174 COKEELATION OF PHYSICAL FORCES. 



As by the artificial structure of a voltaic battery, chemi- 

 cal actions may be made to cooperate in a definite direction, 

 so, by the organism of a vegetable or animal, the mode of 

 motion which constitutes heat, light, &c., may, without extra- 

 vagance, be conceived to be appropriated and changed into 

 the forces which induce the absorption, and assimilation of 

 nutriment, and into nervous agency and muscular power. 

 Indications of similar thoughts may be detected in the writings 

 of Liebig. 



Some difficulty in studying the correlations of vital with 

 inorganic physical forces arises from the effects of sensation 

 and consciousness, presenting a similar confusion to that 

 alluded to, when, in treating of heat, I ventured to suggest, 

 that observers are too apt to confound the sensations with the 

 phenomena. Thus, to apply some of the considerations on 

 force, given in the introductory portion of this essay, to cases 

 where vitality or consciousness intervenes. When a weight 

 is raised by the hand, there should, according to the doctrine 

 of non-creation of force, have been somewhere an expenditure 

 equivalent to the amount of gravitation overcome in raising 

 the weight. That there is expenditure we can prove, though 

 in the present state of science we cannot measure it. Thus, 

 prolong the effort, raise weights for an hour or two, the vital 

 powers sink, food, i. e. fresh chemical force, is required to 

 supply the exhaustion. If this supply is withheld and the 

 exertion is continued, we see the consumption of force in 

 the supervening weakness and emaciation of the body. 



The consciousness of effort, which has formed a topic of 

 argument by some writers when treating of force, and is by 

 them believed to be that which has originated the idea of 

 force, may by the physical student be regarded as feeling is 

 in the phenomena of heat and cold, viz. a sensation of the 

 struggle of opposing molecular motions in overcoming the 

 resistance of the masses to be moved. When we say we feel 

 hot, we feel cold, we feel that we are exerting ourselves, our 



