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 



