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 
