SPONTANEOUS GENERATION. 281 



that others in repeating his experiments obtained life 

 where he obtained none. Eather is the refutation 

 strengthened by such differences. Given two experi- 

 menters equally skilful and equally careful, operating 

 in different places on the same infusion, in the same 

 way, and assuming the one to obtain life while the other 

 fails to obtain it ; then its well-established absence in 

 the one case proves that some ingredient foreign to the 

 infusion must be its cause in the other. 



Spallanzani's sealed flasks contained but small quan- 

 tities of air, and as oxygen was afterwards shown to 

 be generally essential to life, it was thought that the 

 absence of life observed by Spallanzani might have 

 been due to the lack of this vitalizing gas. To dissi- 

 pate this doubt, Schulze in 1836 half filled a flask with 

 distilled water to which animal and vegetable matters 

 were added. First boiling his infusion to destroy what- 

 ever life it might contain, Schulze sucked daily into 

 his flask air which had passed through a series of 

 bulbs containing concentrated sulphuric acid, where all 

 germs of life suspended in the air were supposed to be 

 destroyed. From May to August this process was 

 continued without any development of infusorial life. 



Here again the success of Schulze was due to his 

 working in comparatively pure air, but even in such 

 air his experiment is a very risky one. Germs will 

 pass unwetted and unscathed through sulphuric acid 

 unless the most special care is taken to detain them. 

 I have repeatedly failed, by repeating Schulze's experi- 

 ments, to obtain his results. Others have failed like- 

 wise. The air passes in bubbles through the bulbs, and 

 to render the method secure, the passage of the air must 

 be so slow as to cause the whole of its floating matter, 

 even to the very core of each bubble, to touch the sur- 

 rounding liquid. But if this precaution be observed, 



