103 
1919-20.] Molecular Energy in Gases. 
Examination of the curve shows that it is well represented by the empirical 
equation 
E = 5-2 1 +0-00043 1 2 + 0-0000002* 3 , 
where the energy E is reckoned per gramme-molecule from 0° C., and t is 
the Centigrade scale temperature, or T — 273*1°. 
This gives, for the specific heat at constant volume per gramme-molecule, 
C v = 5-2 + 0-00086* + 0-0000006* 2 . 
At 2000° C., therefore, the specific heat of this mixture is 9 - 3 as against 5*2 at 
0° C., and the energy is about one-third more than it would have been, 
at the same temperature, if the specific heat had not increased. A great 
part, but by no means all, of this increase is due to the triatomic constituents 
of the mixture. In them the specific heat is much more affected by 
temperature than it is in oxygen or nitrogen. But the researches of 
Nernst and others have made it clear that the specific heat of diatomic 
gases also is increased by heating, though to a less degree. On the other 
hand, in monatomic gases (helium, argon, mercury- vapour) no increase 
whatever in the specific heat has been observed, even at temperatures 
exceeding 2000° C. 
It is also well known that much energy is radiated from a non-luminous 
flame or from an exploded gas mixture, in the form of infra-red bands, 
whose wave-lengths correspond to those of the absorption bands of the 
products of combustion. Thus a carbonic oxide flame is observed to radiate 
energy in bands whose wave-lengths have their maxima about 2*7/*, 4r3/z> 
and 14*7 /u, corresponding to the three conspicuous absorption bands of 
carbon dioxide. 
In any gas, at temperatures such as 2000° C., the short-period vibrations 
which produce the lines of the visible spectrum make no more than a 
negligible contribution to the energy. This is illustrated by the absence 
of any measurable increase in the specific heat of monatomic gases at 
high temperatures. 
These facts are obviously consistent with what is, 1 think, the generally 
accepted view that in any gas the lines of the visible spectrum are due to 
vibrations within the atom ; and that the longer-period vibrations which 
produce the infra-red bands, and do make a substantial addition to the 
energy of a hot diatomic or polyatomic gas, consist of to-and-fro movements 
of the atoms that compose the molecule. 
It is well known that the measured specific heat of a monatomic gas is 
fully accounted for, on the kinetic theory, by the energy of translation of 
