CHAP. IX.] *!rHE DYNAMICS OF LIFE. 91 



Their relations to energy are also opposite. As already Opposite 

 stated, the change that converts the substance of animal f^f^c^^ons 

 tissues into waste material essentially consists in oxida- of animals 

 tion, and oxidation always produces heat, or some equi- tables. ^ 

 valent form of energy. The oxidation either of the food, 

 or of the substance of the tissues that is passing away in 

 ■waste, is the source of animal heat, and of the mechanical 

 energy of animal motion. Animals thus give out energy to Animals 

 the organic world. The dynamical function of vegetables ^ergy. 

 is the opposite of this. We have seen that they decompose 

 water and carbonic acid. In the decomposition of water 

 or carbonic acid, or any other product of combustion, a 

 quantity of energy must be taken up, exactly equal to that "Vegetables 

 which was given out when that product was formed by energy, 

 combustion ; and this is true whether the decomposition is 

 effected by an inorganic process, or in the organism of a 

 living vegetable. Thus, whatever quantity of energy 

 becomes actual, as heat, from the oxidation of the substance 

 or the food of an animal, and is given out in the radiation 

 from the skin and in the heated breath ; the same energy 

 must have become potential, and been tahen up, when the 

 vegetables on which that animal has fed^ decomposed 

 water and carbonic acid, and thereby fixed the combustible 

 elements, the hydrogen and carbon of its food. 



But in what form was this energy before it assumed this 

 potential form 1 for vegetables cannot create energy out of 

 nothing, any more than can animals or machines. 



It was in the form of radiance. It is usually said that Dynamic 

 the leaves and other green parts of vegetables decompose vegetables 

 carbonic acid tmder the stimulus of light. But this is an ^^ 4®°°™" 



° posing car- 



maccnrate expression: as well might we say, that the tonic acid, 

 decomposing cell in electro-chemical experiments decom- 

 poses water under the stimulus of the electric current. The 

 decomposing cell, and the green leaf, are alike only the 

 apparatus where the decomposition takes place ; the agent 

 of the electric decomposition is the electric current, and 

 the agent of the decomposition of the carbonic acid 



^ Or, in the case of a carnivorous animal, the vegetables on which its 

 prey has fed. 



