APPENDIX IV. 



ABSORPTION OF SOLIDS IN SOLUTION. 



In the ultra-violet Hartley and Dobbie (loc. cit.) have studied the 

 absorption spectra of solids dissolved in water. They found that groups 

 of chemically related compounds have similar absorption spectra. In 

 the visible spectrum we also find absorption bands, especially when the 

 solute is a colored substance. Iodine is an example of this type. But 

 in the infra-red the writer found iodine transparent beyond i.i /i. As 

 far as is known to the writer, only one other substance, sulphur in CS2, 

 has been examined far out in the infra-red. This was done by Julius, 

 who found that S had no appreciable eflPect upon the transparency of 

 the CS2. 



That a solid in solution should be transparent to infra-red radiation 

 seemed doubtful. It would indicate a resonance of small particles in 

 the optical region, as distinguished from the intramolecular resonance 

 of the solvent. 



After studying so many compounds it seemed imperative to consider 

 this question more thoroughly. It was suspected that this transparency 

 is simply due to the thin cell used and to the slight solubility of the solids. 

 For example, assuming that 0.05 gram of iodine per cubic centimeter 

 is dissolved in CS2, using a cell 0.3 mm. thick, this would be sufficient 

 to form a solid film only 0.03 mm. It seemed that the proper method 

 of answering this question would be to select a solid having a strong 

 absorption band in a region of the spectrum where the solvent has no 

 absorption bands. 



DiPHENYL. C12H10. 



The curves of carbon tetrachloride, CCI4 showed no marked bands 

 up to 6.5 fi. Accordingly this liquid was selected for a solvent. The 

 sample used with diphenyl showed the water band at 2.97 /x and a second 

 band at 4.5 fi. The saturated diphenyl solution showed an additional 

 large band at 3.25 fi, which is the characteristic band of the benzene 

 nucleus. This is of considerable interest, since diphenyl, CgHg — CgHg, 

 is a double benzene ring. The curve, fig. 129, shows that solids in solu- 

 tion do absorb heat waves, and that the selective absorption of a solid 

 in solution and that of the solvent are identical.-' It also shows that the 



*In my preliminary communication on Infra-red Absorption Spectra, Astrophys. 

 Jour., XX, p. 215, 1904, it appeared to me that there might be a difference. 

 126 



