108 LOUIS PASTEUR 



heat is shown by the seeds of several plants. Seeds, 

 like the germs of micro-organisms to which they 

 are analogous, vary greatly in their resistance to 

 heat. Most of them are destroyed by temperatures 

 far below the boiling point; some, like mustard 

 seed, may withstand a few seconds exposure to 

 boiling water. A few kinds of seeds have been 

 found to resist boiling for several hours without 

 destroying their power of germination. 



Another very deceptive circumstance consists in 

 the relation of the development of some germs to 

 oxygen. In some cases infusions have been boiled 

 and found to remain clear so long as they were 

 kept from the air. When supplied with oxygen, 

 under the most careful precautions to prevent con- 

 tamination, they were nevertheless found to develop 

 life. Here seems to be the kind of proof required 

 for the theory of the spontaneous origin of life. 

 More searching investigation, however, solved the 

 problem in a different way. It was shown that the 

 germs in question were not killed by boiling, but 

 remained alive, though inactive, in the sealed tubes 

 because they required oxygen for their develop- 

 ment. When this was supplied under conditions 

 that kept out any germs from the outside, life de- 

 veloped in the infusion. By bringing the infusion 



