218 THE HISTORY OF SCIENCE 



he had a horror of inflicting pain, and always in- 

 sisted, when operations on animals were necessary 

 in the laboratory, on the use of anaesthetics (our 

 command of which had been greatly advanced by 

 Simpson in 1847). Emile Roux said that Pasteur's 

 agitation at witnessing the slightest exhibition of 

 pain would have been ludicrous if, in so great a man, 

 it had not been touching. 



A few months after his daughter's death Pasteur 

 wrote to one of his friends : " I am pursuing as best 

 I can these studies on fermentation, which are of 

 great interest, connected as they are with the im- 

 penetrable mystery of life and death. I am hoping 

 to make a decisive advance very soon, by solving 

 without the least lack of clearness the famous ques- 

 tion of spontaneous generation." Two years previ- 

 ously a scientist had claimed that animals and plants 

 could be generated in a medium of artificial air or 

 oxygen, from which all atmospheric air and all germs 

 of organized bodies had been precluded. Pasteur 

 now filtered atmospheric air through a plug of cot- 

 ton or asbestos (a procedure which had been fol- 

 lowed by others in 1854), and proved that in air 

 thus treated no fermentation takes place. Nothing 

 in the atmosphere causes life except the micro-organ- 

 isms it contains. He even demonstrated that a pu- 

 trescible fluid like blood will remain unchanged in 

 an open vessel so constructed as to exclude atmos- 

 pheric dust. 



Pasteur's critics maintained that if putrefaction 

 and fermentation be caused solely by microscopic 

 organisms, then these must be found everywhere and 

 in such quantities as to encumber the air. He replied 



