380 On some Points in the Chemistry of Acid-manufacture, 



them, as the bursting of such a retort over a fire carries with it 

 great danger. This plan then was thrown aside. Although it 

 could compete with any in cheapness, yet it carried too many 

 inconveniences with it to become practicable. The following 

 numbers may perhaps be interesting : — 



Sulphuric Acid. Sulphuric Acid. 



As^ O^ per cent. As" 0^ per cent, 



(before purification). (after purification). 



1-131 contained 0'34 



1-303 „ 0-48 



0991 „ 0-63 



The above figures show that the decomposition was not per- 

 fect enough for the plan to be made use of on a manufacturing 

 scale. I had found it to answer perfectly in my laboratory ex- 

 periments, and so hoped that, when extended to the working 

 scale. It would be equally successful. 



Hydrochloric Acid.- — A stream of hydrochloric acid gas was 

 passed into the acid requited to be purified, supposing the decom- 

 position in this case would be similar to the above. This, how- 

 ever, was open to the same objections as the former, with the 

 additional one that the expense entailed by the process is too 

 great for any practical purpose. 



In looking over the foregoing processes, one or two natural 

 conclusions force themselves upon me. The process which, as 

 far as I can yet see, we must rely upon for the purification of 

 our acid is that which depends upon the use of sulphuretted 

 hydrogen. Precipitation is much surer, and, indeed, more per- 

 fect, than decomposition. The latter is dependent upon too many 

 conditions. The heat, the completeness of decomposition of the 

 salt, the rapidity with which the liberated bubbles of hydrochloric 

 acid gas pass through the acid, all exercise a great influence upon 

 the success of the process. Whilst sufiSciency of gas is the only 

 requisite in the former case, the latter depends upon too many 

 causes, too many conditions. Certainty in this case must be 

 striven for before rapidity : once gain the former, and we may 

 be sure of the latter following in its own good time. Besides, 

 in the present condition of the law of the country in such mat- 

 ters, sulphuretted hydrogen is a much safer gas to throw into 

 the atmosphere than hydrochloric acid ! 



I have thus given my own experience of the best methods of 

 the purification of sulphuric acid, gathered during a pretty con- 

 tinued search after some sure and practicable method both in 

 respect of efficiency and economy. I have chosen to do this 

 rather than gather together the experiences of others — partly 

 because these experiences are comparatively few and far between, 



