Page:Popular Science Monthly Volume 5.djvu/562

544 employed in producing the wine, beer, and cider, consumed every year, as M. Dumas says, even astronomers would shrink from the task.

This active property of decomposing sugar, and forming alcohol in consequence, does not belong to the cells of brewer's yeast exclusively. Several chemical agents possess the same power, and certain vegetable cells also are adapted to use it. When fruits are placed in a medium filled with oxygen, they absorb this gas, and occasion the release of carbonic acid; if, on the contrary, they are left in carbonic acid or any other inert gas, they effect the production of alcohol. The fruits remain firm and hard, without suffering any external change, but the sugar they contain is transformed in part into alcohol. How is this phenomenon to be explained? In common air, the cell of the fruit is fed by oxygen; if this gas is withheld, it is forced to borrow the materials of nutrition from the fluids that moisten it, that is, from the saccharine juice, and then the latter is decomposed. M. Pasteur has noted that a similar alcoholic fermentation takes place in other vegetable organs, in leaves, for instance, and in every case he has proved that the phenomenon is due to the cells of the vegetables alone, and not to yeast-globules. Far from throwing any doubt on the physiological doctrine of fermentation, these singular facts agree in lending it support, by giving it deeper and more general application.

We have seen that the fermentation of sugar yields alcohol. The latter, brought in contact with certain porous substances, as, for instance, platinum sponge, can absorb the oxygen of the air and transform itself, by oxidation, into acetic acid. A phenomenon of this kind occurs in wine when it sours, the alcohol contained in it being changed into acetic acid; only, the agent in the transformation is in this case a microscopic plant, made up of little elongated globules, some thousandths of a millimetre in diameter. These globules, these mycoderms, develop on the surface of wine exposed to the air, and form a scum which plays the part of storing away a certain stock of oxygen, afterward used to produce acetification in the liquid. This scum, which is called mother of vinegar, only acts while in communication with the air. As soon as it is below the surface, it loses its efficacy, and the production of acetic acid is checked. Thus the development of vinegar in the acetic fermentation is reduced to an oxidation of alcohol, in which microscopic cells are the vehicles of the oxygen.

When milk turns and sours, that phenomenon also is due to the formation of an acid—lactic acid. This substance proceeds from the decomposition of sugar contained in the milk, and this decomposition, again, is a fermentation. The microscopic being that effects it assumes several forms; sometimes it is made up of cells presenting much resemblance to the cells of yeast, sometimes it consists of straight and exceedingly fine rods. Milk also contains casein, which is the substance that composes cheese, and, when the fermentation of the sugar in milk is over, that of the casein begins; after lactic acid, butyric