384 THE CAUSES OF THE XI 
containing the same kind of infusion, and left one 
entirely exposed to the air, and in the mouth of 
the other placed a ball of cotton wool, so that the 
air would have to filter itself through it before 
reaching the infusion, that then, although you 
might have plenty of animalcules in the first flask, 
you would certainly obtain none from the second. 
These experiments, you see, all tended towards 
one conclusion—that the infusoria were developed 
from little minute spores or eggs which were con- 
stantly floating in the atmosphere, and which lose 
their power of germination if subjected to heat. 
But one observer now made another experiment, 
which seemed to go entirely the other way, and 
puzzled him altogether. He took some of this 
boiled infusion that I have been speaking of, and 
by the use of a mercurial bath—a kind of trough 
used in laboratories—he deftly inverted a vessel 
containing the infusion into the mercury, so that 
the latter reached a little beyond the level of the 
mouth of the inverted vessel. You see that he 
thus had a quantity of the infusion shut off from 
any possible communication with the outer air by 
being inverted upon a bed of mercury. 
He then prepared some pure oxygen and nitro- 
gen gases, and passed them by means of a tube 
going from the outside of the vessel, up through 
the mercury into the infusion; so that he thus 
had it exposed to a perfectly pure atmosphere of 
the same constituents as the external air. Of 
