82 Dr. Rainy on the Detection of Arsenic. 



numerous comparative experiments, it follows that arscnious acid is a 

 thousand times more sensitive to Rcinsch's process than arsenic acid ; or, 

 in other words, in order to give a similar deposit, arsenic acid must be 

 present in a thousand times greater quantity. 



An increase in the proportion of the muriatic acid promotes the de- 

 position, so that indications of the presence of arsenic may be obtained 

 with weaker solutions. Thus, a solution of yj^y gave no deposit, when 

 the muriatic acid formed yL, a slight tarnish when the acid was ^, and 

 a distinct steel-grey deposit when the acid was ^. 



In solutions containing yo.^ijo ^^ arsenic acid, I obtained no deposit 

 even when the mixture contained ^ of the muriatic acid of ordinary 

 strength. 



11. These statements will serve to show that Eeinsch's process is not 

 applicable to the detection of arsenic when it is in the state of arsenic 

 acid. They also explain, in a satisfactory manner, how the process be- 

 comes impaired in its delicacy when applied to dilute solutions of arsenious 

 acid, which, by keeping, is gradually changed into arsenic acid. 



12. The injurious effects of nitrates and other compounds, such as the 

 persalts of mercury, is also explained by these facts ; for when such sub- 

 stances are present along with muriatic acid, they readily convert the 

 arsenious acid into arsenic acid, and thus render it much less sensitive 

 to Reinsch's process. 



1 3. In conformity with these views, it might be expected that if arsenic 

 acid were reduced to the state of arsenious acid, it would lie brought into 

 a suitable condition for the application of Eeinsch's process. This can be 

 accomplished by sulphurous acid. If a current of sulphurous acid gas is 

 passed through the mixture, the arsenic acid is changed into arsenious acid, 

 and the process resumes all its original delicacy, as I have ascertained by 

 numerous experiments both on dilute solutions of arsenic acid, and dilute 

 solutions of arsenious acid altered by keeping. 



14. The mixture which is to be examined should first be boiled for a few 

 minutes with the proper porportion of muriatic acid. It should then be 

 allowed to cool, and a current of sulphurous acid gas should be passed 

 through it till it is thoroughly saturated. This is most conveniently done 

 by heating a mixture of sulphuric acid and charcoal in a flask furnished 

 with a suitable tube for conducting the gas. Carbonic acid is produced 

 at the same time, and escapes along with the sulphurous acid; but it 

 does not, in any respect, interfere with the process. 



It is indispensably necessary, however, that the sulphm-ous acid should 

 be kept in contact with the arsenic acid for some time. The reduction of 

 the arsenic acid into arsenious acid is a gradual process. I have sometimes 

 found two hours necessary for its completion. When a sufficient time is 

 elapsed, the superfluous sulphurous acid should be boiled off, till all smell 

 of sulphurous gas is gone, the copper may then be introduced, and the 

 process completed in the usual way. 



15. Deposits bearing a considerable resemblance to the arsenical film 



