28 MAXWELL'S LAW 59 



I could with little trouble have enlarged this table very 

 considerably, since the tables of densities very carefully 

 compiled byPoggendorff, 1 as well as the fuller tables of 

 Boedeker 2 and those prepared by Trau be, 3 would have 

 furnished ample materials. But I fear that I have already 

 included rather too much than too little in the above table. 



For the calculated numbers can claim exactness only 

 for those bodies to which the theory may be applied 

 without hesitation, and thus, strictly speaking, only to 

 gases which conform to Boyle's law. But a series of 

 gases and vapours have been introduced into the table 

 which do not obey this law, at least at the temperature 

 for which the calculations have been made ; some, indeed, 

 cannot exist in the gaseous state at 0. For these bodies 

 the calculated numbers possess no directly real meaning, 

 but they may be used to calculate the real values of the 

 molecular speed at a higher temperature , at which 

 Boyle's law does hold, by simple multiplication by the 

 factor -v/(l + a9), where a = O00367 (see 14). Instead^ 

 of making the calculation for these higher temperatures, 

 which would have had to be different for different bodies, 

 I have preferred to refer all numbers to one and the same 

 temperature, so as to make them comparable with each 

 other. 



29. Equality of Temperature of Different G-ases /fi~ 



Just as in a simple gas which is in equilibrium and at 

 rest with respect to external bodies the state of motion of 

 the molecules is subject to a definite law of distribution of 

 speeds, so the distribution of energy in a mixture of two or / 

 more gases must also be equalised among the different kinds ^/ 

 of molecules in a way regulated by law. 



The law which regulates this more general case was also 

 discovered by Maxwell, and a strict proof of it was given 

 by Boltzmann. The calculation 4 showed that it is the 



1 Pogg. Ann. xlix. 1840, p. 424. 



2 Boedeker, Die gesetzmdssigen Beziehungen zwischen der Dichtigkeit, 

 der specifischen Warme und der Zusammensetzung der Gase, Gottingen 1857. 



3 Landolt and Bernstein's Tables, 1894, 2nd ed. 



4 See 20* of the Mathematical Appendices. 



