Chemistry of Acid-manufacture. 379 



A known amount of sulphuric acid containing a determined 

 percentage of arsenic trioxide was run into a large leaden pan, and 

 a calculated amount of sodium sulphide added. At the bottom 

 of this pan a layer of coke, which had previously been well di- 

 gested with hydrochloric acid to free it from iron and other 

 impurities, was placed, through which the acid, being run upon 

 it, filtered, thus freeing it from the precipitated tersulphide, 

 whilst it was run out from the pan by a tap at the bottom. The 

 precipitate was removed from this filter every night ; but the 

 coke was allowed to remain for along time, two or three months 

 sometimes elapsing before removal. The escaping sulphuretted 

 hydrogen was conveyed away by a process similar to that em- 

 ployed in the sulphuretted-hydrogen method. 



The results obtained by tliis method were very satisfactory, 

 and the expense was extremely moderate — one hundred gallons 

 of sulphuric acid giving only a very minute trace of arsenic after 

 being subjected to this process. 



The deduction I naturally draw from the results of the above 

 methods of purification is, that sulphuretted hydrogen can pre- 

 cipitate the last traces of arsenic in the acid it is required to 

 purify. The only difiiculty lies in the method to be employed, 

 and the means of getting rid of the escaping sulphuretted hy- 

 drogen which has been allowed to be present in excess. It is 

 necessary to state that the above method of decomposition of that 

 gas did not fully answer the purpose required. 



Sodium Chloride. — The next plan tried was purification by the 

 addition of common salt to the acid. It was thus supposed that, 

 according to the general rule, the arsenious acid would be con- 

 verted into the terchloride of arsenic and escape as such. This, 

 however, was found unsuitable, for many reasons. 



In the first place, the decomposition of the salt is not perfect, 

 a considerable amount remaining in the acid as sodium chloride. 

 Next, its action upon the ordinary brown vitriol (as it runs from 

 the chamber) was found not to be so perfect as upon that under- 

 going refining, so that it was necessary to add the salt to the 

 glass retorts after they had been in action for some time. This, 

 of course, entails great inconvenience. The necessity for open- 

 ing the retort after the acid has been for some time in a state of 

 ebullition is at once very disagreeable and very dangerous, whilst 

 the sudden and powerful (partial) action upon the salt makes it 

 an attempt not to be incautiously determined upon. If, on the 

 other hand, the salt is introduced into the retort before the acid 

 has commenced boiling, it collects in considerable amount at the 

 bottom, and thus causes the mortality among the retorts to 

 become a matter of some consideration, as well as making it a 

 dangerous occupation for the man whose business it is to watch 



