216 H. Willy Wien on the Division of Energy in 



uncertainty in their theoretical groundwork, but have the 

 advantage that the deductions from them can be directly com- 

 pared over a very wide range with the results of experience. 

 Their confirmation or contradiction by experiment will there- 

 fore decide the question of the correctness or otherwise of the 

 hypotheses, and thus far be useful as a further development 

 of the molecular theory. 



The law that in an exhausted vessel the radiation is the 

 same as that from a black body at the same temperature as 

 the walls of the vessel, holds also if the radiating body be a 

 gas which is shut off from the vacuous space by transparent, 

 and from the exterior by reflecting walls. 



But this gas must possess a finite absorptive power for all 

 wave-lengths. There remains, however, no doubt that there 

 are gases, such as carbonic acid and water-vapour, which, by 

 mere elevation of temperature, emit heat rays'*. Strongly 

 superheated vapours may be regarded as gases, and by suit- 

 able mixing of different substances, it is possible to conceive 

 of a mixture of gases which possesses a finite absorptive 

 power for all wave-lengths. In this case one must not, how- 

 ever, consider that radiation which gases send out under the 

 influence of electrical or chemical processes. 



If one radiating body be a gas, then Maxwell's law of the 

 division of velocities will hold if we take as our basis the 

 kinetic theory of gases. The absolute temperature will be 

 proportional to the mean kinetic energy of the gaseous 

 molecule. This assumption has been rendered highly probable 

 by the labours of Clausius f and Boltzmann J, and is still 

 further supported by the researches of Helmholtz § on mono- 

 cyclic systems, according to which researches both the kinetic 

 energy and the absolute temperature have the property of 

 being the integrating denominator of the differential of the 

 added energy. 



To avoid the unecessary prolixity which would result from 

 a consideration of the different constituents of a mixture of 

 gases, let us imagine a mixture of such a kind that the 

 homogeneous radiation under consideration is sent out by one 

 only of the gases forming the mixture. 



The number of molecules whose velocity lies between v 

 and v \-dv is proportional to the quantity 



— V -L 

 v^e a2 dv, 



* Paschen, Wied. Ann. 1. p. 409 (1893). 



t Pogg. Ann. cxlii. p. 433 (1871). 



X Wien. Ber. [2] liii. p. 195 (186(5). 



§ Gesammelte Abhandlunc/en, iii. p. 119. 



