VITAL PHENOMENA OF BACTERIA. 81 
If after the sugar is consumed not enough acid has 
been formed to kill the bacteria, a similar change in 
reaction now takes place to that in ordinary culture 
media in the absence of sugar—viz., the acid is neu- 
tralized gradually, and in the end the reaction becomes 
alkaline. 
Among the acids produced the most important is 
lactic acid; also traces of formic acid, acetic acid, pro- 
prionic acid, and butyric acid, and not infrequently 
some ethyl-alcohol and aldehyde or acetone are formed. 
Occasionally no lactic acid is present, and only the other 
acids are formed. 
Various bacteria, as yet incompletely studied, possess 
the property of producing butyric acid and buty]-alcohol 
from carbohydrates. 
Some bacteria also seem to have the power of decom- 
posing cellulose, found in the stomach and intestinal 
contents of herbivorous animals and in marshy soils, 
with the production of marsh-gas. 
Formation of Gas from Carbohydrates and Other Ferment- 
able Substances of the Fatty Series. The only gas pro- 
duced in visible quantity in sugar-free culture media is 
nitrogen. If sugar is vigorously decomposed by bac- 
teria, as long as pure lactic acid or acetic acid is pro- 
duced there may be no development of gas, as, for 
instance, with the B. typhosus on grape-sugar; but 
frequently there is much gas developed, especially in 
the absence of air. About one-third of the acid-pro- 
ducing species also develop gas abundantly, this con- 
sisting chiefly of CO,, which, according to Smith, is 
always mixed with H. Marsh-gas is seldom formed 
by bacteria, with the exception of those decomposing 
cellulose. 
: 6 
