THE QUESTION OF SPONTANEOUS GENERATION. 105 



reader, Gay-Lussac had arrived at the conclusion that, 

 in Appert's process, one condition of the preservation 

 of animal and vegetable substances consisted in the 

 exclusion of oxygen. 



Even this proposition was soon improved upon, 

 and it became a current opinion in science that the 

 smallest bubble of oxygen or of air which might come 

 in contact with a preserve would be sufficient to start its 

 decomposition. The partisans of spontaneous genera- 

 tion the heterogenists thenceforward threw their 

 objections to Pasteur into this form : 



' How can the germs of microscopic organisms be 

 so numerous that even the smallest bubble of air 

 contains germs capable of developing themselves in 

 every organic infusion ? If such were the case the 

 air would be encumbered with organic germs.' M. 

 Pouchet said and wrote that they would form a thick 

 fog, as dense as iron. 



But Pasteur showed that the interpretation of 

 Gay-Lussac's experiment, with respect to the possible 

 alteration of preserves by a small quantity of oxygen 

 gas, was quite erroneous. If, after a certain time, an 

 Appert preserve contains no oxygen, this is simply 

 because the oxygen has been gradually absorbed by 

 the substances of the preserve, which are always more 

 or less chemically oxidisable. But in reality it is easy 

 to find oxygen in these preserves. Pasteur did not 

 fail to perceive that the interpretation given to Gay- 



