THE ORIGIN OF LIFE 25 



watch-glass, developed through changes incidental to its 

 germination in the barley grain itself. 



Spallanzani, some years later (1777), pointed out the 

 slip-shod methods under which most of the so-called 

 experiments had been performed, and supposed that he 

 had proved spontaneous generation impossible by a new 

 and improved technic. He filled flasks with various 

 organic infusions, such as were supposed to be " biogenic," 

 subjected the contents to thorough boiling, hermetically 

 sealed them, and then placed them under what were 

 supposed to be conditions favorable to the development 

 of life, but always with negative results. Instead of 

 carrying conviction with them, these experiments of 

 Spallanzani were severely criticized by Treviranus on the 

 ground that the atmosphere so essential to life had been 

 excluded from the fluids. To overcome this objection, 

 Spallanzani gently tapped his flasks so as to produce 

 minute cracks through which air might enter. When 

 this was done, life invariably appeared and decomposi- 

 tion occurred. 



The problem remained in about the same state until 

 Schultze in 1836 improved the method by which air was 

 admitted to the flasks. He filled them but half full of 

 putrescible fluids, boiled them thoroughly to destroy 

 such life as they might already contain, and then daily 

 sucked into them a certain quantity of air that passed 

 through a series of bulbs containing concentrated sul- 

 phuric acid or strong alkalies by which any germs of life 

 that might be in the air should be destroyed. The cul- 

 ture flasks were kept from May to August, air being 

 passed into them daily, yet without the appearance of 

 life or putrefaction in the contained fluid. 



Schwann in the following year (1837) performed a 

 similar experiment with the same result, passing the air 

 admitted to the flasks through highly heated tubes 

 instead of through acids and alkalies. 



Schroeder and van Dusch, in 1854, discovered that if 

 the mouth of the flask containing a putrescible fluid was 



