Mr. Hume on the Detection of Arsenic. 298 



Experiment 3. — In this case I substituted the carbonate 

 of barytes, and the product was a true neutral arsenite of 

 larytcs. 



As neither of these carbonates is soluble in water aloner, 

 there can be no error from an excess ; 1 have, therefore, 

 prescribed a few grains, meaning that there should, in all 

 cases, be rather more than the arsenic can take up ; and 

 hence, these modifications of my test may be considered of 

 some value, and, in particular circumstances, may possess 

 some advantages over the more soluble alkalies. 



From these and some other experiments on this plan', 

 which I have not now leisure to describe, T ajn inclined td 

 believe, that white oxide of arsenic would decompose all the 

 earthy carbonates, forming with their respective bases so- 

 luble and, perhaps, crvstallizable salts. 



The arsenite of baryles, on being properly reduced by 

 evaporation, afforded regular crystals, the precise figure of 

 which 1 did not notice; and the small quantity T had pre- 

 pared is not now in my possession, having given it to 's 

 scientific friend for examination. 



Lest the carbonate of magnesia might contain any por- 

 tion of the precipitant or sulphate of potash, I took especial 

 care, by repeated washings with boiling distilled water, to 

 render it unexceptionable on this score. To the carbonates 

 of lime and barytes, however, there could be no such ob- 

 jection, as both were natural productions, — the first being 

 pure marble finely levigated; and the last was the commoa 

 native carbonate of barytes, containing oxide of iron to be 

 sure, but in all respects sufficiently pure for the purpose, as 

 the iron could have no material efftct. 



1 have not had time to try the carbonate of strontites, 

 although I can scarcelv doubt of the same successful issue, 

 whenever this earth shall be sulij^^clcd to the same process. 



With common pipe-clay T also effected an union, but I 

 did not persevere further than merely to assure myself 

 that arsenic did not exist alone in the solution ; and I was 

 unwilling to prepare for my puipose an artificial carbonate 

 of alumioe, wliicti, notwithstanding the utmost attention, 

 ca;Miot be reaclilv divested of toreian matters. 



The viagnesid in the solution bv the first experiment 

 was readily delected by lime-water, and also by a solution 

 of caustic pota'ih. in the second experiment I found the 

 oxalate of ammonia fully to answer my purpose, by proving 

 the existence of lime. As to the third experiment, any of 

 the soluble sulphates, or sulphuric acid alone, will render 

 ihe barytes ilislinct, and form,l believe, a precipitate rather 

 T 4 more 



