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 formation 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 diminution was very slow it is true, being apparent only after weeks or perhaps 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 supposed 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 conductors, 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.