29 MAXWELL'S LAW 61 



means if we consider the gases, not as already mixed,. but 

 as initially separated. And here we employ a law that is 

 the result of experiment, viz. that if two gases of the same 

 temperature are mixed with each other, the temperature 

 remains unchanged ; and we make an assertion which is 

 little likely to meet with opposition, viz. that if two gases 

 whose molecules have the same mean value for the energy of 

 their to-and-fro motion are mixed with each other, the mean 

 energy of both kinds of molecules will remain unchanged. 

 That the total amount of energy in the mixture remains 

 unchanged follows from the law of the conservation of 

 energy. An inequality of the mean energy on mixture 

 could therefore only arise if one of the gases suffered 

 diminution of its energy below the mean value in order to 

 raise that of the other above that amount, which would be 

 very improbable. This analogy would therefore justify us 

 in assuming also for unmixed gases the proposition first 

 laid down by Clausius, 1 that two gases have the same 

 temperature when the mean energy of r rectilinear motion of 

 the molecules is the same for both gases. 



It is not, perhaps, superfluous to observe that the law 

 laid down for the state of a gaseous mixture applies only to 

 the condition of equilibrium. Hence those cases are ex- 

 cluded in which the mixture of gases is accompanied by 

 chemical reactions. The law of the equality of molecular 

 energy need not therefore be applicable to such gaseous 

 mixtures as chemically combine in a flame ; even if the 

 original two gases which combine in the flame possessed 

 equal temperatures, it is still possible, and indeed prob- 

 able, that the molecules of the products of combustion 

 possess a far higher temperature and greater energy than 

 the molecules of the primitive gases while uncombined, as 

 Srnrth^lLs_^jbS^umes. "^ thermometer which is held in a 

 flame would then indicate, not the temperature of com- 

 bustion, but the mean temperature of the burnt and 

 unburnt gases existing in the flame. 



1 Pogg. Ann. c. 1857, p. 370 ; Abhandl. 2. Abth. 1867, p. 247 ; transl. Phil. 

 Mag. [4] xiv. 1857, p. 108. 



2 Phil. Mag. [5] xxxvii. 1894, p. 245. 



