20 ABSORPTION SPECTRA OF SOLUTIONS. 



solutions exert considerable absorption in the ultra-violet region, as the 

 spectrum is faint compared with the comparison spectrum. If this absorp- 

 tion increases with rise of temperature, as is very probable, the difference 

 noted might well be produced by a difference in temperature of only a 

 very few degrees. 



COBALT CHLORIDE IN ETHYL ALCOHOL WITH WATER. (See Plate 8.) 



The concentration of cobalt chloride throughout was 0.088. The per- 

 centages of water, beginning with the solution used in making the strip 

 nearest the comparison spectrum, were 0, 1, 2, 3, 4, 5, 5.5, 6, 6.5, 7, 7.5, 

 8, 9, 10, 11, and 12. The duration of the exposures to the Nernst lamp and 

 the spark was, respectively, 1^ and 3 minutes; the slit was adjusted to a 

 width of 0.01 cm. The common depth of absorbing layer was 2.0 cm. 



The four solutions containing the least amounts of water show the 

 bands at X 3100 and X 3600 to be very intense, making the absorption for 

 the solution in pure ethyl alcohol complete from X 3800 to the end of 

 the ultra-violet. The bands, however, disappear rapidly on addition of 

 water, so that in the strip corresponding to 6 per cent of water they can 

 hardly be noticed. This strip transmits light as far out as X 2600, while 

 on the strip made with the solution containing 12 per cent of water trans- 

 mission extends to A 2475. 



The band in the green behaves very much the same as it does in methyl 

 alcohol, being, however, somewhat fainter in ethyl than in methyl alcohol. 

 Its middle for the solution containing 12 per cent of water is at X 5200. 

 For the solution in pure ethyl alcohol it is not possible to determine its 

 middle, as it unites with the strong absorption band in the orange and red, 

 this solution showing complete absorption from about I 4930 to X 7200. 



The absorption band in the red narrows very rapidly with addition of 

 water. Its limits for the solution containing 3 per cent of water are X 5750 

 and X 7000. In the strips which correspond to the solutions containing 

 from 5 to 7.5 per cent of water, the band breaks up into a rather compli- 

 cated spectrum. Absorption bands may be noticed having centers at 

 X 5910, X 6060, X 6240, X 6400, and X 6700.' Of these, the last is the strong- 

 est and widest. The one at X 6400 is very faint, the one at X 5910 a little 

 more intense; while those at X 6060 and X 6240 are fairly intense, compar- 

 ing favorably with the bands in the same region seen on Plate 4. 



Referring to the description of the negatives from which Plate 5 was 

 made, it will be recalled that, although the absorption in the red showed 

 signs of breaking up into finer bands, these did not appear very distinctly. 

 Indeed, the only ones that could be made out were those at X 5910 and 

 X 6060. The water added to solutions of cobalt chloride in ethyl alcohol 

 must hence play an important part in the developments of these bands, 

 and it is barely possible that the faint development of the bands noted 

 in Plate 5 may have been due to slight traces of water which it is impos- 

 sible to remove from the ethyl alcohol, or which might have found its way 

 into the alcohol in the process of pouring the solution into the cell and expos- 

 ing this in the spectrograph. It is probable that the activity of water 

 in producing changes in the absorption spectrum depends not upon the 



