Du. Raist on the Detection of Arsenic. 83 



arc produced by boiling copper in solutions containing free sulphur, sul- 

 phuretted hydrogen, and the combinations of sulphuretted hydrogen with 

 bases, sulphuret of copper being thus formed as a thin film on the 

 metal. The presence of muriatic acid is in no respect necessary for this 

 reaction. 



I thought it possible that sulphurous acid might act in a similar manner, 

 and give rise to a coating of sulphuret of copper. On trial, I found this 

 supposition incorrect ; for the copper undergoes no change in its colour 

 or lustre when heated in a watery solution of sulphurous acid. 



If, however, muriatic acid and sulphurous acid are present together in 

 a solution, the copper is speedily changed. It assumes a steel colour of 

 a blueish tinge, dependent on the decomposition of the sulphurous acid 

 and the consequent formation of a thin film of sulphuret of copper. In 

 this reaction we have SO2 + 2 HCl. + 3 Cu= SCu + 2 HO + 2 CI Cu 

 — 1 atom sulphurous acid + 2 atoms hydrochloric acid + 3 atoms copper, 

 yield 1 atom sulphuret of copper, 2 atoms water + 2 atoms protochloride 

 of copper. 



This bears a close analogy to the action of hydrochloric acid — arsenious 

 acid and copper in Reinsch's process in which 3 HCl + 3 Cu + AsOs= 

 3C1 Cu + 3 HO + As, or 3 atoms hydrochloric acid + 3 atoms copper 

 + 1 atom arsenious acid yield 3 atoms protochloride of copper + 3 atoms 

 water + 1 atom metallic arsenic. It is also interesting to observe, that 

 while the combined action of copper and muriatic acid can decompose 

 arsenious acid and sulphurous acid, on the more stable compounds, arsenic 

 acid and sulphuric acid, they act on the one very feebly and on the other 

 not at all. 



16. I have been led into these observations of sulphurous acid, in 

 order to obviate an objection which might be made to its employment, in 

 bringing the arsenical solution into a fit state for Reinsch's process. It 

 may be objected that the presence of sulphurous acid may itself cause a 

 deposit on the copper, and thus mislead the experimenter. This difiiculty, 

 however, is obviated at once by boiling off the superfluous sulphurous acid 

 before the copper is introduced. This will remove every ambiguity. But 

 besides this, the subsequent testing, which is indispensable in every ease, 

 will afford complete security against error. 



17. In a former part of the paper I mentioned that dilute solutions of 

 arsenious acid, when long kept, give a white cloudiness with sol. of nitrate 

 of silver, whereas it is well known that nitrate of silver gives no preci- 

 pitate whatever in solutions of perfectly pure arsenious acid. The occa- 

 sional production of this cloudiness in solutions of arsenious acid has 

 frequently been noticed. I am not aware that any attempt has been made 

 to account for it. I have satisfied myself that it arises from the presence 

 of arsenic acid, produced in these solutions, as I have already explained, 

 by exposure to the air. The arseniato of silver, in its ordinary forn), no 

 doubt is of a deep brown or red ; but the colour varies greatly with the 

 state of dilution of the fluid — when conccntvatecl, it is of a reddish brown 



