PASTEUR, KOCH, AND OTHERS 289 
ciently porous toadmit 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 arc 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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