S2 (16) 



ergy, which at any time may become actual as force. 

 This food the animal now appropriates ; 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 

 oxidized matter, the fallen weight, to the plant to be 

 again de-oxidized, to be again raised. The plant then 

 is to 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 con- 

 version, be intimately correlated to it ? 



But approaching our question still more closely, let 

 us, in illustration of the vital forces of the animal econ- 

 omy, 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 mo- 

 tion ; 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 ob- 

 viously 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 body is precisely such as 

 would be set free by the combustion of this food out- 

 side of it, it is fair to assume that it thus originates. To 



