16 MORRIS LOEB 



under the play of the various forms of energy which pertain 

 to them; and these actions must be supposed to take place 

 in tridimensional space during perceptible time. 



Let us turn now to the consideration of energy. We talk 

 of kinetic energy, or energy performing work, and of poten- 

 tial energy, or energy which is capable of performing work, 

 but not yet doing so. In the case of the mechanical energy 

 of motion, the potential form involves change in time but not 

 in other obvious ways. It keeps on existing in the shape of 

 a tendency to move, attraction or the like, while the kinetic 

 form involves both a change in respect to time and to the 

 directions of space. We recognize this in the word speed, 

 or velocity, which expresses the distance traversed in com- 

 parison with the amount of time consumed. I think this 

 energy of motion to be the easiest to conceive, and conse- 

 quently the most natural to imagine as at the bottom of all 

 phenomena of energy. Certain it is that physicists are finding 

 more and more reason to believe that all the various forms 

 of energy are really only manifestations of different sorts of 

 direct motion. 



For example, we find that what we know as heat or light 

 may be simply interpreted as a manifestation of the motions 

 of the molecules. If we heat a substance, its molecules may 

 be supposed to move more rapidly in straight lines and vi- 

 brate or oscillate more rapidly. The oscillations they are able 

 to impart to neighboring particles that may be incapacitated 

 from directly taking up the rectilinear motions. If these mo- 

 tions or oscillations are very rapid, they affect our sense of 

 temperature. They may partly belong to the molecule as a 

 whole, partly to its atoms. 



All sorts of molecules may be supposed to receive the straight 

 motions in equal degree; but each molecule seems to respond 

 only to certain kinds of oscillations. So we can throw a set 



