OF MOVEMKNT IN ANIMALS. 



39 



what modi. i does this force pass before it becomes mechanical 



work ? 



In steam engines, heat is the necessary medium between 



the oxidation of the fuel and the developed mechanical work. 



It is ver} 7 probable that the same 



thing takes place in the muscles. 



The chemical action produced by 



the nerve within the fibre of the 



muscle disengages heat from it : 



this heat in its turn is itself 



partially transf trnu'd into work. 



We say partially, since accord- 

 ing to the second principle of 



tliermo-dynamics, heat cannot be 



entirely transformed into me- 

 chanical work. 



Certain facts seem to justify 



these views : thus, by warming 



a muscle, we change the form of 



it, and may see it contract in 



length as it expands in breadth. 



These effects disappear when the 



muscle is cooled. 



Muscular fibre is not singular 



in its power of transforming heat 



into work. India-rubber, for in- 

 stance, has an analogous property, 



and this substance may be made 



to imitate the muscular phe- 

 nomena to a certain degree. If 



we take a strip of india-rubber 



(not vulcanised), and, drawing it 



between the fingers, stretch it out 



to ten or fifteen times its original 



length, we see that it becomes 



white, and of a pearly lustre. 



will become sensibly warm, and it will tend energetically to 



return to its original condition, so that if we let go either of 



its ends, it will instantly resume its former length, and fall to 



Put. (. Transformation of heat into 

 work by a strip of iiid.a rubber. 



At the same time the strip 



