precipi- 

 tate appears in the most distinct manner, and becomes even 

 more characteristic from a comparison with the white pre- 



* It is necessary, as Dr. Roget has observed in the paper already quoted, 

 that the quantity of ammonia should not be too large; for in that case the 

 precipitate is redissolvcd. But, even tlien, it may be made to reappear, by 

 the addition of nitric acid in buHicient quantity to saturate the alliaii. In 

 thi'i cake, however, the prcci|>it<itc is not permanent, owing, I find, to its 

 being soluble in the nitrat of ammonia wliich is formed in the process. 

 Carbonat of ammonia has also the property of producing and redisiolving 

 the precipitate. 



The fixed alluliet in cxcets have not the power of rcdistolving the preci. 

 pitau. 



cipltate. 



