12 POPULAB EKEOKS. 



to kill the germs which the air contained. Under 

 these conditions the liquid was found to keep for 

 any length of time without fermentation. The 

 presence of living germs in the air was proved by 

 actually collecting them by drawing air through 

 plugs of gun cotton; the numerous minute bodies 

 which are usually floating in the air were rendered 

 visible to those who doubted their existence by 

 admitting a beam of light into an ordinary darkened 

 room. The fact that the visibility of the beam 

 depended wholly on the reflection of the light from 

 the suspended particles in the air was proved by 

 keeping the room closed until all the particles had 

 subsided, when the beam was no longer visible, 

 though admitted by a small opening as before. By 

 these and many other ingenious experiments Pas- 

 teur proved the almost universal presence of living 

 germs, and their ability to originate all known cases 

 of fermentation and putrefaction. A debate with 

 Pouch et and others on this subject before the 

 French Acadamy was carried on for months during 

 the years 1861-62, and resulted in a triumphant 

 vindication of the principle for which Pasteur con- 

 tended. It was not until after the Franco- Prus- 

 sian war, however, that Liebig, declining to dis- 

 cuss the question longer, virtually admitted that the 

 fermentation of liquids was due to the presence of 

 living ferments derived from the air as claimed by 



