SPONTANEOUS GENERATION. 479 



such differences. Given two experimenters equally skillful and equal- 

 ly careful, operating in different places on the same infusions, 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 quantities of air, 

 and as oxygen was afterward 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 dissipate this 

 doubt, Schulze in 1836 half-tilled a flask with distilled water, to which 

 animal and vegetable matters were added. First boiling his infusion 

 to destroy whatever 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 pro- 

 cess was continued without any development of infusorial life. 



Here, again, the success of Schulze was due to his working in com- 

 paratively 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 experiments, to obtain his 

 results. Others have failed likewise. 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 core of each bubble, to touch the surrounding liquid. But, 

 if this precaution be observed, water will be found quite as effectual 

 as sulphuric acid. By the aid of an air pump, in a highly-infective 

 atmosphere, I have thus drawn air for weeks without intermission, 

 first through bulbs containing water, and afterward through vessels 

 containing organic infusions, without any appearance of life. The 

 germs were not killed, but they were effectually intercepted, while 

 the objection that the air had been injured by being brought into con- 

 tact with strongly corrosive substances was avoided. 



The brief paper of Schulze, published in Poggendorfs Annalen for 

 1830, was followed in 1837 by another short and pregnant communi- 

 cation by Schwann. Redi, as we have seen, traced the maggots of 

 putrefying flesh to the eggs of flies. But he did not and he could not 

 know the meaning of putrefaction itself. He had not the instrumen- 

 tal means to inform him that it also is a phenomenon attendant on the 

 development of life. This was first proved in the paper now alluded 

 to. Schwann placed flesh in a flask filled to one-third of its capacity 

 with water, sterilized the flask by boiling, and then supplied it for 

 months with calcined air. Throughout this time " there appeared no 

 mould, no infusoria, no putrefaction ; the flesh remained unaltered, 

 while the liquid continued as clear as it was immediately after boiling." 



