bonic acid, it appears that any vegetable cells which are capable of extracting their needed supply of oxygen from organic combinations may, by this manifestation of their vital activity, act as ferments, and the true ferments are distinguished from these, not by a difference in their specific action, but from the fact that they are capable of carrying on the functions of nutrition and assimilation with much greater activity without a supply of atmospheric oxygen. Pasteur has likewise proved that the alcoholic ferments develop rapidly in the presence of air, but that their function as ferments is impaired by this ready supply of oxygen. In the absence of air, on the other hand, as in an atmosphere of carbonic acid, they take their supply of oxygen from organic substances, as sugar, and their function as ferments is increased. When the life of the bacteria or other organized ferments is destroyed, the processes of fermentation and putrefaction cease, and this takes place at a temperature of from 122° to 140°, according to observations made in the course of the controversy in regard to spontaneous generation. After the organized ferments are killed, fermentation or putrefaction can not take place until the living ferments are again introduced. The canned articles of food which are now so common in the markets are an illustration of the application of this principle. In their preparation heat is applied, which kills the bacteria—the active agents of fermentation—and the cans are then sealed to prevent the introduction of a fresh supply of germs from the atmosphere. The popular notion that canned articles of food are preserved by excluding the atmospheric oxygen, which has been derived from the application of Liebig's chemical theory of fermentation, is without foundation. The experiments of Schwann, Pasteur, and Tyndall conclusively prove that articles which are peculiarly liable to undergo putrefactive changes, as urine, and an endless variety of vegetable and animal infusions, can be kept without change for months and years when abundantly supplied with free oxygen, if proper precautions are taken to exclude the living organisms that are the real cause of fermentation. These experiments have likewise proved that the germs of the bacteria of fermentation and putrefaction are widely distributed in the air, and the supposed cases of spontaneous fermentation, or putrefaction, are readily explained by the "seeding" of the fermenting substances with germs derived from the atmosphere.
As fermentation is strictly a physiological process, the fermented product may be looked upon as the residuum of what is required in the nutritive processes of the bacteria of fermentation.
The variations in the quality of ensilage, to which attention has already been directed, are readily explained by differences in the condition of the crops, as to maturity and development, and the manner in which it is packed in the silo, all of which must have an influence on the performance of the nutritive functions of the bacteria, and cor-
