Prof. Challis on Newton's "Foundation of all Philosophy." 291 



hammer on striking the bar of iron is suddenly arrested in its 

 course, and the force which stops it is the upward resistance of 

 the superficial parts of the bar. But as this resistance does a 

 great deal of work in a very short time, the constituent atoms 

 must run into closer proximity to supply the requisite force, the 

 resistance of pressure being universally greater the greater the 

 density of the atoms. On the cessation of the shock the atoms 

 tend to return, through a succession of vibrations, to their nor- 

 mal positions; but the repetition of the blows maintains and 

 intensifies the vibrations. The vibratory motions of the atoms 

 in the sether causes vibrations of the aether, the dynamical action 

 of which is heat. (This assertion rests on my mathematical 

 theory of heat.) Heat-vibrations are always accompanied by 

 light-vibrations. (This also is a mathematical deduction.) The 

 light-vibrations become sensible to the eye when of sufficient 

 intensity. The same explanation applies, so far as regards the 

 production of heat, to the experiment of rubbing two pieces of 

 ice together. In both experiments mechanical force is employed 

 to produce the heat ; both are simple instances of the law of the 

 dynamical equivalence of heat. Necessarily, therefore, regard 

 being had to that law, heat is a mode of force. I cannot imagine 

 a reason why in drawing an inference respecting the quality of 

 heat from these facts, the word "force" should be avoided. The 

 foregoing explanation of these instances may be considered to be 

 some approximation towards an a priori theory of the law of 

 dynamical equivalence. 



Again, the generalization which Professor Thomson has based 

 on the law of the dynamical equivalence of heat, is open to 

 weighty objections both in respect to the philosophic principles 

 it involves, and the conclusions drawn from it. I have already 

 urged that a theory cannot be based upon a particular experi- 

 mental law, the province of theory being to account for a law 

 by mathematical deduction from universal and fundamental hy- 

 potheses. On this ground I object to calling the above-men- 

 tioned generalization " The Dynamical Theory of Heat," nothing 

 having hitherto been published in science to which those terms 

 properly apply except the theory of heat which I have proposed 

 in the Philosophical Magazine for March 1859. This diversity 

 of view as to the foundation of a dynamical theory of heat leads 

 to widely different results. For instance, Professors Thomson 

 and Tait deduce from " the contemplation of dynamical energy 

 and its laws of transformation in dead matter," that " all energy 

 tends ultimately to become heat, which cannot be transformed 

 into any other modification j " and consequently that "we must 

 conclude that when all the chemical and gravitation energies of 

 the universe have taken their final kinetic form, the result will 



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