228 CRITIQUES AND ADDRESSES. [x 



in the water with which the infusion is made, or in the 

 superjacent air. Now the vitality of all germs is de- 

 stroyed by heat. Therefore, if I boil the infusion, cork it 

 up carefully, cementing the cork over with mastic, and 

 then heat the whole vessel by heaping hot ashes over it, 

 I must needs kill whatever germs are present. Con- 

 sequently, if Redi's hypothesis hold good, when the 

 infusion is taken away and allowed to cool, no animal- 

 cules ought to be developed in it ; whereas, if the ani- 

 malcules are not dependent on pre-existing germs, but 

 are generated from the infused substance, they ought, 

 by and by, to make their appearance. Needham found 

 that, under the circumstances in which he made his 

 experiments, animalcules always did arise in the 

 infusions, when a sufficient time had elapsed to allow 

 for their development. 



In much of his work Needham was associated with 

 Buifon, and the results of their experiments fitted in 

 admirably with the great French naturalist's hypothesis 

 of " organic molecules," according to which, life is the 

 indefeasible property of certain indestructible molecules 

 of matter, which exist in all living things, and have 

 inherent activities by which they are distinguished from 

 not living matter. Each individual living organism is 

 formed by their temporary combination. They stand to 

 it in the relation of the particles of water to a cascade, 

 or a whirlpool ; or to a mould, into which the water is 

 poured. The form of the organism is thus determined by 

 the reaction between external conditions and the inherent 

 activities of the organic molecules of which it is com- 

 posed ; and, as the stoppage of a whirlpool destroys 

 nothing but a form, and leaves the molecules of the water, 

 with all their inherent activities intact, so what we call 

 the death and putrefaction of an animal, or of a plant, is 

 merely the breaking up of the form, or manner of asso- 



