FOUNDATIONS 7 



Filtrable Viruses. The living virus of foot-and-mouth 

 disease was found by Loeffler and Frosch (1898) to pass 

 through filters which arrest any known bacteria. Since that 

 date a number of other living poisons have been found 

 to be filtrable, including those of measles, smallpox, and 

 molluscum contagiosum. 



Many pathologists now write with facility of " invisible 

 micro-organisms," so it is of value to examine this very 

 important field. M. Arthus, in a most interesting book 

 published last year, writes thus : " We admit that the 

 causal microbe exists, but suppose that it is so small that 

 it eludes microscopic search, and this hypothesis for it is 

 nothing more than a hypothesis is supported by the facts 

 elicited in the study of pleuro-pneumonia of cattle." The 

 author goes on to say that in the serous exudation of this 

 disease fine granules can be seen with the highest powers of 

 the microscope ; they can be stained, though with difficulty, 

 and they can also be cultivated on very special media. 

 Arthus asks whether we may not see in the virus in question 

 a link between the visible and cultivable organisms on the 

 one hand, and the invisible organisms on the other 

 hand. 



We find no word here of these most minute of known 

 living units having any phases other than the filtrable. 

 And this is the usual view, though it is proved to be an error 

 by what is seen in simple water cultures of molluscum 

 bodies : see Chapter VI. 



