208 REPORTS ON THE STATE OP SCIENCE. 



instance by combustion or by compression; the assumption that this 

 is the case is involved in any statement of volumetric heat as a definite 

 physical quantity. The pressure energy in the final state of equilibrium 

 is certainly shared equally between the different kinds of molecules, but 

 the atomic energy is not necessarily equally shared. It is known, for 

 example, that the steam molecules, after an explosion of hydrogen and 

 air, carry, on the average, more energy than the nitrogen molecules, 

 though the pressure energy is the same. 



The process of attaining equilibrium after an explosion, which has 

 just been described, would (if heat loss were arrested) result in n 

 rise of temperature, and in the ordinary case of rapid cooling it would 

 retard the cooling. It would, therefore, be indistinguishable as regards 

 pressure or temperature effects from continued combustion or after- 

 burning. 



Stated in terms of the molecular theory, the first question as to 

 which there is difference of opinion is whether the radiation from a 

 flame arises from gas which is in equilibrium or whether it comes from 

 molecules which still possess a larger share than tiiey will ultimately 

 (in the equilibrium state) be entitled to, of the atomic energy which 

 resulted from their formation. If the products of combustion of a non- 

 luminous Bunsen flame were heated — say, by passing through a hot 

 tube — to the average temperature of the flame, would they emit 

 substantially the same amount of radiation? In order to clear the 

 ground for the discussion of this question it will be convenient, first, 

 to state two or three points about which there will probably be general 

 agreement. First, there is here no question of the origin of luminosity, 

 for the luminous part of the radiation from the flame possesses practi- 

 cally no energy. Secondly, the radiation, whether in the heated gas or 

 in the flame, arises almost entirely from the compound constituents 

 C0 2 and H o 0; in neither case does any come from the molecules of 

 nitrogen or of excess oxygen. And, thirdly, the powerful absorption of 

 cold C0 2 for the radiation from a CO flame, and of water vapour for 

 that from a hydrogen flame, will probably lead all to admit that these 

 gases when heated will emit some radiation of the same type. The 

 only question is, how much? 



E. von Helmholtz was of opinion that the radiation in a flame comes 

 mainly from molecules which have just been formed, and which are 

 therefore still in a state of vigorous vibration. Pringsheim, Smithells, 

 and others take the same view. This is practically equivalent to saying 

 that this radiation, like the radiation of higher frequency which gives 

 luminosity, is due to chemical action and not to purely thermal causes. 

 On the other hand, Paschen and some others have maintained that 

 the radiation from a flame is purely thermal, or that it arises from gas 

 which has attained the normal or equilibrium state and is substantially 

 the same as that which would be emitted if the products of combustion 

 were heated. 



It will readily be seen that the difference between the two opinions 

 really turns on the question of the time taken by a gas which is not 

 initially in, or has been disturbed from, the equilibrium state to attain 

 that state. All will concede that the CO, or steam molecule will 



