ENEEGY OF CHEMICAL AFFINITY 7 



liberated. We may say, for example, that in the elements 

 carbon and oxygen, so long as they remain separate, a certain 

 amount of energy remains latent. We call this potential 

 energy. When the carbon and oxygen atoms are allowed to 

 come together and unite, this potential energy of chemical 

 affinity is liberated as kinetic energy, and manifested in the 

 form of light and heat. 



It is from the potential energy of chemical affinity that the 

 energy of a living organism is immediately derived. Protoplasm, 

 the fundamental constituent of both plants and animals, contains 

 chemical compounds of extremely complex structure, composed 

 of many elements and containing a large amount of potential 

 energy locked up in them. Moreover, these proteids, as 

 they are termed, are extremely unstable bodies, readily breaking 

 up on oxidation into simpler and more stable combinations and 

 thus liberating energy. It is the presence of these unstable 

 proteids which confers upon protoplasm its peculiar fitness to 

 form what has been so aptly termed by Huxley " The physical 

 basis of life." They play the part of the gunpowder in a 

 cartridge, ready to produce a manifestation of energy as 

 soon as the proper stimulus is applied. 



It is, then, the breaking up of proteids, or of some other 

 complex substances, usually by recombination of their con- 

 stituents with oxygen, which furnishes the constant supply of 

 energy which an organism requires. This process, however, 

 can only go on so long as the supply of combustible matter on 

 the one hand and of oxygen on the other is adequately main- 

 tained, and this brings us to the consideration of two of the 

 most important functions which every living organism must 

 perform, nutrition and respiration. 



Under the head of nutrition we must include all those 

 processes which are concerned in building up the body, in 

 making good the waste of substance necessitated by the expendi- 

 ture of energy and thus providing new stores of fuel for the 

 use of the organism. The first step in nutrition is the taking 

 into the body of suitable food material. In the case of the 

 typical animal this material must contain in some form or 

 other all the necessary supply of potential energy, locked up in 

 more or less complex and unstable chemical compounds such 

 as it can obtain only from the bodies of other organisms. The 

 green plant, on the other hand, by virtue of the chlorophyll 



