72 MICROBES, FERMENTS, AND MOULDS. 
ganized ferment has been ascertained, that ferment is 
necessary. This minute being produces the transforma- 
tion which constitutes fermentation by breathing the 
oxygen of the substance to be fermented, or by ap- 
propriating for an instant the whole substance, then 
destroying it by what may be termed the secretion 
of the fermented products. Three things are necessary 
for the development of the ferment: nitrogen in a 
soluble condition, phosphoric acid, and a hydrocarbon 
capable of fermentation (such as grape sugar). Finally, 
every organized ferment of fermentation or putrefac- 
tion is borne about in the air, as may be shown by 
experiments. 
II. VecrtaBLe NATURE OF FERMENTS oR YEAST. 
Yeast, or ferments, are in their organization 
closely allied to the fungi of which we spoke in the 
preceding chapter under the name of Microsporon. 
Many botanists still assign them to the class of fungi 
under the name of Saccharomycetes ; yet, as they live 
in liquids, or at any rate on damp substances, like the 
Algz, which are species of water-fungi, it is now 
almost agreed to place them in the same category as 
the latter, which they resemble in their whole organi- 
zation, except in the absence of chlorophyl. This 
last characteristic, the only one by which they ap- 
proximate to fungi, is common both to them and to 
microbes or bacteria, which are only ferments of 
