102 Proceedings of the Royal Society of Edinburgh. [Sess. 
XII. — Molecular Energy in Gases. By Sir J. A. Ewing, 
K.C.B., F.R.S. 
(Abstract of an Address delivered to the Royal Society of Edinburgh 
by request of the Council, May 3, 1920.) 
(MS. received May 3, 1920. Read May 3, 1920.) 
Experiments on the specific heats of gases and on their infra-red emission 
and absorption have gone a good way towards showing how the molecules 
take up energy when a gas is heated, and have raised questions as to how 
far the observed facts may be explained on the basis of “ Newtonian 
dynamics,” and what particulars suggest or necessitate a resort to the 
Quantum Theory. It may be useful to state some of these questions 
without attempting categorical answers, which indeed cannot be offered 
until the physical concepts underlying the Quantum Theory have become 
more definite. 
It is now established that when diatomic and polyatomic gases are 
strongly heated the specific heats become notably greater, and that the 
increase of specific heat is associated with infra-red radiation. Both of 
these effects are conspicuous when an explosive mixture is fired in a closed 
combustion vessel or in the cylinder of a gas-engine. 
In the explosion, for instance, of a rich mixture of coal-gas and air, the 
highest temperature reached is only about 2000° C., although the heat 
developed by the explosion would be sufficient to raise the products of 
combustion to a very much higher temperature if the specific heat of the 
products remained the same as when they are cold. The experiments of 
B. Hopkinson and others show that nearly all the heat has been generated 
by the time the maximum temperature is reached. It is recognised that no 
substantial part of the effect can be ascribed to “ afterburning ” or delayed 
combustion. The observed defect of temperature is sufficiently explained by 
increase of specific heat as the gas becomes hot. The internal energy E, 
instead of being proportional to the temperature, increases at an augment- 
ing rate. This is well illustrated by data contained in the First Report of 
the British Association Committee on Gaseous Explosions (1908). A curve 
is given there for the internal energy of a gaseous mixture containing 5 per 
cent, by volume of carbon dioxide and 12 per cent, of water-vapour, the 
remainder being nearly all nitrogen. The curve is based on experimental 
results obtained by several independent observers using different methods. 
