experimentally with gases which were intentionally much diluted, pure sulfur dioxid with a large excess of air being used, and it could be demonstrated that the reaction did not fully cease, under certain circumstances, until the sulfur dioxid was almost completely converted into sulfuric acid. Strangely enough, even when double the theoretical amount of air was present, it seemed to have no untoward influence upon the reaction, indeed it rather appeared as if the large excess of oxygen exercised a favorable effect upon the quantity of sulfur trioxid obtained from a given quantity of the dioxid. From this it followed that the earlier conceptions, according to which the dilution of the sulfur gas was unfavorable for the contact-process, must be submitted to a critical examination.
From now on the experiments were carried out upon the gases which came directly from the pyrites-burners. For this purpose the gases were brought directly from the pyrites-burners to the laboratory through a long lead pipe. This pipe acted as a long dust catcher, and the gases in their passage through it were thoroughly freed from every mechanical impurity, such as ashes, burner-dust, etc. The gases were further passed through several bottles filled with sulfuric acid, before they reached the contact mass. The experiments were very satisfactory, for quite as favorable results were attained as had been the case with the mixture of pure gases. No diminution in the activity of the contact mass could be observed, although the experiments extended over several days, and the hope seemed well justified that in this simple manner it would be possible to manufacture sulfuric acid directly from the burner-gases without loss of sulfur.
The experiments were now carried out on a larger scale. Here, however, it soon appeared that the activity of the contact-mass rapidly diminished in strength and finally ceased. The results were the same, even after the gases had been not only purified as in the laboratory experiments, by cooling in long tubes and washing repeatedly with sulfuric acid, but in addition had been passed through a dry coke and asbestos filter; they were then as pure as was technically possible. We were, therefore, obliged to consider the experiments on a large scale as a failure.
Although by these unlooked-for results a hard blow was given to the hope of success, nevertheless further experiments were undertaken in the laboratory for the purpose of investigating the cause of the apparently inevitable deterioration of the contact-mass.
The surprising observation was soon made that there are substances which, when present in exceedingly small quantities, are capable of inhibiting the catalytic action of platinum to an extraordinarily great degree. Among these substances are first of all arsenic, mercury and phosphorus, while antimony, bismuth, lead, iron, zinc and the other sub-
