144 FERMENTATION lation of winch to a certain amount arrests the process. 5. Butyric Fermentation. To- ward the close of lactic fermentation butyric acid makes its appearance, accompanied by the evolution of hydrogen and carbonic acid, par- ticularly when sugar of milk and lime are em- ployed. The formation is represented by the following equation : 2C 3 H 6 3 = C 4 H 8 2 + 2C0 2 Lactic acid. Butyric acid. 4H 6. Viscous or Mucous Fermentation. When the juices of beet root and carrot are left in a warm place for a few days, they spontaneously pass into the viscous state, for which reason this has been called the viscous fermentation. During the process there is an escape of car- bonic acid and hydrogen, as in the case of butyric fermentation, and the formation of mannite, gum, and lactic acid. It has been described as taking place under the influence of a peculiar ferment composed of minute spherules, which are probably a species of pe- nicillium. It is doubtful whether this should be considered as a distinct species, or as an incident in lactic or butyric fermentation. 7. Putrefactive Fermentation. This occurs when bodies containing nitrogenous compounds decompose spontaneously in a limited amount of air. When the decomposing substance is freely exposed to the air, and there is not too much moisture present, eremacausis or slow combustion takes place (see EREMACATJSIS) ; but if the access of air is much obstructed, as when the decaying body is submerged in water, a more complex reaction takes place, in which several very offensive gases are evol- ved, prominent among which is sulphuretted hydrogen, the gas which gives the odor of rotten eggs. Phosphuretted hydrogen, carbu- retted hydrogen, ammonia, free nitrogen and hydrogen gases, and acetic, lactic, butyric, and valeric acids, as well as several noxious com- pounds, the nature of many of which is not perfectly understood, are also formed. The putrefaction which takes place soon after the death of a person or animal generates poisonous matter of great virulence. It is, however, the opinion of Dr. Lionel Beale that the peculiar matter which is the most poisonous is engen- dered at about the time of death, and perhaps a few hours before. (" Disease Germs, their Nature and Origin," London, 1872.) Com- plete exclusion of the air prevents putre- faction. If fermentable liquids are first boiled and sealed tightly in close jars, they may be kept for an indefinite time without undergo- ing either vinous or putrefactive fermentation. The commencement of the process is a matter which is involved in some obscurity. A piece of wood or animal tissue undergoing erema- causis, if supplied with sufficient moisture and nearly excluded from the air, immediately be- gins to putrefy. Whether the ferment is the decaying matter itself, or consists of living or- ganisms, is a question that has not been decided. Pasteur regards putrefaction as a peculiar spe- cies of fermentation caused by animal organ- isms of the genus vibrio, of which there are six known species ; and he also regards each of them as having the power of exciting a particu- lar mode of putrefaction. If a putrescible liquid holding air in solution is sealed in a glass vessel and left to stand for a time, certain infu- soria, monas crepusculum and bacterium tcrmo, are first developed. They absorb oxygen from the air and evolve carbonic acid, and then die and fall to the bottom as a sediment. If germs of the vibrio are present, they become devel- oped, and the process of putrefaction com- mences. These vibrions, according to Pasteur, cannot exist in a liquid which contains oxygen. If the putrescible liquid is exposed to the air, the monads and bacteria are first developed, and forming a pellicle on the surface prevent the access of oxygen to the interior. Putrefac- tion then commences, but the products are par- tially decomposed by the influence of the layer of infusoria, and receiving oxygen are converted into water, carbonic acid, and ammonia. Pas- teur also regards the slow oxidation of animal and vegetable matters, such as moistened saw- dust, as dependent upon the influence of the lower cryptogamic and infusorial organisms, without the presence of which he thinks dead organized matter would be subject to but little change. There is a tendency at the present time to regard all kinds of fermentation as due to the development of living organisms, either animal or vegetable, depending princi- pally upon the nature and condition of the fer- menting liquid. According to Pasteur, it is always accompanied by an incessant inter- change of molecules between the fermenting substance and the living cells which develop themselves within it. In the souring of wine, a growth of mycoderma aceti forms on the surface, and has the power of condensing the oxygen of the air, like that of platinum black, or of the blood globules, and conveying it to the liquid on which it rests. Pasteur also says that the germs which cause the fermentation of grape juice come from the exterior of the fruit. He finds with the microscope organized corpuscles attached to the grape skins, which he regards as germs of the ferment. He more- over holds that alcoholic fermentation may be conducted without the presence of atmospheric oxygen, and in an atmosphere comp6sed en- tirely of carbonic acid; in accordance with which idea he has invented and patented ap- paratus for brewing, by which atmospheric air is excluded during fermentation, one great ad- vantage of which he claims is that the germs of other ferments which produce lactic, acetic, and butyric acids are excluded, and beer yeast or true alcoholic ferment alone allowed to act, by which a greater percentage and also better quality of product is obtained, and in a more economical way. Experiments have been made by Pasteur and others in which boiled must and other fermentable liquids have been sub-
