INTRODUCTION. 19 



appear in the infusions so treated. The objection raised 

 by Treviranus, viz., that the high temperature to which 

 the infusions had been subjected had so altered them 

 and the air about them that the conditions favorable to 

 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 decom- 

 position 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 decom- 

 position 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 conducting 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 Hoff- 

 mann and in 1861 Chevreul and Pasteur demonstrated 

 that the precautions taken by the preceding investiga- 

 tors 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 infu- 



