25 



According to the equation there should be, with a given concen- 

 tration of copper salt, two simple methods of bringing about such a 

 displacement of etiuilibrium, namely, by increasing the concen- 

 tration of the bromide ion and by decreasing the "active mass" of 

 the water. The addition of any other bromide to a solution of 

 cupric bromide would make the solution browner in both ways, for 

 it would not only increase the concentration of the bromide ion 

 but it would also decrease the "active mass" of the water, since it 

 lowers the vapor pressure. 



When salts other than bromides are added to the cupric l)romi(le 

 this second influence alone is at work and we find that a relatively 

 slight change of color is produced. Indeed we should expect from 

 the above equation that the first influence would be much the 

 more powerful unless n is a very large number. The (|uestion is. 

 What will the result be of adding to equal quantities of cupric 

 bromide diflt'erent solutions with the same concentration of bromide 

 ion but with different vapor pressures? In order to answer this 

 question the following experiment was made: 



The same quantity. of cupric bromide was added to 10 cubic centi- 

 meters of a thrice-normal solution of each of the following bromides: 

 KBr, NH^Br, NaBr, LiBr. In these solutions the concentrations of 

 bromide ion are nearly alike, being somewhat greater in the case 

 of KBr, somewhat less in the case of LiBr. If this were the only 

 factor determining the color we should expect the solution in KBr 

 to be the most brown, that in LiBr the least. 



The facts are just the opposite. The NH^Br and NaBr solutions 

 have nearly the same color, that of KBr is less brown, while the 

 LiBr solution is very much l)rowner. In all these solutions the 

 blue is so faint as compared with the Ijrown that the relative brown- 

 ness of the solutions can be found by means of a colorimeter. 

 Colunnis of LiBr, NaBr, and KBr solutions, respectively 14, IS. 5, 

 and 20 millimeters thick, appeared equally brown. 



These experiments were repeated several times, a different 

 quantity of cupric bromide being used in each instance. The 

 results were the same. A small change in the concentration of 

 cupric bromide in any solution changes only the density of the 

 color but not its character. 



It is ol)vious that something influences the color besides the 

 concentration of the bromide ion. When we examine the vapor 

 pressures we find a very considerable difference in tlu' four solu- 



