INTRODUCTION. 19 



spontaneous generation no longer existed, was met by 

 Spallanzani by gently tapping one of the flasks, that 

 had been boiled, against some hard object until a minute 

 crack was produced ; invariably organisms and decompo- 

 sition 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 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 

 was robbed of its living organisms by being caused to 

 pass through strong acid or alkaline solutions no de- 

 composition appeared, 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 passing it through highly-heated tubes ; 

 whereas Schroder and Dusch, by means of cotton-wool 

 -interposed between the boiled infusion and the outside 

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

 isms by the simple process of filtration. In 1860 

 Hoffmann and in 1861 Chevreul and Pasteur demon- 

 strated that the precautions taken by the preceding in- 

 vestigators for rendering the air which entered these 

 flasks free from bacteria were not necessary ; that all 

 that was necessary to prevent the access of bacteria to 

 the infusions in the flasks was to draw out the neck of 

 the flask into a fine tube, bend it down along the side 

 of the flask, and then bend it up again a few inches 

 from its extremity, and leave the mouth open. The 



