But alas for our mechanical model consisting of the cloud of little elastic solids flying about among one another.  Though each particle have absolutely perfect elasticity, the end must be pretty much the same as if it were but imperfectly elastic.  The average effect of repeated and repeated mutual collisions must be to gradually convert all the translational energy into energy of shriller and shriller vibrations of the molecule.  It seems certain that each collision must have something more of energy in vibrations of very finely divided nodal parts than there was of energy in such vibrations before the impact.  The more minute this nodal subdivision, the less must be the tendency to give up part of the vibrational energy into the shape of translational energy in the course of a collision; and I think it is rigorously demonstrable that the whole translational energy must ultimately become transformed into vibrational energy of higher and higher nodal subdivisions if each molecule is a continuous elastic solid.  Let us, then, leave the kinetic theory of gases for a time with this difficulty unsolved, in the hope that we or others after us may return to it, armed with more knowledge of the properties of matter, and with sharper mathematical weapons to cut through the barrier which at present hides from us any view of the molecule itself, and of the effects other than mere change of translational motion which it experiences in collision.

To explain the elasticity of a gas was the primary object of the kinetic theory of gases.  This object is only attainable by the assumption of an elasticity more complex in character, and more difficult of explanation, than the elasticity of gases—­the elasticity of a solid.  Thus, even if the fatal fault in the theory, to which I have alluded, did not exist, and if we could be perfectly satisfied with the kinetic theory of gases founded on the collisions of elastic solid molecules, there would still be beyond it a grander theory which need not be considered a chimerical object of scientific ambition—­to explain the elasticity of solids.  But we may be stopped when we commence to look in the direction of such a theory with the cynical question, What do you mean by explaining a property of matter?  As to being stopped by any such question, all I can say is that if engineering were to be all and to end all physical science, we should perforce be content with merely finding properties of matter by observation, and using them for practical purposes.  But I am sure very few, if any, engineers are practically satisfied with so narrow a view of their noble profession.  They must and do patiently observe, and discover by observation, properties of matter and results of material combinations.  But deeper questions are always present, and always fraught with interest to the true engineer, and he will be the last to give weight to any other objection to any attempt to see below the surface of things than the practical question, Is it likely to prove