24 BACTERIOLOGY. 
Helmholtz, in 1843, repeated and confirmed 
Schwann’s experiments with calcined air. He found 
that the free admission of air so treated to boiled 
organic infusions was not capable of producing fer- 
mentation of any kind. 
Again, it was objected to these experiments that the 
heating of the air had perhaps brought about some 
chemical change which hindered the production of fer- 
mentation. Schroeder and von Dusch, in 1854, then 
showed that by a simple process of filtration, which 
has since proved of inestimable value in bacteriological 
work, the air can be mechanically freed from germs. 
By placing in the mouth of the flask containing the 
boiled solutions a loose plug of cotton, through which 
the air could freely circulate, it was found that all 
suspended micro-organisms could be excluded, and 
that air passed through such a filter, whether hot or 
cold, did not cause fermentation of boiled infusions. 
Similar results were obtained by Hoffmann in 1860, 
and by Chevreul and Pasteur in 1861, without a cotton 
filter, by drawing out the neck of the flask to a fine 
tube and turning it downward, leaving the mouth open. 
In this case the force of gravity prevents the suspended 
bacteria from ascending, and there is no current of air 
to carry them upward through the tube into the flask 
containing the boiled infusion. 
Tyndall later showed (1876), by his well-known in- 
vestigations upon the floating matters of the air, that 
in a closed chamber, in which the air is not disturbed 
by currents, all suspended particles settle to the bottom, 
the superincumbent air being optically pure, as is 
proved by passing a ray of light through it. He dem- 
onstrated that the presence of living organisms in 
