FUNCTIONS OF THE NUTRIENTS 163 



of energy as applied to machines are as fully applicable 

 to the activities of animal life. 



238. The animal organism does not originate energy. 

 It is safe to go farther and say that the animal organism 

 does not originate energy. Among the fundamental 

 conceptions upon which all our knowledge of chemical 

 and physical laws rests is this, that energy and matter 

 are indestructible, and, moreover, that the sum total 

 of these in the universe is unchangeable. If, then, the 

 horse expends the muscular energy necessary to draw a 

 load of one ton over 10 miles of road, the equivalent of 

 this must have been supplied to his body from some out- 

 side source. He could not create it. We know that this 

 is so, and we also know it is conveyed to the animal 

 in the food. 



239. The nature of energy. A definition of energy is 

 difficult. It can be illustrated by pointing out some of 

 its manifestations. It is a common observation to see a 

 blacksmith hammer an iron rod until it is red hot. The 

 motion of the hammer-head descending with great veloc- 

 ity was suddenly arrested when it came in contact with 

 the rod. The hammer-head was driven by energy sup- 

 plied from two sources, gravity or the energy of position, 

 and energy exerted through the blacksmith's arm, that 

 is, energy supplied through oxidation in the blacksmith's 

 body. When the hammer met the iron rod on the anvil, 

 the mass motion ceased. The operating energy was not 

 annihilated, but it appeared in another form. The motion 

 of the hammer-head (kinetic energy), a mass of matter, 

 was communicated to the molecules of the iron rod, and 

 as the vibrations of the molecules increased in rapidity, 

 the rod grew hotter and hotter. Here we have another 

 manifestation of energy, viz., the motion of the molecule. 



