52 LOUIS PASTEUR 



seen at all. Some of the ultramicroscopic forms 

 pass through niters (the so-called filterable viruses) 

 and their existence is inferred only by the effects 

 which they produce. 



Like most other unicellular organisms, bacteria 

 multiply by fission. At times some species such 

 as the hay bacillus and bacilli of tetanus and 

 anthrax may form spores which are very resistant 

 to heat, dryness, and destructive chemicals. The 

 fact that as spores some bacteria may withstand 

 boiling even for several hours, proved to be a very 

 troublesome circumstance in the controversy over 

 the spontaneous generation of life as we shall see 

 in a later chapter. 



Under favorable conditions the multiplication of 

 bacteria may proceed with great rapidity. Since 

 divisions in some cases may follow one another 

 every twenty or thirty minutes, the number arising 

 from a single individual in the course of a few 

 days is enormous. Cohn estimates that if a bac- 

 terium divides once in an hour it would produce 

 over three and one-half million descendants at the 

 end of twenty-four hours. Assuming its size to be 

 that of a Bacillus proteus, which is about %oo of 

 a millimeter long by %ooo of a millimeter thick, 

 the volume of this mass of bacteria after twenty- 



