616 report— 1884. 



as according to Boscovich, endowed with forces of mutual positive and jiegatn > 

 attraction, varying according to some definite function of the distance, we cannot 

 avoid the question of impacts, and of vibrations and rotations of the molecules 

 resulting from impacts, and we must look distinctly on each molecule as being 

 either a little elastic solid, or a configuration of motion in a continuous all-pervad- 

 ing liquid. I do not myself see how we can ever permanently rest anywhere short 

 of this last view ; but it would be a very pleasant temporary resting-place on the 

 way to it, if we could, as it were, make a mechanical model of a gas out of little 

 pieces of round perfectly elastic solid matter, flying about through the space 

 occupied by the gas, and colliding with one another and against the sides of the 

 containing vessel. This is, in fact, all we have of the kinetic theory of gases up to 

 the present time, and this lias done for us, in the hands of Clausius and Maxwell, 

 the great things which constitute our first step towards a molecular theory of 

 matter. Of course from it we should have to go on to find an explanation of the 

 elasticity and all the other properties of the molecules themselves, a subject vastly 

 more complex and difficult than the gaseous properties, for the explanation of 

 which we assume the elastic molecule ; but without any explanation of the pro- 

 perties of the molecule itself, with merely the assumption that the molecule has the 

 requisite properties, we might rest happy for awhile in the contemplation of the kinetic 

 theory of gases, and its explanation of the gaseous properties, which is uot only 

 stupendously important as a step towards a more thorough-going theory of matter, 

 but is undoubtedly the expression of a perfectly intelligible and definite set of 

 facts in nature. But alas for our mechanical model consisting of the cloud of 

 little elastic solids flying about amongst 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 of 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 iu 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 1 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 dis- 

 cover 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 



