Page:Popular Science Monthly Volume 25.djvu/235

Rh an accumulation of experimental evidence that could not be controverted. He proved that sugar was acted upon by a variety of ferments, each giving its own peculiar product, and that the different kinds of fermentation, properly so called, as the alcoholic, the lactic, the acetic, the viscous, the butyric, and putrefactive, were each the result of the vital activity of distinct and specific organisms.

Ferments are now generally divided into two classes: 1. The so-called soluble or chemical ferments, as acids and diastase, which "invert" cane-sugar and transform it into dextrose, or change starch into dextrin. These soluble ferments, according to Dumas, "always sacrifice themselves in the exercise of their activity," but they do not produce fermentation in the strict sense of the term. 2. The true ferments, which, through the investigations of Pasteur, are now known to be living organisms that produce fermentation as a function of their vital activity. Unlike the soluble ferments, these living organisms increase at the expense of the substances fermented. The true fermentations are therefore purely physiological processes, which are defined by Pasteur as "the direct consequence of the processes of nutrition, assimilation, and life, when they are carried on without the agency of free oxygen," or, "as a result of life without air."

The organized ferments, which belong to the class of fungi, may be divided into two groups, the saccharomycetes, or budding fungi—the active agents of alcoholic fermentation, of which yeast may be taken as the type—and the schizomycetes, or fission fungi, which include the lactic, the butyric, and similar ferments, and the organisms that produce putrefaction; most of them are of the form known as bacteria, and they multiply rapidly by subdivision. It is probable that all the members of both groups propagate by means of spores, as well as by their special processes of budding and fission, but there are many species in which reproduction by spores has not been observed. The living organisms (bacteria) found in samples of fresh ensilage belong to the group of schizomycetes. Thus far no members of the group of saccharomycetes (yeast or alcoholic ferments) have been observed, by me, in samples from the interior of the silo that had not been exposed to the air. When a large surface of ensilage is exposed to the air, after the silo is opened, a variety of ferments may make their appearance, and with them several species of molds, but they are evidently produced from germs derived from the air.

The mold-fungi are not included in the class of ferments, as Pasteur has proved that they act as ferments under exceptional conditions only, and even then they do not produce active fermentation. The alcoholic ferments have been studied more thoroughly than the others, from their importance in the manufacture of beer, wine, etc., but many of the facts developed in their investigation are undoubtedly applicable to other ferments.

From Pasteur's experiments with fruits in an atmosphere of