ABSORPTION AND EMISSION CENTERS. 3 



tion take place. During these reactions it is possible that the atoms may exist 

 in the so-called "nascent " condition, and their electrons may then be subjected 

 to very little, if any, damping. In such reactions it might be expected that 

 the gas would be greatly ionized. This, however, is not the case, as was shown 

 by Henry 1 and Whiddington. 2 



Several workers, including Galitzin and Wilip, 3 have studied the absorp- 

 tion spectra of bromine at different temperatures and pressures. As the tem- 

 perature is raised the fine band absorption spectrum changes, becomes more and 

 more indistinct, and finally disappears at high temperatures. Evans 4 has 

 shown that the temperature of the disappearance of the absorption bands is 

 higher the greater the pressure. The disappearance of the absorption lines 

 is closely connected with the dissociation of the diatomic bromine molecules, 

 and the disappearance of absorption at high temperatures can be explained 

 by assuming that the monatomic molecules have no absorption between X 3500 

 and X 6800. 



Heated bromine and iodine vapors give an emission spectrum that coin- 

 cides with the absorption banded spectrum. Evans believes that the absorp- 

 tion spectra disappear as soon as the emission due to the dissociation and 

 recombination of the diatomic molecules is equal to the absorption of the 

 undissociated state. Evans gives the following table: 



1 Spectrum still present at 1320 C. 



- Tube becomes opaque, collapsing at 1400 C. 



Sulphur Bands. 



Graham 5 has investigated the absorption spectra of sulphur vapor between 

 530 and 900 C, the pressure being in general about 10 mm. of mercury. 

 Above 580 C. the dissociation is considered to be from S 8 to S 2 . At or below 

 520 C, Graham believes that there are intermediate compounds formed 

 between S 8 and S 2 . The following are the wave-lengths of some of the sul- 

 phur bands: 



1 Proc. Camb. Phil. Soc, 9, 319 (1897). 



2 Ibid., 15, 189 (1909). 



3 Memoires de 1'Academie des Sciences de St.-Petersbourg, 17, 1-112 (1906). 

 Astrophys. Journ., 32, 2S1 (1910). 



6 Proc. Roy. Soc, A, 84, 311 (1910). 



