INTRODUCTION. 23 



to which the infusions had been subjected had so al- 

 tered them and the air about them that the conditions 

 favorable to spontaneous generation no longer existed, 

 was promptly met by Spallanzani when he gently tapped 

 one of the flasks that had been boiled against a hard 

 object until a minute crack was produced ; invariably 

 organisms and decomposition appeared in the flask thus 

 treated. 



From the time of the experiments of Spallanzani 

 until as late as 1836 but little advance was made in the 

 elucidation of this, at that time, obscure problem. 



In 1836 Schulze attracted attention to the subject by 

 the convincing nature of his investigations. He showed 

 that if the air which gained access to boiled infusions 

 be robbed of its living organisms by first passing it 

 through strong acid or alkaline solutions no decom- 

 position occurred, and living organisms could not be 

 detected in the infusions. Following quickly upon 

 this contribution came Schwann, in 1837, and somewhat 

 later (1854) Schroder and Dusch, with similar results 

 obtained by somewhat different means. Schwann 'de- 

 prived the air which passed to his infusions of its living 

 particles by conducting it through highly heated tubes ; 

 whereas Schroder and Dusch, by means of cotton-wool 

 interposed between the boiled infusions and the outside 

 air, robbed the air passing to the infusions of its organ- 

 isms by the simple process of filtration. In 1860 Hoff- 

 mann and in 1861 Chevreul and Pasteur demonstrated 

 that the precautions taken by preceding investigators 

 for rendering the air which entered these flasks free 

 from bacteria were not necessary; that all that was 

 required to prevent the access of bacteria to the infu- 

 sions in the flasks was to draw out the neck of the flask 



