240 Transactions of the American Institute. 



In study ino; tlie j^liciiornena of living beings, it is important also 

 to bear in mind the different and at the same time the coordinate 

 purposes subserved by the two great kingdoms of nature. The food 

 of the plant is matter whose energy is all expended; it is a fallen 

 weight. But the plant-organism receives it, exposes it to the sun's 

 ray, and, in a way yet mysterious to us, converts the actual energy 

 of the sunlight into potential energy within it. The fallen weight 

 is thus raised, and energy is stored up in substances which now are 

 alone competent to become the food of the animal. This food is not 

 such because any new atoms have been added to it ; it is food because 

 it contains within it potential energy, which at any time may become 

 actual as force. This food the animal now a])propriates; he brings 

 it in contact with oxygen, and the potential energy becomes actual; 

 he cuts the string, the weight falls, and what was just now only 

 attraction, has become actual force; this force he uses for his own 

 purposes, and hands back the oxydized matter, the fallen weight, to 

 the plant to be again de-oxydized ; to be again raised. The plant then 

 is to be regarded as a machine for converting sunlight into potential 

 energy ; the animal, a machine for setting the potential energy free 

 as actual, and economizing it. The force which the plant stores up 

 is undeniably physical ; must not the force which the animal sets 

 free by its conversion, be intimately correlated to it? 



But approaching our question still more closely, let us, in illustra- 

 tion of the vital forces of the animal economy, choose three forms of 

 its manifestation in which to seek for the evidences of correlation ; 

 these shall be heat, evolved within the body ; muscular energy or 

 motion ; and lastly, nervous energy, or that form of force which, on 

 the one hand, stimulates a muscle to contract, and on the other, 

 appears in forms called mental. 



The heat which is produced by the living body is obviously of the 

 same nature as heat from any other source ; it is recognized by the 

 same tests, and may be applied for the same purposes. As to its 

 origin, it is evident, that since potential energy exists in the food 

 which enters the body, and is there converted into force, a portion of 

 it may become the actual energy of heat. And since, too, the heat 

 produced in the T3ody is ])recisely such as would be set free by the 

 combustion of this food outside of it, it is fair to assume that it thus 

 originates. To this may be added the chemical argument that while 

 food capable of yielding heat by combustion is taken into the body, 

 its constituents arc completely or almost com})lete]y, oxydized before 



