20* MAXWELL'S LAW 393 



eluding the following law, first established byClausius (see 29), 

 and in taking it as definition of equality of temperature, viz. two 

 different gases are at the same temperature when the mean kinetic 

 energy of the molecules of both if mas is ine same. 



If both gases are also under the same pressure, and have there- 

 fore equal amounts of kinetic energy in unit volume, the further\k*- 

 conclusion of Avogadro's law, discussed in 31, holds good, viz.'' 

 that two different gases at the same temperature and pressure con- 

 tain in equal volumes eqiial numbers of molecules. 



21*. Polyatomic Molecules 



The foregoing considerations can, strictly speaking, claim 

 applicability only to gases whose molecules have no internal 

 motions ; for atomic motion was left out of account, and our con- 

 clusions are thus justified only for gases whose molecules consist 

 each of a single atom. 



For .sys*ep nf po|ya,tr>rm'n molecules the investigation is cer- 

 tainly jspmewhat more complicated ; but for these media too the 

 distribution of the motion among trie individual molecules may be 

 found by the same method used before, and also the law of distri- 

 bution of speeds among the constituents of the molecules, i.e. 

 among the atoms. 



Let a molecule m consist of a number of similar or different 

 atoms nil, Wz> - which, in addition to the molecular velocities 

 u, v, w, execute special atomic motions- with the velocities 

 tli, t>i, H>i, U 2 , t) 2 , tt> 2 , . The magnitudes of these last velo- 

 cities must satisfy the equations 



= S.mit, = S.nw, = S.mw, 



when the summation denoted by S is extended over all the atoms 

 forming a molecule. The equations 



cS.w = "Si.mw 



also hold good, in which a, b, c, as before, denote the velocities 

 of flow of the gas, and the summation 2 is extended over all 

 molecules. 



But the equation for the kinetic energy takes in this case an 

 essentially different shape. While the motion of the molecules 

 investigated before was not constrained by molecular forces, the 



