The Nature of Animal Life. 1 7 



of an old-fashioned eight-day clock. Some of the 

 energy of my arm is converted into the potential energy of 

 the weight ; that is, the raised weight is now in a position 

 of advantage, and capable of doing work. It has energy of 

 position, or potential energy. If the chain breaks, down falls 

 the weight, and exhibits the energy of motion. But, under 

 ordinary circumstances, this potential energy is utilized in 

 giving a succession of little pushes to the pendulum to 

 keep up its swing, and in overcoming the friction of the 

 works. Again, the energy of an electric current may be 

 utilized in decomposing water, and tearing asunder the 

 oxygen and hydrogen of which it is composed. The 

 oxygen and hydrogen now have potential energy, and, if 

 they be allowed to combine, this will manifest itself as the 

 light and heat of the explosion. These examples will 

 serve to illustrate the nature of the changes which energy 

 undergoes. These are of the nature of transferences of 

 energy from one body to another, and of transformations 

 from one mode or manifestation to another. The most 

 important point that has been established during this 

 century with regard to energy is that, throughout all its 

 transferences and transformations, it can be neither created 

 nor destroyed. But there is another point of great im- 

 portance. Transformations of energy take place more 

 readily in certain directions than in others. And there is 

 always a tendency for energy to pass from the higher or 

 more readily transformable to the lower or less readily 

 transformable forms. When, for example, energy has 

 passed to the low kinetic form of the uniformly distributed 

 molecular motion of heat, it is exceedingly difficult, or 

 practically impossible, to transform it into a higher and 

 more available form. 



Now, both animals and plants are centres of the trans- 

 formation of energy ; and in them energy, notwithstanding 

 that it is being raised to a high position of potentiality, is 

 constantly tending to be degraded to lower forms. Hence 

 the necessity of some source from which fresh stores of 

 available energy may be constantly supplied. Such a 



