FEhMENTATION. 549 
fortuitous ferments of disease. These ferments, which, it 
is to be remembered, are living organisms, have their 
activity suspended by temperatures below 10 degrees C., 
and as long as they are reduced to torpor the beer remains 
untainted either by acidity or putrefaction. The beer of 
low fermentation is brewed in winter, and kept in cool 
cellars; the brewer being thus enabled to dispose of it at 
his leisure, instead of forcing its consumption to avoid the 
loss involved in its alteration if kept too long. Hops, it 
may be remarked, act to some extent as an antiseptic to 
beer. The essential oil of the hop is bactericidal: hence 
the strong impregnation with hop juice of all beer intended 
for exportation. 
These low organisms, which one might be disposed to 
regard as the beginnings of life, were we not warned that 
the microscope, precious and perfect as it is, has no power 
to show us the real beginnings of life, are by no means 
purely useless or purely mischievous in the economy of 
nature. They are only noxious when out of their proper 
place. They exercise a useful and valuable function as the 
burners and consumers of dead matter, animal and vege- 
table, reducing such matter, with a rapidity otherwise 
unattainable, to innocent carbonic acid and water. Fur- 
thermore, they are not all alike, and it is only restricted 
classes of them that are really dangerous to man. One 
difference in their habits is worthy of special reference 
here. Air, or rather the oxygen of the air, which is 
absolutely necessary to the support of the bacteria of 
putrefaction, is, according to Pasteur, absolutely deadly to 
the vibrios which provoke the butyric acid fermentation. 
This has been illustrated by the following beautiful obser- 
vation. 
A drop of the liquid containing those small organisms is 
placed upon glass, and on the drop is placed a circle of 
exceedingly thin glass; for, to magnify them sufficiently, 
it is necessary that the object-glass of the microscope 
should come very close to the organisms. Round the edge 
of the circular plate of glass the liquid is in contact with 
the air, and incessantly absorbs it, including the oxygen. 
Here, if the drop be charged with bacteria, we have a zone 
of very lively ones. But through this living zone, greedy 
of oxygen and appropriating it, the vivifying gas cannot 
penetrate to the center of the film. In the middle, there- 
