6 ABSORPTION SPECTRA OF SOLUTIONS. 



law, which states that the effect of the solvent upon absorption is simply 

 to shift the position of the absorption bands, these being located nearer 

 the region of long wave-lengths the greater the dispersion of the solvent, 

 seems to imply that the molecule of the dissolved substance exists in the 

 free state in solutions. If this view be taken, then the effect of the sol- 

 vent should not be very great, especially if solutions in solvents having 

 similar optical properties are compared. If, on the other hand, a salt 

 when it goes into solution always forms some sort of a compound with 

 the solvent, we might expect to find radical differences in the absorption. 

 It is evident that a study of the absorption spectra of a salt when 

 dissolved in various solvents ought to bring out many points of interest 

 as bearing upon the question of the nature of solutions. 



APPARATUS. 



For visual examination of solutions a small, direct-vision, grating, 

 pocket spectroscope was always at hand, and was found very useful for 

 the purpose of examining solutions in order to determine what particular 

 range of concentrations it was desirable to work with. Judging from the 

 color of the solution as seen by the unaided eye was found to be very 

 unsatisfactory, since many solutions have very wide absorption bands 

 which may give the solution quite a decided tint, even when the absorp- 

 tion is so feeble that it would be almost impossible to obtain a satisfactory 

 photographic record of it. On the other hand, when solutions have nar- 

 row, intense absorption bands, like the salts of the rare earth metals, the 

 solution may show practically no color to the unaided eye, and still the 

 absorption bands may be quite intense as seen in the spectroscope. 



For photographing the spectra, the vertical grating spectrograph used 

 by Jones and Uhler was employed. In its original form as used by them 

 films 2.5 by 7 inches were employed, on which the spectrum from A 2000 

 to A 6300 could be registered. 



In the present work it was decided to remodel it so as to allow the 

 whole spectrum from h 2000 to A 7600 to be photographed, and accord- 

 ingly the camera and the camera end of the box were enlarged so as to 

 hold films 2.5 by 9 inches. Owing to the fact that the grating has only 

 10,000 lines to the inch, it was not possible to add the extra 2 inches to 

 the red end of the camera, which, if it could have been done, would have 

 left the grating-axis very near the middle of the spectrum; the 2 inches 

 being actually added to the ultra-violet end, which necessitated turning 

 the grating-axis farther away from the slit, thus placing it in a point of 

 the spectrum some little distance beyond the visible violet. The spec- 

 trum is, however, near enough to normal, even at the extreme red end, 

 to make any correction unnecessary, unless measurements of a very high 

 degree of accuracy are required. 



For holding the aqueous solutions, the cell illustrated in figure 66, 

 page 172, of "Hydrates in Aqueous Solution," was employed throughout, 

 while for non-aqueous solvents the cell shown by Plate 22 of the same work 

 was used. 



