236 CRITIQUES AND ADDBESSES. [x. 



maintains tha't the atoms of sugar, tartrate of ammonia, 

 yeast-ash, and water, under no influence but that of free 

 access of air and the ordinary temperature, re-arrange 

 themselves and give rise to the protoplasm of Bacterium. 

 But the alternative is to admit that these Bacteria arise 

 from germs in the air ; and if they are thus propagated, 

 the burden of proof that other like forms are generated 

 in a different manner, must rest with the assertor of 

 that proposition. 



To sum up the effect of this long chain of evidence : 



It is demonstrable that a fluid eminently fit for the 

 development of the lowest forms of life, but which 

 contains neither germs, nor any protein compound, 

 gives rise to living things in great abundance if it is 

 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 experimental 

 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 sus- 

 pended in the atmosphere in myriads. 



