CHAPTER XXIV 



FILTERABLE VIRUSES 



In describing bacteria it was stated that they varied 

 in size from 50;^ to y^ inch; the average pathogenic 

 bacterium is about lo;^ inch long. It is obvious that 



a h c d 



¥\g. 41. — Different types of bacteriologic filters: a, Kitasato; 6, 

 Berkefeld; c, Chamberland; d, Reichel (McFarland). 



such tiny organisms readily pass through ordinary paper 

 filters and through absorbent cotton filters. It is pos- 

 sible, however, to construct filters which entirely hold 



12(i 



