70 CROONIAN LECTURES 



of lieat is known, the equivalent amount of 

 work which the substance can do may be 

 calculated. 



By far the greater part of the potential 

 energy or tension which goes in is ultimately 

 changed into warmth. Other modes of motion, 

 as electricity and mechanical work, take but a 

 small part of the total income. The balance- 

 sheet at present can give an idea of the form 

 which the account will ultimately take ; but it 

 cannot tell the items with any approach to 

 accuracy. 



The difficulties which surround the determi- 

 nation of these items at present may be seen 

 by looking for a few moments where our 

 knowledge now is regarding the origin of 

 muscular motion. 



According to the earliest ideas, muscular 

 motion was produced by the soul. 



According to transition ideas, the motion was 

 considered to be created by a definite some- 

 thing, a vital force or fluid, capable of being 

 increased or diminished according as stimulants 

 (nervous or other) acted on it or not ; and 

 finally, capable of quitting the matter of the 

 muscle a short time after the body was dead. 



