CHAPTER XXIV 



FILTERABLE VIRUSES 



IN describing bacteria it was stated that they varied 

 in size from 5^00 to ^ inch; the average pathogenic 

 bacterium is about 10^00 mcn l n g- It is obvious that 



a b c d 



Fig. 40. 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 



124 



