

ON GASEOUS EXPLOSIONS. 207 



amounts to nearly 3 calories per gramme molecule, or to 12 foot-pounds 

 per cubic foot per degree Centigrade. 



The other part of the energy produces no external physical effect 

 except radiation, and at ordinary temperatures, when there is no radia- 

 tion, its existence and amount are inferred from the fact that when 

 work is done or heat put into the gas the corresponding increase in 

 pressure energy amounts to only a fraction of the whole. The internal 

 motions to which this suppressed energy corresponds may be pictured 

 as of a mechanical nature, such as the vibrations of spring-connected 

 masses or as rotation about the centre of gravity of the molecule, but 

 there is not the same reason as exists in the case of the transitional 

 energy for supposing that they are really of this character. They may 

 be, and indeed probably are, electrical phenomena, at any rate in part. 

 Any radiation from the gas must take its origin in this internal motion, 

 and so much of that motion as gives rise to radiation must be of a 

 periodic character and have a frequency equal to that of the radiation 

 emitted. It will be convenient to call the whole energy which is internal 

 to the molecule 'atomic energy,' and that part of it which gives rise 

 to radiation may be called ' vibrational energy. ' The vibrational energy 

 may be imagined as due to high-frequency vibrations within the mole- 

 cule, and the rest of the atomic energy as due to slower movements — 

 perhaps rotations of the molecule as a whole — which do not produce 

 any disturbance in the aether. This remaining energy may conveniently 

 be called ' rotational,' it being understood that the motion to which it 

 corresponds is not necessarily a physical rotation, but is some internal 

 motion which gives no external physical effects. 



"When the gas is in a steady state the various kinds of energy 

 will bear definite ratios to one another, dependent on the temperature 

 and pressure. It may be expected, however, that after any sudden 

 change of temperature or pressure the gas will not at once reach the 

 steady state of equilibrium corresponding to the new conditions. For 

 instance, it may be that in the rapid compression of a gas the work done 

 goes at first mainly to increasing the translational energy. If in such 

 case the compression be arrested, and if there be no loss of heat, this 

 form of energy will be found in excess; and a certain time, though 

 possibly a very short time, will elapse before the excess is transformed 

 by collisions into atomic energy and the state of equilibrium attained. 

 This change would be manifestos a fall of temperature or of pressure 

 without any change of energy. 



If, on the other hand, the gas be heated by combustion, the first 

 effect is undoubtedly an increase in the energy of those molecules, 

 and of those only which have been formed as the result of the combus- 

 tion ; and it is probable that in the first instance the energy of the 

 newly formed molecules is mainly in the atomic form. Before 

 equilibrium can be attained there must be a process of adjustment, in 

 the course of which the energy of the new molecules will be shared 

 with inert molecules, e.g., the nitrogen in an air-gas explosion, 

 while the translational form of energy will increase at the expense of 

 the atomic energy. The final state of equilibrium reached will be the 

 same at the same temperature, whether the gas was heated in the first 



p 2 



