260 Mr. W. T. David on Thermal 



The energy in this part of the radiation is, however, generally 

 small, for the number of negative ions is generally only a 

 small proportion of the number of molecules in the gas *.. 

 The second part of the radiation is luminescence. It is due 

 to the shaking up of the intra-atomic corpuscles by ionic 

 bombardment, and also by ionization and recombination, 

 which is continually going on as explained above. The 

 energy in this part of the radiation is also generally small. 

 The third part is the thermal radiation which is emitted if 

 the molecules possess low-frequency vibrations like those of 

 00 2 and water-vapour. The emission of this infra-red radia- 

 tion maybe due to molecular collisions or ionic bombardment 

 of the molecules. The energy in this part of the radiation 

 may be very considerable. 



Pasclien's Experiments. 



Paschen, as we mentioned above, heated C0 2 in a metal 

 tube with open ends, and measured the radiation emitted by 

 the hot gas by means of a bolometer. The arrangement of 

 his apparatus was somewhat as shown in fig. 1, but the rock- 

 salt prism used to disperse the radiation is not shown. The 



Fig. 1. 



K 



b h^^^rri - -" N_r~_n:^_p_7~_ 



\/W/,W»/.,/////////////S////M//»/.\ 



bolometer at B does not "see" any part of the hot tube, it 

 "sees" only a small cone N of the gas through the small 

 aperture at A. The emission from the gas was measured at 

 various temperatures between 150° C. and 500° C, and after 

 passing it through a rock-salt prism he found that the radia- 

 tion of wave-length 4'4yu- emitted by a thickness of 7 cm. of 

 gas was only a little below that of a black body at the same 

 temperature. He concluded that C0 2 has a pure temperature 

 emission — that the intra-molecular vibrations are excited 

 during molecular collisions. The writer doubts whether 

 this conclusion drawn from these experiments is justifiable. 



* In a metal, on the other hand, the number of free corpuscles is 

 comparable with the number of atoms it contains. 



