HISTORICAL. 



II 



words, the absorption depends upon the form of the internal energy of 

 the vibrating mass, i. e., on its structure. 



LaubenthaP found for the absorption spectra of solutions of the 

 chlorides of the alkali metals that the absorption bands shift toward 

 the red with increasing atomic weights, and that the shifts are propor- 

 tional, so that the ratio between the wave-lengths of the two absorption 

 bands of each spectrum is constant for each group of metals. The 

 absorption spectra are thus brought in line with the emission spectra, 

 and also the densities and melting points, as having their origin in the 

 same fundamental cause. 



In addition to these observations we have those of Stockl- on solutions 

 of fuchsine, cyanine, and iodine, in which the maximum of the absorp- 

 tion band depends upon the solvent. This corresponds to my own 

 work on iodine in solution.^ 



To crown all this we have the " electromagnetic theory of selective 

 absorption in isotropic nonconductors " of Planck,* in which Stockl's 

 observations are applied to the question of intra-molecular resonance. 

 This is of great interest, since it is the first theoretical recognition of the 

 possible unification of the selective absorption of a solution and the 

 selective absorption of the solvent. Whether there is a distinction 

 between the two is to be discussed later on. It will be sufficient to 

 notice that in my work on iodine solutions (loc. cit.) the selective 

 absorption of iodine (solute) is lost at 7.3 /x in the infra-red, where 

 solid iodine has a large absorption band, while the band continues in the 

 visible spectrum, just as though there was a resonance of small particles 

 in the optical region which is out of tune in the infra-red. ( See, how- 

 ever. Appendix IV, on transparency of solutions in the infra-red.) 



SOLVENTS. 



In the visible spectrum the absorption of but few solvents has been 

 investigated. Schonn^ investigated methyl, ethyl, and amyl alcohol for 

 columns of liquid 1.6 to 3.7 meters, and found a shifting of the maxima 

 to the red, as follows : 



'Laubenthal: Ann. der Physik, 7, p. 851, 1902. 



^Stockl: Inaug. Diss., Tubingen, 1901. 



^Coblentz : Phys. Rev., xvi, i, 1903. 



*Planck : Sitzungsber. d. Akad. d. Wiss., Berlin. Nos. 22 and 25, 1903. 



^Schonn : Ann. der Physik (2), 6, p. 267, 1879. 



