the Radiation of Heated Gases. 465 



Arsenic. — The vapour of this substance glows distinctly 

 when heated in nitrogen. In hydrogen also the glow is dis- 

 tinct, but fainter than in nitrogen. 



To sum up, then, it appears that besides iodine, the vapours 

 of bromine, chlorine, sulphur, selenium, and arsenic can all 

 be made more or less incandescent by heating to the tempe- 

 rature at which glass combustion-tube softens, and the light 

 emitted by each of these glowing vapours appears to give a 

 perfectly continuous spectrum ; whilst the corresponding- 

 absorption spectra are selective. Thus there is no such close 

 relation between emission and absorption as is implied by 

 KirchhofFs law of radiating bodies. There seems, however, 

 to be a general relation between the total absorbing and 

 radiating power for the visible rays : those vapours which are 

 highly coloured and absorb strongly in the visible spectrum 

 also radiate conspicuously in that part of the spectrum, whilst 

 colourless non-absorbing vapours, such as phosphorus, emit 

 no perceptible light when heated. 



That the glowing in these cases in no way differs from the 

 glowing of heated solids seems, to say the least, extremely 

 probable, for there is no evidence whatever that chemical 

 changes accompany the luminosity; and there is besides the 

 fact that when direct combination does occur between the 

 vapour and the gas in which it is heated, as in the case of 

 iodine in hydrogen, and possibly also arsenic in hydrogen, 

 there is no luminous effect at all. 



It may be questioned, however, whether molecular disso- 

 ciation may not be concerned in the radiation, or alternate 

 dissociation and reaggregation of the atoms of the molecules. 

 For, according to the kinetic theory, at a given temperature 

 and pressure the vapours may contain a certain proportion of 

 free atoms distributed among the more complex molecular 

 groups, but the individuality of these uncombined atoms will 

 continually change whilst the proportion remains the same, 

 for there will be a constant reaction or interchange going on 

 between the atoms and the molecules. The emission of light 

 may be supposed to depend on this act of union or disunion 

 of the atoms, the radiant energy being indirectly derived from 

 the heat supplied to the system to maintain the temperature. 



Thus in the case of the diatomic gases iodine, bromine, 

 and chlorine, a proportion of the molecules I 2 , Br 2 , Cl 2 may 

 dissociate into 21, 2Br, 2C1, and sulphur vapour may similarly 

 dissociate from S 6 to 3S 2 , and so on. From recent determi- 

 nations of the vapour densities of the halogens, it appears 

 that iodine begins to dissociate between 600° and 700° 0., at 

 a pressure of 1 atmosphere *. Chlorine, on the other hand, 

 * Crafts and Meier, Ber. deut. chem. Ges. xiii. 



