208 Theory of a Non-Conservative Gas [CH. ix 



depend on the frequency of the particular light-waves under consideration. 

 Hence, speaking somewhat loosely, we may say that light of one frequency 

 will be emitted mainly by molecules moving with a certain velocity, light of 

 another frequency by molecules moving with another frequency, and so on. 

 Obviously violet light will in general be emitted by molecules moving faster 

 than those which emit red light. Something of this kind has been 

 noticed by Schuster and Hemsalech* in the case of the light emitted by the 

 electric arc. The conditions here are of course widely different from those 

 contemplated by our simpler theory, but the cases are sufficiently parallel for 

 a comparison to be of interest. The point which is of primary importance is 

 that our hypothesis seems capable by itself of accounting for the observations 

 of Schuster and Hemsalech : it is not necessary to introduce a further 

 hypothesis to the effect that the light-radiating molecules are in a "special 

 state." 



Light produced by heating. 



249. From the considerations just mentioned, it is clear that from our 

 hypothesis we should expect that as the temperature of a gas is increased, 

 the light first observed would be that of lowest frequency, provided of 

 course that this light is the result solely of molecular collisions. This 

 prediction cannot be verified by direct observation, for when a gas is heated 

 until it emits light, it is not possible to decide whether this light is due to 

 molecular collisions only or to chemical or electric action. But the analogous 

 phenomenon in the case of a solid is one of everyday observation, and 

 there is no reason to suppose that the behaviour of a gas would not be 

 similar. 



In connection with the radiation from heated gases, the experiments of 

 Paschen f and others must be mentioned. It has been found that when 

 certain gases (e.g. H 2 0, CO 2 ) are heated, there is an emission of rays of 

 definite wave-length, this wave-length being many times greater than that 

 of ultra-red light. 



Light produced by Electric Action. 



250. When a gas is radiating light the emission seems to be produced, 

 in almost all cases which can be examined in the laboratory, by chemical or 

 electrical action J. 



* Phil. Trans. A. cxcm., p. 189. 



t See a paper by Paschen (Wied. Ann. L. p. 409) for a summary of evidence on this question. 



J Pringsheim (Wied. Ann. XLV. p. 428) is of opinion that no light is ever produced in gases 

 by collisions alone. This view is contested by Paschen (Wied. Ann. L. p. 409) and Smithells, 

 Phil. Mag. [5], xxxvn. p. 244. See also Pringsheim, Rapports presentes au Congres International 

 de Physique, Paris (1900), p. 100. 



