614 



REPORT — 1884. 



i! 



t!li 



by tho happy discovery of Tail and Dewar,' that the loofrtli of flie fivo path of tlin 

 residual inolociiles of air in a jjood raodern vacuum may amount to .several 

 inches. (!lawsius' and Maxwcirs explanations of the dillusion of gases, and of 

 thermal conduction in pases, tlieir charniiuf^ly intcllifrihlo conclusion tliat in frascs 

 the dilfusion of hent is just a little more rapid than the dilfusion of molecules, 

 because of tho interclianjjre of eneriry in collisions between molecules,- whih- the 

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

 Maxwell's explanation of the viscosity ol jrases, with the absolute nuniericid 

 relBtiims which tho work of those two frreat discoverers found among tho tlu'ie 

 properties of diilusioM, thernial conduction, and viscosity; have annexed to tlie 

 domain of science a vast and ever-growinir province. 



Uich as it is in practical results, the kinetic theory of gases, as hilherlo 

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

 suggestion towards explaining the ])roperties in virtue of which the atoms or 

 molecules mutually intluence 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 lind liumford's 

 conclusion regarding the heat generated in boring a brass gun: * It appears to nie 

 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 whs 

 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 dehned ii 

 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, Sec, 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 tliat 

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

 to be repulsion is produced ? Two bodies ily 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 hod repelled one another, and might not 

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

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

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

 curves, fly asunder again. 



Joule, Clausius, and Maxwell, and no doubt Daniel Bernoulli himself, and 1 

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

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

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



' Proc. R. S. E. March 2, 1874, and July 5. \^in. 



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

 the diffusion of lieat must be chiefly by interchange of energies between tlie 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 tho material or thermal diffusivities of gases. Thus the diffusivity oC 

 common salt through water was found by Fick to bo 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. I'ottomley to be about -002 square centi- 

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

 experiments, range from '098 (the interdiffusivityof carbonic acid and nitrous oxide) 

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

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

 of gases, are "OSO for carbonic acid, 'Ifi 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). 



