REVIEW OF EARLIER WORK. 161 



AQUEOUS SOLUTIONS. 

 INTRODUCTORY. 



The importance of the absorption spectra of salts as affected by the pres- 

 ence of certain other salts, in their bearing on the hydrate theory as proposed 

 by Jones, has already been pointed out.* It has been mentioned that our 

 attention was directed to certain of these color changes by Dr. G. N. Lewis. 



The chief reactions that we have studied are those between cobalt chloride, 

 copper chloride, and copper bromide, when treated with calcium chloride, 

 calcium bromide, or aluminium chloride. It is well known that aqueous 

 solutions of cobalt chloride are purplish-red in color. When a strong, aque- 

 ous solution of cobalt chloride is treated with a fairly concentrated, aqueous 

 solution of calcium chloride, the color of the cobalt solution is changed 

 from purplish-red to blue. This same result is obtained if calcium bromide 

 is used instead of calcium chloride. Similar color changes occur when solu- 

 tions of aluminium chloride are added to solutions of cobalt chloride. Indeed, 

 smaller amounts of aluminium chloride are required to effect the same color 

 changes than are necessary when either of the salts of calcium is used. 



If any one of the above-named dehydrating agents is added to a fairly 

 dilute solution of cupric chloride or bromide, marked color changes result. 

 A fairly concentrated solution of cupric chloride or bromide is greenish- 

 brown, while dilute solutions are blue. As more and more water is added to 

 the more concentrated solutions, we have all gradations between the above 

 colors. The addition of calcium chloride, calcium bromide, or aluminium 

 chloride, to a blue solution of the copper salt changes the color to green, and 

 if sufficient of the dehydrating agent is added the color becomes greenish- 

 brown. 



In the present investigation these color changes have been studied quan- 

 titatively. The absorption spectra of the substances separately, and when 

 mixed in known quantities, have been observed by means of a direct-reading 

 spectroscope, and the wave-lengths of the absorption bands read off and 

 recorded. What is of far greater importance, however, is the photographic 

 record of such absorption bands, and the changes in the position of these 

 bands as varying amounts of one or another dehydrating agent is added 

 to the salt in question. These photographs have been taken by means of 

 the spectrograph, to be described later. 



Before taking up in detail the work that is here recorded, a brief dis- 

 cussion of some of the more important investigations in this same field seems 

 desirable. 



*Amer. Chem. Journ., 34, 291 (1905). Journ. de Chim. phys., 3, 494 (1905). 



