Dr. Raixy on the Detection of Arsenic. 81 



time. I have uniformly found arsenic acid in such solutions, if concen- 

 trated to a .small bulk by evaporation. The residual liquor reddens litmus 

 and gives a brick-red precipitate with nitrate of silver. But this conver- 

 sion of arsenious into arsenic acid is restricted ivithin very narroio limits, 

 as the arsenic acid increases the change goes on more slowly ; and when the 

 arsenic acid amounts to one part in ten or fifteen thousand, no further 

 conversion takes place. Hence, the whole, or nearly the whole of the 

 arsenious acid may be converted into arsenic acid in a very dilute solution, 

 while in a concentrated solution, though the same absolute quantity is so 

 converted, it will bear a trifling proportion to the arsenious acid which 

 still remains unchanged. I have recently ascertained that solutions of 

 arsenious acid, containing animal matter, do not undergo this change, but 

 remain, after an interval of several months, as sensitive as ever to 

 Reinsch's process. 



9. I cannot assign the cause of this change with absolute certainty, 

 but there are strong reasons for believing that it depends on some prin- 

 ciple communicated from the air : for it occurs more readily if the solution 

 is exposed to the air in an open vessel, or kept in a vessel only partially 

 filled and often shaken. It is natural to suppose that it is the oxygen of 

 the air ishich, in these circumstances, unites directly with the arsenious 

 acid. This opinion is in accordance with the partial conversion of sul- 

 phurous acid into sulphuric acid, when its aqueous solution is in contact 

 with atmospheric air. Still I can adduce no proof of the correctness of 

 this supposition, and it is conceivable that chlorine, or some nitrous com- 

 pound in the atmosphere may be the real agent. 



10. Supposing, however, that arsenious acid, in very dilute solutions, 

 is gradually changed into arsenic acid, will this account for the fact, 

 that such solutions gradually become less sensitive to Reinsch's test? 

 This leads us to examine how solutions of arsenic acid, prepared in the 

 usual way, are affected by that test. Is arsenic deposited on copper as 

 readily in solutions containing arsenic acid, as in solutions containing 

 arsenious acid ? I believe it is generally supposed that there is no ma- 

 terial difference. Mr. Taylor, in his valuable work on Toxicology, repre- 

 sents the process as no less applicable to arsenic acid and its combinations 

 than to arsenious acid. 



I made several experiments on this subject several years ago, and 

 having operated on very dilute solutions, such as one part of arsenic acid 

 in ten thousand parts of water, I was led to conclude, prematurely, that 

 arsenic cannot be detected by Reinsch's process, when it is in the state of 

 arsenic acid. This inference was erroneous ; for the process succeeds 

 partially when tried with moderately dilute solutions; still the difference 

 between arsenious and arsenic acid is very great, for a solution of one 

 part of arsenic acid in fifteen hundred parts of fluid, containing one 

 tenth of ordinary muriatic acid, gives 7io depo.sit when boiled with 

 copper, while one part of arsenious acid, as already stated, gives a 

 distinct deposit when diffused in one miUion parts of a similar fluid. From 



Vol. III.— No. 2. 2 



