4 ABSORPTION SPECTRA OF SOLUTIONS. 



The work done on the absorption spectra of solutions by Jones and 

 Uhler, 1 Jones and Anderson, 2 and Jones and Strong, 3 which extended over 

 five years, and in which some 6,000 solutions were studied, all involved the 

 photographing of the various spectra. In this way the positions of the 

 various absorption lines and bands were determined. 



A question even more fundamental than the positions of the lines and 

 bands is their intensities, and the relative intensities of different parts of 

 the same band. The photographic method gave only a means of dealing 

 qualitatively with this problem. Some general idea could be gained of the 

 relative intensities of the various lines and bands on the photographic plate, 

 but these changed with the time of exposure, the intensity of the light used, 

 and with other conditions, so that we were able to learn very little about 

 the intensities of the various lines and bands by means of the photographic 

 method. 



Further, the photographic plate is sensitive only between the wave-length 

 2,000 Angstrom units and 7,600 a.u., 4 which is a comparatively small part of 

 the spectrum. It is especially important to work also into the region of the 

 infra-red. 



A method was used which dealt quantitatively with the intensities of the 

 various lines and bands. This same method, instead of being limited by 

 the above-named wave-lengths, could be used down into the infra-red to 

 wave-lengths as great as 20,000 a.u. to 30,000 a.u. Indeed, the method can 

 be used for even greater wave-lengths, if solvents can be found that are 

 transparent to the longer waves. This method involves the use of the radio- 

 micrometer. 



The description of the instrument which we built, the method of work, 

 and the results thus far obtained, will be found on pp. 29 to 93. 



^arn. Inst. Wash. Pub. 60. Amer. Chem. Journ., 37, 126, 207 (1907). 



2 Cam. Inst. Wash. Pub. 110. Amer. Chem. Journ., loc. cit. 



3 Carn. Inst. Wash. Pub. 130 and 160. Amer. Chem. Journ., loc. cit. 



throughout this paper we have employed this expression to designate Angstrom units. 



