PASTEUR, KOCH, AND OTHERS 289 



ciently porous to admit of exchange of air, but matted closely 

 enough to entangle the floating particles. He showed also 

 that many of the minute organisms do not require free oxygen 

 for their life processes, but are able to take the oxygen by 

 chemical decomposition which they themselves produce from 

 the nutrient fluids. 



Jeffries Wyman, of Harvard College, demonstrated that 

 some germs are so resistant to heat that they retain their 

 vitality after several hours of boiling. This fact probably 

 accounts for the difference in the results that have been 

 obtained by experimenters. The germs in a resting-stage 

 are surrounded by a thick protective coat of cellulose, 

 which becomes softened and broken when they germinate. 

 On this account more recent experimenters have adopted a 

 method of discontinuous heating of the nutrient fluid that is 

 being tested. The fluids are boiled at intervals, so that the 

 unusually resistant germs are killed after the coating has been 

 rendered soft, and when they are about to germinate. 



After the brilliant researches of Pasteur, the question of 

 spontaneous germination was once again regarded as having 

 been answered in the negative; and so it is regarded to-day 

 by the scientific world. Nevertheless, attempts have been 

 made from time to time, as by Bastian, of England, in 1872, 

 to revive it on the old lines. 



Tyndall.--John Tyndall (1820-1893), the distinguished 

 physicist, of London, published, in 1876, the results of his ex- 

 periments on this question, which, for clearness and ingenuity, 

 have never been surpassed. For some time he had been 

 experimenting in the domain of physics with what he called 

 optically pure air. It was necessary for him to have air from 

 which the floating particles had been sifted, and it occurred 

 to him that he might expose nutrient fluids to this optically 

 pure air, and thus very nicely test the question of the 

 spontaneous origin of life within them. 

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