844 Prof. L. T. More on the Postulates and 



the disembodied energy of radiation in vacuo. In the past, 

 we have created many kinds of aether and many forms of 

 radiant energy , and we tried to endue them with some likeness 

 to ponderable matter ; now, it is proposed to make matter 

 with a remarkable likeness to a vacuum. 



In our ambition to subordinate matter to light and electriciiy, 

 we have forgotten that explanations of heat are likely, at 

 some time, to be just as importunate. According to the 

 kinetic theory of heat, temperature is the kinetic energy of 

 small masses composing the system. As yet no one has 

 announced that the temperature of a man rises or falls 

 according as he runs faster or slower. According to the 

 theory of relativity, his mass and space dimensions must 

 be modified to account for this fluctuation in temperature. 

 If it be argued that " this modification affects only the laws 

 for rapid motions," we may cite the case of a moving system 

 of bullets, in a battle, whose velocity is quite comparable to 

 the velocities of heat molecules in a gas at ordinary tem- 

 perature. Thus in our mechanical explanation of heat energy 

 we wisely leave the laws of mechanics of bodies of a per- 

 ceptible size unchanged and create a heat molecule whjse 

 kinetic energy is not mechanical but thermal. Would it not 

 be advisable to follow the same plan for electro-kinetics and 

 radiant energy and create a mechanical model of an electro- 

 optical molecule whose dimensions are subject to the Lorentz- 

 FitzGerald modification, and leave the mechanics of pon- 

 derable bodies as they are ? Besides the modification in 

 mechanical laws which, Einstein claims, must be made to 

 explain optics and electrodynamics, he applies the concepts 

 of relativity to the motion of stars and claims that here, too, 

 the change is necessary. This criticism is on a different 

 footing, for we are now dealing with pure mechanical 

 problems. The discussion of celestial mechanics will be 

 taken up later in the paper. 



We are being misled by our desire to unite formally the 

 different branches of physics by a single mechanical ex- 

 pression into the idea that radiation and heat and mechanics 

 are one in essence because they have a mutually convertible 

 attribute, energy. Newton, undoubtedly, bases mechanics 

 on the law of inertia, and no amount of argument derived 

 from phenomena of light and electricity will alter our con- 

 viction that it is a satisfactory and an adequate postulate. 

 But the law of action and reaction is as necessary as the law 

 of inertia. Mechanics becomes as shifting sand unless we 

 can reduce phenomena to a balanced or static system of 

 reactions equal to, and simultaneous with, actions. If 



