28 Introduction 



hog-cholera, African horse-fever, infectious anemia or swamp 

 sickness of horses, fowl plague, small-pox, cow-pox, sheep- 

 pox, horse-pox, swine-pox, and goat-pox are at some or all 

 stages able to pass through the Berkefeld or diatomaceous 

 earth filters, and some of them through the much less porous 

 unglazed porcelain or Chamberland filters. Thus there is 

 opened a new world that is ultramicroscopic, but still teems 

 with invisible living organisms. 



Such organisms, interesting as they must be to the biolo- 

 gist and pathologist, cannot yet be known or described, and, 

 therefore, seem to be, for the most part, beyond the scope of 

 the present writing. 



