SULFURIC ACID. 29 



stances, which are liable to be present in the burner-gases, are only 

 injurious to the extent to which, when present in large quantities, they 

 cover up and choke the contact mass. The injurious action of arsenic, 

 for example, is so great that when present to the amount of only one or 

 two per cent, of the platinum in the contact-mass, the latter becomes com- 

 pletely inert. Hence by these investigations, it was incontrovertibly 

 proved that there are substances which are capable of exerting a specific 

 action, 'poisonous/ one might almost call it, upon the contact-mass. The 

 question now was as to whether there was present in the burner-gases, 

 in spite of the efforts at purification, such a substance. 



There was, in fact, in these gases a trace of white fume of sulfuric 

 acid which could not be removed, and this was found still to contain 

 arsenic derived from the pyrites. But even if the failure of the pro- 

 cess on a large scale was owing to a now known cause, the remedy was 

 not apparent. At that time the complete precipitation of this white 

 fume, the so-called 'huttenrauch/ was considered by the most distin- 

 guished experts as technically impossible. 



Although after such long and careful work, the prospects of future 

 success had become very slight, nevertheless, since the cause of the 

 previous failures was known, fresh energy was applied to the solution 

 of the new problem. This was nothing less than the attempt to free 

 the burner-gases completely from all impurities, so that finally there 

 should remain absolutely nothing but the pure gases, that is, sulfur 

 dioxid, oxygen and nitrogen. 



With an enormous expenditure of time, care, money and patience, 

 experiment after experiment was undertaken in the effort to reach this 

 goal, and it may well be said without exaggeration that it has been 

 one of the most difficult problems of modern industry which has had 

 to be solved, in order to render possible the present revolution in the 

 manufacture of sulfuric acid. It would carry us too far to go into 

 the particulars of the various experiments. Even since the process has 

 been put into operation on a large scale, it has demanded several 

 years of the most assiduous work, before it has become possible to look 

 upon the purification of the burner-gases as absolutely assured. The 

 great difficulty of the task lay in the fact that it was a continual strug- 

 gle with an invisible enemy, as one might say, and that every mistake 

 paid the penalty of permanent damage to the plant as regards the 

 amount of production. The final result of these labors was that it was 

 in fact found possible to free the burner-gases from every trace of every 

 impurity, if after appropriate treatment and cooling, they are made to 

 undergo a thorough, systematically continued scrubbing with water or 

 sulfuric acid. This must be continued until both optical and chemical 

 examination of the gas assures us of complete purification from every 

 injurious substance. How this thorough scrubbing is carried out witb 



