5 



ATLAS OF ABSORPTION SPECTRA. 



158. Cobalt Chloride in Acetone Cont'd. 



It is thus seen that the photographic 

 center of the band of absorption in 

 the green was displaced by about 

 200 Angstrom units as the solvent 

 was changed from water to methyl 

 alcohol. A still greater displacement 

 was produced by changing from the 

 one alcohol to the other, the concen- 

 trations of the two solutions being 

 very nearly equal. 



The empirical data given above serve 

 to illustrate* the general fact that 

 the position and character of a 

 given region of absorption or of 

 transmission of a chosen colored 

 salt can be varied, in general, over 

 wide ranges by suitable changes in 

 the solvent used. 



Fig. 94 shows the way in which the 

 limits of absorption change when 

 water is added to solutions of anhy- 

 drous cobalt chloride dissolved in 

 absolute acetone. The depth of the 

 cell was 2 cm. The solutions were 

 made up in the following manner : 

 A certain arbitrary volume of water 

 was poured into a measuring flask 

 and then the flask was filled up to 

 its calibration mark by running into' 

 it from a burette the recjuisite amount 

 of a mother-solution composed of 

 anhydrous cobalt chloride and abso- 

 lute acetone. When water is gradu- 

 ally added to such a mother-solution 

 the resulting liquid changes by de- 

 grees from deep blue through light 

 blue and then through an almost 

 colorless condition to faint pink. 



The percentages by volume of the 

 water in the solutions under consid- 

 eration were, o, 2, 4, 6, 8, 10, and 12. 

 The concentration of the mother- 

 solution was 0.015 normal. 



The photographic strip nearest to the 

 comparison spectrum corresponds to 

 the solution which was anhydrous. 

 The next strip pertains to the solu- 

 tion which contained 2 per cent of 

 water, and so on, across the entire 

 spectrogram. The mother-solution 

 absorbed completely all radiations 

 between 0.20/n and 0.333^11. The con- 

 tinuous background was very much 

 weakened as far as about 0.361/x. 

 The solution transmitted freely 



158. Cobalt Chloride in Acetone Cont'd. 



from this wave-length to near 

 0.552/1. A strong absorption band 

 commenced at 0.552/1 and extended 

 into the red. 

 The photographic strip pertaining to 

 the solution which contained the 

 smallest measured amount of water 

 transmitted from 0.333/1 to about 

 0.566/1. The change in absorption 

 due to the addition of water to the 

 anhydrous mother-solution is, there- 

 fore, more noticeable in the ultra- 

 violet than in the yellow. The photo- 

 graphic boundary of the ultra-violet 

 absorption band changed but little, 

 as the percentage of water present 

 in the solutions increased from 2 to 

 12, and this is due to the intense 

 ultra-violet absorption of the pure 

 acetone. (See No. 148.) On the 

 other hand, acetone possesses no ab- 

 sorption band in the visible spectrum, 

 and hence the limits of transmission 

 in the green and yellow, as shown 

 by the several strips of the spec- 

 trogram, represent correctly the 

 changes in absorption consequent 

 upon the addition of successive in- 

 crements of water. 



I S9- Cobalt Chloride in Ethyl Alcohol. 

 See No. 158. 



160. Cobalt Chloride in Methyl Alcohol. 

 See No. 158. 



161. Cobalt Glass. 

 Fig. 85, pi. 21. 



.\ plane-parallel sheet of ordinary blue 

 cobalt-glass was ground to the form 

 of a wedge and then polished. A 

 prism of colorless glass was at- 

 tached at the sides to the cobalt prism 

 with its refracting edge parallel to 

 that of the colored glass. The two 

 wedges were in contact over their 

 hypothenuse planes, and hence the 

 outer plane surfaces were nearly 

 parallel. The object in using the 

 colorless glass wedge was, obviously, 

 to correct for the dispersion of the 

 cobalt-glass prism. The lack of 

 agreement between the contiguous 

 edges of the two photographic strips 

 shows that the angle of the color- 

 less prism ought to have been at 

 least twice as large as that of the 

 blue prism. The angle of the cobalt- 



*See also No. 165. 



