Feb., 1916] Spectra of Some Halogeji Compounds 119 



focal length were waxed on to the ends to assist in focusing the 

 light, passing as a parallel beam of light through the tube, upon 

 the slit of the spectrometer. The source of illumination was a 

 Nernst glower. The tube was heated by a long gas burner 

 set between the water cooled jackets. Two chief difficulties 

 arose in the use of the apparatus; one was the condensation 

 of the vapor on the cold glass ends, the other was leakage at 

 high temperatures. Various schemes for improvement were 

 tried, but the time alloted to this portion of the work was too 

 meager to obtain satisfactory results except possibly in one case. 



Several trials were made with stannic chloride, a substance 

 which vaporizes readily at a low temperature. Heat was applied 

 rapidly and subsequently a volume of vapor was produced 

 which appeared to cut off all the light coming through the 

 spectroscope, except a broad band in the red end. By making 

 comparison with the emission spectrum for this compound, it 

 seems probable that the absorption and emission spectra 

 are complementary. 



The emission spectra, being produced under electrical 

 excitation, are due to ionization set up by some external agency. 

 Since there is no such external, source of ionization in the 

 experiments just described, it might be concluded that there 

 would be no ions present and hence, if certain lines of reasoning 

 relative to the production of absorption spectra were followed 

 out, no absorption spectra could be expected. An examination 

 of the inside of the tube, after using mercuric iodide or stannic 

 chloride for example, showed a bright metallic mirror at the 

 water-cooled ends. This proved that dissociation must have 

 occurred and therefore presumably some ionization; a consid- 

 erable number of experiments by various investigators (Rich- 

 ardson, Willows, Beattie, Schmidt, Sheard et al.) have shown 

 the existence of negative ions of iodine and bromine when their 

 salts have been heated under potential.* 



*It is of interest in this connection to cite the experiments of Lenard (Annalen 

 der Physik, 17, page 197, 190.5) made with beads of the fused salts of the alkali 

 metals supported upon platinum wires in the Bunsen flame. The emitted light 

 was found to be strongly colored, the color depending upon the metal. An exam- 

 ination of the absorption spectra of the fused salts showed that the color of the 

 transmitted light was complementary to that of the emitted light, as should 

 follow from KirchhofT's law. The salts were colorless, however, when cold 

 showing that some sort of dissociation resulted from the high temperatures, 

 metallic ions being set free which had the property of absorbing and emitting 

 radiations of the same frequency. In the case of most of the salts examined the 

 color was found to depend upon the metal, i. e., upon the cations; the borates and 

 silicates were marked exceptions, however, the color being due to the anions, the 

 nature of the metal being immaterial. 



