614 report — 1884. 



by the happy discovery of Tait and Dewar, 1 that the length of the free path of the 

 residual molecules of air in a good modern vacuum may amount to several 

 inches. Clausius' and Maxwell's explanations of the diffusion of gases, and of 

 thermal conduction in gases, their charmingly intelligible conclusion that in gases 

 the diffusion of heat is just a little more rapid than the diffusion of molecules, 

 because of the interchange of energy in collisions between molecules, 2 while the 

 chief transference of heat is by actual transport of the molecules themselves ; and 

 Maxwell's explanation of the viscosity of gases, with the absolute numerical 

 relations which the work of those two great discoverers found among the three 

 properties of diffusion, thermal conduction, and viscosity ; have annexed to the 

 domain of science a vast and ever-growing province. 



Rich as it is in practical results, the kinetic theory of gases, as hitherto 

 developed, stops absolutely short at the atom or molecule, and gives not even a 

 suggestion towards explaining the properties in virtue of which the atoms or 

 molecules mutually influence one another. For some guidance towards a deeper 

 and more comprehensive theory of matter, we may look back with advantage to 

 the end of last century and the beginning of this century, and find Rumford's 

 conclusion regarding the heat generated in boring a brass gun: ' It appears to me 

 to be extremely difficult, if not quite impossible, to form any distinct idea of any- 

 thing capable of being excited and communicated in the manner the heat was 

 excited and communicated in these experiments, except it be motion/ and Davy's 

 still more suggestive statement : ' The phenomena of repulsion are not dependent 

 on a peculiar elastic fluid for their existence. . . .' ' Heat may be defined a 

 peculiar motion, probably a vibration, of the corpuscles of bodies, tending to 

 separate them. . . .' 'To distinguish this motion from others, and to signify the 

 causes of our sensations of heat, &c, the name repulsive, motion has been adopted.' 

 Here we have a most important idea. It would be somewhat a bold figure of 

 speech to say the earth and moon are kept apart by a repulsive motion ; and yet, 

 after all, what is centrifugal force but a repulsive motion, and may it not be that 

 there is no such thing as repulsion, and that it is solely by inertia that what seems 

 to be repulsion is produced ? Two bodies fly together, and, accelerated by mutual 

 attraction, if they do not precisely hit one another, they cannot but separate in 

 virtue of the inertia of their masses. So, after dashing past one another in sharply 

 concave curves round their common centre of gravity, they fly asunder again. A 

 careless onlooker might imagine they had repelled one another, and might not 

 notice the diflerence between what he actually sees and what he would see if the 

 two bodies had been projected with great velocity towards one another, and either 

 colliding and rebounding, or repelling one another into sharply convex continuous 

 curves, fly asunder again. 



Joule, Olausius, and Maxwell, and no doubt Daniel Bernoulli himself, and I 

 believe every one who has hitherto written or done anything very explicit in the 

 kinetic theory of gases, has taken the mutual action of molecules in collision as 

 repulsive. May it not after all be attractive ? This idea has never left my mind 



1 Proc. R. 8. E. March 2, 1874, and July 5. 1875. 



- On the other hand in liquids, on account of the crowdedness of the molecules, 

 the diffusion of heat must be chiefly by interchange of energies between the mole- 

 cules, and should be, as experiment proves it is, enormously more rapid than the 

 diffusion of the molecules themselves, and this again ought to be much less rapid 

 than either the material or thermal diffusivities of gases. Thus the diffusivity of 

 common salt through water was found by Fick to be as small as -0000112 square 

 centimetres per second : nearly 200 times as great as this is the diffusivity of heat 

 through water, which was found by J. T. Bottomley to be about -002 square centi- 

 metres per second. The material diffusivities of gases, according to Losclnnidt's 

 experiments, range from -098 (the interdiffusivity of carbonic acid and nitrous oxide) 

 to '642 (the interdiffusivity of carbonic oxide and hydrogen) ; while the thermal 

 diffusivities of gases, calculated according to Clausius' and Maxwell's kinetic theory 

 of gases, are -0S9 for carbonic acid, -16 for common air or other gases of nearly the 

 same density, and 1-12 for hydrogen (all, both material and thermal, being reckoned 

 in square centimetres per second). 



