VITAL PHENOMENA OF BACTERIA. 63 
visible change, when sugar is added decompose it, with 
the production of gas. The term fermentation is dif- 
ferently used by different authors. Some call every 
kind of decomposition due to bacteria a fermentation, 
speaking thus of the putrefactive fermentation of albu- 
minous substances; others limit the term to the process 
when accompanied by the visible production of gas; 
others, again, take fermentation to mean only the 
decomposition of carbohydrates, with or without gas- 
production. 
Fermentation may be defined as a chemical decom- 
position of an organic compound, induced by living 
organisms or substances contained within them (organ- 
ized ferments), or by chemical substances thrown off 
from the bacteria (unorganized or chemical ferments 
or enzymes). In the first the action is due to the 
growth of the organisms producing the ferment,’ as 
in the formation of acetic acid from alcohol by the 
action of the vinegar-plant, and in the second the 
enzyme causes a structural change without losing its 
identity, as in digestion. These enzymes even when 
present in the most minute quantities have the power 
of splitting up or decomposing complex organic com- 
pounds into simpler, more easily soluble and diffu- 
sible molecules. We can only speak of chemical fer- 
ments when it can be demonstrated that the fermenta- 
tion continues in the absence of all living bacteria. 
1 Buchner (Berichte d. detitsch. chem. Gesellsch., xxx. 117-124 and 1110- 
1113) has shown that even in those cases of fermentation in which, until 
lately, we have believed the organized cel] itself was necessarily concerned 
that the cell protoplasm squeezed from its capsule is able to cause the same 
changes as the organized cells. This brings fermentation by unorganized and 
organized ferments very closely together, the one being a substance thrown 
off from the cell, the other a substance ordinarily retained in the cell. The 
increase of both ceases with the death of the bacteria producing them, 
