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 



