88 LOUIS PASTEUR 



ried by it. His flasks, unlike those of Needham, 

 remained clear for a long time, and showed no 

 trace of living organisms. Spallanzani concluded 

 that organisms appearing in boiled infusions do not 

 arise spontaneously, but are brought in from the 

 atmosphere. Needham objected that the heat used 

 by Spallanzani altered the vegetative force of the 

 infusions, and that boiling drove off the air neces- 

 sary to produce life. The controversy came to no 

 decisive issue, and parties to both sides of the ques- 

 tion continued their discussions. 



Schulze, in 1836, conceived the idea of supply- 

 ing boiled infusions with air which had been drawn 

 through sulphuric acid in order to rid it of any 

 living germs which it might possibly carry. Infu- 

 sions supplied with air in this way remained several 

 months free from decay. Schwann, in the follow- 

 ing year (1837) varied the experiment by supply- 

 ing his infusions with air which had been passed 

 through a heated tube. As these infusions re- 

 mained free from organic life, he concluded that it 

 is not air that causes life to develop, but something 

 in the air which is destroyed by heat. In 1854, 

 Schroeder and Dusch obtained the same results by 

 drawing into their infusions air which had simply 

 been filtered through a plug of cotton wool. Thus 



