FERMENTS, FERMENTATIONS, AND LIFE. 277 



condition of mankind. We may now regard it as firmly 

 established; but let us not forget that its establishment 

 has cost two centuries of investigations and labors. Leeu- 

 wenhoek, in the middle of the seventeeth century, was the 

 first to reveal the microscopic world of the air, and to con- 

 jecture its momentous functions. What severe toil, what 

 struggles and tedious trials, since the observations of the 

 Dutch micrographer to the time of the experimental 

 studies of our contemporary and compatriot, Pasteur ! 



