158 CORRELATION OF PHYSICAL FORCES. 



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 super- 

 vening 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 

 expressions are intelligible to beings who are capable of expe- 

 riencing similar sensations ; but the physical changes accom- 

 panying these sensations are not thereby explained. Without 

 pretending to know what probably we shall never fully know, 

 the actual modus agendi of the brain, nerves, muscles, &c., we 

 may study vital as we do inorganic phenomena, both by ob- 

 servation and experiment. Thus Sir Benjamin Brodie has 

 examined the effect of respiration on animal heat by inducing 

 artificial respiration after the spinal cord has been severed ; in 

 which case he finds the animal heat declines, notwithstanding 

 the continuance of the chemical action of respiration, carbonic 

 acid being formed as usual ; but he also finds that under such 

 circumstances the struggles or muscular actions of the animal 

 are very great, and sufficient probably to account for the force 

 eliminated by the chemical action in digestion and respiration ; 

 and Liebig, by measuring the amount of chemical action in 

 digestion and respiration, and comparing it with the labour 

 performed, has to some extent established their equivalent 

 relations. 



Mr. Helmholtz has found that the chemical changes which 

 take place in muscles are greater when these are made to 

 undergo contractions than when they are in repose ; and that, 

 as would be expected, the consumption of the matter of the 

 muscle, or, in other terms, the waste of excrementitious 

 matter thrown off, is greater in the former than in the latter 

 case. 



M. Matteucci has ascertained that the muscles of recently 



