TUANSACTIONS OF SKCTIOX A. 



61.') 



Miict! 1 lirsl road Daw'.-' ' Uupulsivi' Moliim,' iibciit tliirty-livc yi'iirs a<ro, hut I 

 never iiindc any tliinj,' ol' i1. at all events have not done so until tip-tlay (June l(i, 

 lSri4), (if this can he said to he makinfr anytliinj,'' of it), when in end'.iav(Uirin>; to 

 ]ire])ari' the present uddre.-fS I notice that Joule's and my own old e\])erirnent.s ' on 

 the tliermal ell'ect of j^ases expundinj,' iVom a liii^h ju'essure vessel througii a porous 

 |)lu;r, proves the le.s3 dense fjas to have },'reater inti'in.-ie yW('///(«/ enerijfy than the 

 denser jras, if we asaunie the ordinary hypothesis re^ardin;;- the leniperatur- of a }.'as, 

 acciirdinfr to whieh two ^^ases are of etjual temperatures ' wiieii the liinetic enerj^ie.s 

 of tht'ir constituent molt.'cuU's are of equal averajre amounts pei' molecide. 



Think of tiie thinf,' tiuis. Innif^ine a jrreat nndtitnde of particles enclosed hy a 

 houiulary \vhi(li may he pusiieil inwards in any part all roinid at ])leasnre. Now 

 station an enjrineer corjjs of .Maxwell's army of sortin;.-' demons all round the 

 enclosure, with orders to ])ush in the houndary dili^iently everywhere, when 

 none of the hi.'sieffed troops are near, and to do nothin^r when any of them 

 are seen ajiproaehin^', and until after thev have turned ajrain inwanls. The 

 result, will he that with exactly the same siun of kinetic and potential 

 eue.yies of the same (inclosed nndtitude of particles, the thron!.c has been 

 caused to be denser. Now Joule's and my own old experimi-nts ou the ettlux 

 of air prove that if the crowd be common air, or oxyjren, or nitrogen, or 

 carbonic acid, the temperature is a little hijiher in the denser than in the ran-r 

 condition when the enernies are the same, liy the hypothesis, equality of tempera- 

 ture between two dillerent gases or two portions of the same gas at diil'erent 

 densities means equality of kinetic energies in the sami; munber of molecules of 

 the two. l'"n)ni our observations proving the temperature to be higher, it there- 

 fore follows that the potential tMiergy is smaller in tlie condensed crowd. This — 

 always, lioweviT, under protest as to the temperatiu'e hypnthesis — jjroves .some 

 degree of attraction among the molecules, but it does not prove ultimate attraction 

 between two molecules in collision, or at distances much less than the average 

 mutual di.stance of neare.-^t neighbours in the nniltitude. The C(dlisional force 

 migiit be repulsive, as generally suppo-sed hitherto, and yet attraction might predo- 

 minate ill the whole! reckoning of ditferenco between the intrinsic potential energies 

 of the more dense and less dense multitudes. It is, however, remarkable that the 

 explanation of the propagation of sound through gases, ami even of the positive 

 tiuid pressure of a gas against the sides of the containing vessel, according to the 

 kinetic theory of gases, is quite independent of the question whether the ultimate 

 collisional force is attractive or repulsive. Of course it must be understood that if 

 it is attractive, the particles must be so small that they hardly ever meet — they 

 would have to be infinitely small to 7icve7' meet — that, in fact, they meet so seldom, 

 in com])arison with the number of times their courses are turned through large 

 angles Ijy attraction, that the influence of these purely attractive collisions is 

 preponderant over that of the comparatively very rare im])acts from actual contact. 

 Thus, after all, the train of speculation suggesttnl by Davy's 'Repulsive Motion' 

 does not allow us to escape from the idea of true repulsion, does not do more than 

 let us say it is of no consequence, nor even say this with truth, because, if there 

 are ini])acts at all, the nature of the force during the impact, and the etlects of the 

 mutual ini]>acts, however rare, cannot be evaded in any attempt to realise a con- 

 ception of the kinetic theory of gases. Ami in fact, unless w(^ are satislied to 

 imagine the atoms of a gas as mathematical points endowed with inertia, and, 



' ]!cpul»lished in Sir W. Thomson's Mathi'm<ittriil and P/i'/glcal Pctjicrs, Vol. I. 

 Article Xl-lX. p. ;{81. 



- That tins is a mere hypothesis has been scarcely remarked by the founders 

 themselves, nor by almost any writer on the kinetic theorj' of gasfs. No one has 

 yet examined the ([uestion : what is the condition as re,<;ards av(n-a<,'e distribution of 

 kinetic energy, which is ultimately fultillcd by two portions of gaseous matter, 

 separate<l by a thin elastic septum which absolutely prevents intorditl'usion of matter, 

 while it allows interchange of kinetic energy by collisions accainst itself .' Indeed 

 I do not know but that the present is the viay tirst statement which has ever been 

 published of this condition of the problem of equal temperatures between two 

 gaseous masses. 



