3G4 LAY SERMONS, ADDRESSES, AND REVIEWS. [xr. 



exposed to ordinary air, while no such development 

 takes place if the air with which it is in contact is 

 mechanically freed from the solid particles which ordi- 

 narily float in it and which may be made visible by 

 appropriate means. 



It is demonstrable that the great majority of these 

 particles are destructible by heat, and that some of 

 them are germs or living particles capable of giving 

 rise to the same forms of life as those which appear 

 when the fluid is exposed to unpurified air. 



It is demonstrable that inoculation of the experi- 

 mental fluid with a drop of liquid known to contain 

 living particles gives rise to the same phenomena as 

 exposure to unpurified air. 



And it is further certain that these living particles 

 are so minute that the assumption of their suspension 

 in ordinary air presents not the slightest difficulty. 

 On the contrary, considering their lightness and the 

 wide diffusion of the organisms which produce them, 

 it is impossible to conceive that they should not be 

 suspended in the atmosphere in myriads. 



Thus the evidence, direct and indirect, in favor of 

 Biogenesis, for all known forms of life must, I think, 

 be admitted to be of great weight. 



On the other side, the sole assertions worthy of at- 

 tention are that hermetically-sealed fluids, which have 

 been exposed to great and long-continued heat, have 

 sometimes exhibited living; forms of low organisation 

 when they have been opened. 



The first reply that suggests itself is the probabil- 

 ity that there must be some error about these experi- 

 ments, because they are performed on an enormous 

 scale every day with quite contrary results. Meat, 

 fruits, vegetables, the very materials of the most fer« 

 mentable and putrescible infusions, are preserved to 



