The manager of a well directed brewery, which was built according to the latest improvements and provided with ice-cooling arrangements, found that the alcoholic fermentation of lager beer did not advance with proper regularity. The beer did not clarify well, it remained turbid and had a tendency to assume a disagreeable odor and taste. Microscopic examination of the yeast, however, showed the same to be bottom yeast. After some time its action apparently diminished, or rather, the fermentation, which began well, ceased, and at the same time a white foam formed in the center of the vat. The manager observing this, again submitted it to microscopic examination. The instrument revealed a number of much smaller forms of fungi, similar to those of young yeast, and some which were excessively large, a variety never found in bottom yeast. Fully appreciating the microscopic examination, and aware of the danger which the spread of the fungi could cause, the manager resorted to all known means to retard its pernicious influence. Fresh yeast was employed, and the fermenting vats throughly cleaned, both inside and out, but the phenomena reappeared, showing that the transmission took place through the air. A microscopic examination of a gelatinous coating on the wall of the fermenting room further explained the matter. Beginning at the door of the ice cellar, the walls were covered with a gelatinous mass, which, even when placed beneath the microscope, showed no definite organic structure; however it contained numerous threads of fungi. Notwithstanding the precautions which were taken for cleanliness, these germs traveled from the ceiling through the air into the fermenting liquid and there produced a change, which would ultimately have caused the destruction of all the beer. For a third time and by altogether different means, it was demonstrated that the air was the bearer of these germs. The whole atmosphere was infected, and a simple change of air was by no manner of means sufficient, as has already been shown. In addition, these observations throw considerable light on the means by which contagious diseases are spread, for often a room, a house, or the entire neighborhood appears to be infected. It must also be remembered how, in times of plague, large fires were resorted as to a method of purifying the air.