THE PROGRESS OF PHYSICS. 137 



tion, as in the particles of a heated bar of iron ; wheth- 

 er the potential energy be expressed in a visible ar- 

 rangement of bodies, as in the stone resting on the 

 roof-edge, or in invisible arrangements, as in the 

 mutual relations of particles in an explosive ; we sum 

 up all the different forms in the one conception of 

 energy or power. 



The convenience of this concept " Energy " to sum 

 up groups of sense-impressions is obvious, but it must 

 be borne in mind that in using the term we are simply 

 making an abstraction which proves useful in the 

 rapid discussion of the forms or modes of motion 

 which we see and measure. Clerk Maxwell said in 

 his remarkable little book Matter and Motion: " We 

 are acquainted with matter only as that which may 

 have energy communicated to it from other matter, 

 and which may in turn communicate energy to other 

 matter," and again, " Energy, on the other hand, we 

 know only as that which in all natural phenomena 

 is continually passing from one portion of matter to 

 another." But, as Karl Pearson points out, these 

 statements do not carry us far. " The only way in 

 which we can understand matter is through the en- 

 ergy which it transfers. . . . The only way to un- 

 derstand energy is through matter. Matter has been 

 defined in terms of energy, and energy again in terms 

 of matter." 



" The activity of the material universe," says Prof. 

 Oliver Lodge, " is due to, or represented by, or dis- 

 played in, the continual interchanges of energy from 

 matter to ether and back again, accompanied by its 

 transformation from the kinetic to the potential form 

 and vice versa" * 



*" Modern Views of Matter," Internal. Monthly, I. (1900), 

 p. 500. 



