3 o POPULAR SCIENCE MONTHLY. 



the purifying fluids is a matter of indifference in the final result. 

 Thorough scrubbing and wet filtration, each by itself, or both together, 

 bring us at last to the same point. Only a few of the difficulties which 

 appeared in the application of gas purification on a large scale may 

 be mentioned here. 



For the success of the process it had proved necessary that the 

 gases should be cooled slowly. It is a curious fact, and one as yet 

 without due explanation, that the sulfuric acid fume from the pyrites- 

 burners is far more difficult to remove when the gases are cooled rapidly 

 than when they are cooled slowly. 



For this purpose long iron conductors were used, which were kept 

 cool by the circulation of the air. As far as our knowledge went at that 

 time, these iron conductors could occasion no untoward influence 

 upon the contact-process; for when dry pyrites are used in the burners, 

 the sulfuric acid in the gases has a concentration of at least ninety per 

 cent. Should this act on the iron at all, it would result in the forma- 

 tion of sulfur dioxid, and this could of course do no damage in the 

 process. 



But now in spite of the fact that the gases were freed from every 

 mechanical impurity, so that the optical test, which was at that time 

 considered sufficient, betrayed not the slightest sign of any solid or 

 liquid particles (for the sake of absolute certainty the gases were finally 

 passed through a kind of wet cloth filter, somewhat like a filter press), 

 still the contact-mass gradually diminished in activity. This diminu- 

 tion was very slow it is true, being apparent only after weeks or per- 

 haps months, but it was nevertheless certain to occur. It was only 

 after long and difficult labor that the presence of arsenic was again 

 proved to be present in the contact mass, and this, after it was sup- 

 posed that every trace of the element had been eliminated in the process 

 of purification. The contact-mass, however, showed unmistakably that 

 arsenic was in evidence, and the suspicion arose that the cause of all 

 the trouble might be due to the action of the condensed sulfuric acid 

 upon the iron cooling-conductors. Further investigation showed that 

 this was probably the case, and that by this action some gas which 

 contained arsenic must have been formed. This gas was probably 

 arsin, the hydrid of arsenic. 



A change in the arrangements was now made so that the condensed 

 sulfuric acid could no longer come into contact with the iron con- 

 ductors, and from this time on the contact-mass remained undiminished 

 in its activity. It appears from this that, contrary to the generally 

 received ideas, hydrogen can be evolved by the action of concentrated 

 sulfuric acid upon iron, and that when arsenic is present, arsin may be 

 also formed. 



