12 THE BEGINNINGS OF LIFE. 



of the vanes of the wheel against the various fluids 

 was ascertained with the greatest care. The uniform 

 results obtained in these experiments enabled Mr. Joule 

 most satisfactorily to establish the mechanical equiva- 

 lent of what has been termed the unit of heat. He 

 found that the energy of a body weighing one pound 

 which had fallen from a height of 772 feet was exactly 

 equal to the quantity of molecular motion or heat 

 which suffices to raise the temperature of one pound of 

 water by one degree of the Fahrenheit scale 1 . 



It is needless for us to follow further the ultimate 

 developments of this doctrine with which the names 

 of Clausius, Rankine, Thomson., and Helmholtz are 

 associated. We have called attention to the experi- 

 ments and reasonings by which it has been shown that 

 an exact relation of equivalence exists between the 

 motion of masses produced by mechanical force, and 

 the motion of the particles of bodies manifesting itself 

 as heat produced by friction. Heat, therefore, has been 

 indubitably established to be a c mode of motion; 3 and 

 there is the very best reason for believing that all the 

 other forces or affections of matter are similarly re- 

 lated to motion, whilst they are also mutually con- 

 vertible. Each alike may arise from, or may give origin 

 to motion either directly or indirectly. 



1 The ' unit of heat ' therefore, or that amount of heat which will 

 raise a pound of water i Fahr., is equal to 772 'foot-pounds,' if we 

 call the actual energy of a body weighing one pound which has fallen 

 one foot, ^.foot-pound. 



