IN THE LAST HALF- CENTURY. 85 



' The total energy of any body or sys- 

 tem of bodies is a quantity which can 

 neither be increased nor diminished by 

 any mutual action of such bodies, though 

 it may be transformed into any one of the 

 forms of which energy is susceptible.' It 

 follows that energy, like matter, is inde- 

 structible and ingenerable in nature. The 

 phenomenal world, so far as it is material, 

 expresses the evolution and involution of 

 energy, its passage from the kinetic to 

 the potential condition and back again. 

 Wherever motion of matter takes place, 

 that motion is effected at the expense of 

 part of the total store of energy. 



Hence, as the phenomena exhibited by 

 living beings, in so far as they are mate- 

 rial, are all molar or molecular motions, 

 these are included under the general law. 

 A living body is a machine by which en- 

 ergy is transformed in the same sense as a 

 steam-engine is so, and all its movements, 

 molar and molecular, are to be accounted 



