INVISIBLE MICROORGANISMS. 67 



during filtration. The time of filtration should not exceed one hour' 

 The filtrate should remain free 'f rom all visible bacteria as shown by 

 microscopic examination and cultural tests. The filtrate should 

 possess the specific disease-producing qualities of the unfiltered material. 

 Animals infected with the filtrate should yield material which, after 

 filtration, will in its turn possess the attributes of the original unfiltered 

 material. 



