116 PHYSIOLOGY OF THE BACTERIA. 
to act like all other living beings, and to respire 
like the other inferior organisms deprived of chlo- 
rophyll — that is to say by absorbing oxygen and 
eliminating carbonic acid. This is, mdeed, the 
opinion of a great number of botanists. But, 
according to Pasteur, it is not so with the bacteria. 
When we examine what occurs in putrefaction, 
we find that at first certain species are developed 
(Jonas crepusculum, Bacterium termo, etc.), which 
absorb all the oxygen dissolved in the liquid, and 
come to the surface where they form a thick veil ; 
after this, other species of vibrioniens appear, 
which are developed in a medium entirely de- 
prived of free oxygen, by borrowing this gas 
from the fermentable matters contained in the 
liquid. These chemical decompositions constitute 
putrefaction. 
The first of these organisms, regarding the na- 
ture of which Pasteur has long been uncertain, 
are aérobies : they live in contact with the air, and 
have need of oxygen. The second, anaérobies, 
not only have no need of oxygen, but are killed 
by it. 
These differences in the respiration of organ- 
isms belonging to the same group are not admitted 
by a great number of recent observers. Hoff- 
mann, among others, says expressly: ‘“ These little 
beings cannot live without air, I should say with- 
out oxygen: if this gas is wanting, they cease to 
move and do not multiply at all. If a drop of 
liquid full of bacteria is placed upon a glass slip, 
then covered by a piece of thin glass, the active 
