378 Mr. H. A. Smith on some Points in the 



Section II. Methods of removal. 



Two precautions have to be taken into account in the methods 

 employed for the removal of arsenic from sulphuric acid. 



1st. The substance or agent which is used in the purifying 

 process must have no deleterious effect on any article in the 

 manufacture of which the acid is required. 



2ndly. We must prevent our works, as far as in us lies, from 

 becoming a nuisance to our neighbours. 



The item of expense is naturally a matter of course. 



The following purifying agents have been carefully tried, with 

 the following results. 



Sulphuretted Hydrogen in a gaseous state. — This is, I believe, 

 in use in many cases, but, as far as I can make out, with very 

 variable results. For my own part I found it infringed the 

 second requisite of a purifier, inasmuch as it became a '^nuisance,*' 

 and, which was much more serious, was too expensive. The 

 plan employed in this case was a very simple one. 



A large flat leaden pan was employed covered with a wooden 

 top, and having access to a chimney by means of a long fliue. 

 About 20 feet from the pan this flue was led through a fire, 

 which, decomposing the escaping sulphuretted hydrogen, not 

 only prevented a great escape to the atmosphere, but allowed 

 the deposited sulphur to be recovered. 



Whether from defective draught, or from the unsuitability of 

 the process to the required purpose, this plan did not answer ; 

 whilst the use of acid to liberate the gas, together with the dif- 

 ficulty in regulating the supply, made the expense of working 

 too great. 



Instead of using' separate acid to liberate the sulphuretted 

 hydrogen, I tried to use that which was to undergo purification, 

 by the addition first of 



Sulphide of Iron. — This plan is, of course, only admissible in 

 certain cases. For acid employed in wire-workmg, galvanizing, 

 or similar work, purification by the simple addition of ground 

 sulphide of iron is quite safe, and completely answers the pur- 

 pose required ; whereas that containing arsenic cannot be used, 

 its action having a deleterious effect upon the iron. Acid 

 purified by this means, however, cannot be used in bleaching, 

 dyeing, or printing; so that its field of usefulness is extremely 

 limited ; still, as a purifying agent, it was all that was required. 



Sodium Sulphide. — As this was a substance capable of being 

 used in most cases, more care was expended on it than on either 

 of the former. The sulphide of sodium employed was made 

 from black ash, and, although contaminated with a little lime, 

 answered the purpose completely. The method of application 

 was as follows : — 



