PHYSICS 65 



does not convey to us the precise situation. When we find 

 that heat, Hght, electricity, and other subjects are special 

 divisions of mechanics, we have not proven that now all the 

 phenomena of these subjects are cases of matter in motion, 

 but that the simple and assumed laws connecting force, matter, 

 and motion apply to cases other than purely mechanical cases 

 and that the concepts of work and energy originally based 

 upon cases of simple motion can be used to interpret other 

 phenomena. The fundamental laws that were true for large 

 masses are extended to the molecule, to the atom and now 

 even to the electron. Wherever we find force or energy, to 

 this is applied these laws of mechanics. 



The case of heat just cited is a good example. Heat to 

 us at first is a mere sensation, we feel it. We certainly 

 have no a priori reason for supposing that a law of mechanics 

 can be applied to it. As another example, when we extend 

 these mechanical laws and principles to radiation, to light, 

 we find that a beam of light must have momentum and must 

 therefore exert pressure on all bodies that this light falls 

 upon. It may be noted that the fact that light should have 

 momentum and pressure was predicted by Maxwell nearly 

 30 years before its discovery. Not only was he able to pre- 

 dict the effect, but to actually compute the magnitude of it. 

 This result too was obtained by this generalization of the 

 principles and laws of mechanics. 



Sometimes in extending the scope Of a fundamental 

 concept we find something which effects the concept itself. 

 As a result of the applications of the concepts of energy and 

 work to heat, Vv^e find an additional property of energy, often 

 called the "dissipation and degradation of energy." The 

 application of mechanical principles showed that heat is a 

 form of energy, and the second law of thermodynamics states 

 that by no means can heat be converted into work unless the 

 heat passes from a body at a higher temperature to another 

 at a lower temperature. Since all activities tend to produce 



