544 THE POPULAR SCIENCE MONTHLY, 



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 re- 

 main 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 ma- 

 terials 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 vegeta- 

 ble 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 sup- 

 port, 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 in- 

 stance, platinum sponge, can absorb the oxygen of the air and trans- 

 form 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 thou- 

 sandths of a millimetre in diameter. These globules, these myco- 

 derms, 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 efiicacy, 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. 



\yhen 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 re- 

 semblance to the cells of yeast, sometimes it consists of straight and 

 exceedingly fine rods. Milk also contains casein, which is the sub- 

 stance that composes cheese, and, when the fermentation of the sugar 

 in milk is over, that of the casein begins ; after lactic acid, butyric 



