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 completely 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 process 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 distinguished 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 struggle 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 with