594 CHARLES R. VAN HISE 



observed. From such observations the existence of energy is inferred. 

 Wherever forces act upon matter and work is done, energy must exist. 

 Further reasoning shows us that bodies may possess energy which is 

 latent and is not exerting force. Hence many physicists have defined 

 "energy" without introducing the words " force " or "work." Thus, 

 according to Holman, " energy" is power to change the state of motion 

 of a body.'^ If energy be recognized as the primary thing, then 

 "force" can be defined in terms of energy. According to Holman, 

 "force" is that action of energy by which it produces a tendency to 

 change the state of motion of bodies.^ Similarly, the word "energy" 

 may be introduced into the definition of the word work. Thus 

 Holman says "work" is that action of energy by which it produces 

 motion in a free body, or produces or maintains the motion of a body 

 against resisting forces. ^ 



An "agent" is any portion of the substance of the earth which 

 may exert force and thus expend energy to perform geological work. 

 Thus ether, air, water, and rock are agents. 



The next step in the comprehension of geological processes is a 

 consideration of the kinds of energies, forces, and agents, and their 

 relations. 



Kinds of energy and force. — Ultimately the forms of energy may 

 be reduced to a few, and possibly to a single kind. Indeed, some 

 physicists believe that all forms of energy are really but different 

 manifestations of kinetic energy. But the number of elementary 

 kinds of energy in the universe is a problem for the physical philoso- 

 pher, not the geologist. The geologist is concerned in all the kinds of 

 energy which he observes at work. These are: (i) gravitation energy, 

 (2) heat, (3) elasticity energy, (4) cohesion energy, (5) chemical energy, 

 (6) electrical energy, (7) magnetic energy, (8) radiant energy (including 

 radiant heat, radiant light, and electro-magnetic radiation). ^ 



From another point of view energy may be classified into kinetic 

 energy and potential energy. Under static conditions of all the 

 parts of a system any or all of the kinds of energy above named may 

 be exerting force, but so long as no motion occurs and no work is 

 done they are all potential. When anywhere in the system move- 



1 Silas W. Holman, Matter, Energy, Force and Work, 1898, p. 20. 



2 Ibid., p. 41. 3 Ibid., p. 17. 4 Ibid., p 37. 



