258 NATURE AND LIFE. 



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 trans- 

 formation 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 vine- 

 gar 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 sub- 

 stance proceeds from the decomposition of sugar contained 

 in the milk, and this decomposition, again, is a fermenta- 

 tion. 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 acid is produced. 

 Examining with a microscope the casein transforming into 

 butyric acid, we observe in it little rods, two thousandths 

 of a millimetre in diameter, and of a length from two 

 to five times as great ; this is the butyric ferment, which, 

 concurrently with other microscopic vegetable growths, 

 determines in various cheeses the slow production of 



