94 THE PATH OF SCIENCE 



Thus evolved the idea that heat is a mode of motion, the 

 motion of the molecules; that a hot body is one in which the 

 molecules are moving energetically; and that the latent heat 

 of evaporation of water is the energy absorbed in giving 

 rapid motion to the molecules leaving the liquid surface. 



As a result of the work of Nicolas Carnot on the theory 

 of the steam engine and of Julius Mayer and James Joule 

 on the transformation of mechanical work into heat, the law 

 of the conservation of energy was enunciated, often known 

 as the first law of thermodynamics: "Energy can neither be 

 created nor destroyed, but it may be changed from one form 

 to another." 



This principle, simple as it seems, has been one of the chief 

 guiding principles of physics ever since it was first stated. 

 Motion, heat, light, and electricity— all are forms of energy, 

 and they can be transformed into each other. Indeed, the 

 science of physics deals primarily with this transformation. 

 With the discovery by Einstein that mass and energy also 

 are interchangeable, that the motion of a particle involves 

 a change in its mass— a change that becomes great only when 

 its velocity approaches that of light— and, still more impor- 

 tant, that the destruction of mass liberates enormous quan- 

 tities of energy, the understanding of the transformations of 

 energy became a knowledge of the physical laws of the uni- 

 verse. The great principle that governs transformation of 

 energy is the second law of thermodynamics: In those trans- 

 formations, energy loses potential. Heat, for instance, can- 

 not of itself pass from a colder to a warmer body. Mechani- 

 cal effect cannot be derived by cooling matter below the 

 temperature of the coldest surrounding objects. The tend- 

 ency of energy transformations is to diminish the difference 

 in energy levels. The quantity of energy transferred, divided 

 by the temperature, is called the entropy. And the second 

 law of thermodynamics can be stated in the terms that the 

 entropy of any closed system tends to increase. In mechanics 

 and electricity the potential always decreases if no outside 

 energy is added. A transformation in which the entropy 



