

Radiation by Rarefied Gases. 505 



the current-work increases continuously with diminishing 

 pressure of gas. 



5. The useful optical effect of the radiation (here given by 

 the ratio of the intensities of the radiation passing through 

 the alum plate and the total radiation respectively) is very 

 high for some of the gases at low pressure (about 90 per 

 cent, for nitrogen). But the useful optical effect of the 

 work spent is not very great (about 8 per cent, for nitrogen 

 of 0*1 millim. pressure). 



6. The intensity of total radiation must be considered as a 

 secondary effect of the discharge, and depends upon the mole- 

 cular constitution of the gas. 



7. Whatever views we hold concerning the nature of the 

 gaseous discharge, this investigation appears to confirm the 

 hypothesis of Hittorf, E. Wiedemann, and others, that the 

 radiation is not a pure function of the temperature of the 

 gases, but must be regarded as anomalous (" irregular,'* 

 " luminescence "). 



If we call " irregular " a radiation in which the spectro- 

 scopic distribution of the energy is anomalous, there are 

 certain facts observed by Prof. Angstrom which lead to the 

 conclusion that the radiation in question is irregular. The 

 radiation did not show any relation to the absorptive power 

 of the gas at ordinary temperatures. Again, the radiation, — 

 which in nitrogen at 2 millim. pressure is still rich in dark 

 rays, — rapidly changes in quality when the pressure de- 

 creases, and at 1 millim. consists almost exclusively of light 

 radiation. Prof. Angstrom supposes that the radiation of the 

 gas during electric discharge consists of two parts, one of 

 them regular, the other irregular. With decreasing pressure 

 the former decreases, whilst the irregular radiation increases in 

 proportion as the motions are less obstructed by the mass of 

 the gas. At constant pressure a certain portion of the energy 

 in each molecule is converted into radiation ; as the strength 

 of the current increases, the number of active molecules, and 

 hence also the radiation, increases in the same proportion as 

 the current. The number of active molecules being relatively 

 small, the damping effect of the rest may be taken as constant, 

 and the composition of the radiation remains practically 

 unaltered as the current increases. On increasing the pres- 

 sure, however, the damping effect changes, the anomalous 

 dispersion is more easily transformed into a normal one, and 

 the radiation becomes richer in infra-red rays. A greater 

 proportion of the energy supplied is spent in heating, and 

 for the same current-work the total radiation decreases with 

 increasing pressure. 



Phil. Mag. S. 5. Vol. 35. No. 217. June 1893. 2 N 



