Very often the whole work of the brewer is jeopardized by the unsuspected presence of diseased ferments, a remedy for which is only devised after the evil has evoked the complaints of customers. In such a case the brewer avails himself of the kindness of some other brewer to obtain a change of yeast—a custom which is recognized and valued in the trade, since all managers of breweries have an interest in keeping it up. The brewer whose produce is most satisfactory recognizes the fact that unforeseen circumstances may compel him at any moment to change his yeast. We have frequently had occasion to show that this necessity for a change of yeast depends, in most cases, on some change brought about by the presence of diseased ferments, the multiplication of which has resulted fortuitously from some unconscious neglect during the process of brewing, or from climatic influences. When we reflect that yeast is a living being, and that the medium which serves as its aliment, and the water in which it lives, are remarkably well adapted for the development of a vast number of other microscopic beings, the comparative purity of yeast should surprise us even more than its deterioration does. Now by means of microscopical observations we might often detect the existence of the evil long before we are warned of it by a defective working, which invariably entails great losses.[18] In proof of this remark we may cite the following facts. In the month of September, 1871, we were permitted to go through a large London brewery, in which the microscopical study of yeast was altogether unknown. We were allowed to make certain experiments in the presence of the managers of the establishment. We first examined porter yeast, which was collected in a channel that received the yeast as it worked out of the fermenting vessels. One of the ferments of disease abounded in this yeast, as may be seen in the accompanying sketch, which was taken on the spot (Fig. 1). It was evident that the working of the porter was extremely unsatisfactory, and had, perhaps, been so for a long time; indeed, we were told that they had obtained a change of yeast from another London brewery that same day. We made a point of examining this yeast with the microscope. It was beyond comparison purer than the preceding yeast. [Illustration: Fig. 1.] It is evident that if these brewers had been in the habit of using the microscope they might have detected the unsoundness of their produce before the time when they actually made the discovery, which, no doubt, was forced upon them by the complaints of their customers, or some other annoying circumstance, that led to their obtaining a change of yeast. We next obtained permission to examine the yeast of the other beers undergoing fermentation, especially those of white beers, such as ale and pale ale. In the sketch which we made of these yeasts one may detect the presence of the filaments peculiar to turned beer (Fig. 2). [Illustration: Fig. 2.]