SALTS OF COBALT. 23 



cell-depths, we are concerned only with the rate of change of dissociation 

 with concentration, and not with the actual value of the dissociation for 

 any given concentration. A comparison of the series of concentrations 

 fulfilling the condition of "molecules constant" for various strong elec- 

 trolytes used in the present investigation shows that they differ from each 

 other by amounts that are small compared with experimental errors in 

 the measurements of dissociation. Hence, the assumption made in the 

 present case would not introduce any sensible error. 



The concentrations used in making the negative for A were 1.53, 1.14, 

 0.885, 0.695, and 0.570; the corresponding depths of absorbing layer of 

 solution were 6, 9, 13, 18, and 24 mm., respectively. For B the con- 

 centrations were 0.70, 0.575, 0.434, 0.329, 0.253, 0.197, and 0.158; the cor- 

 responding depths of cell were 3, 4, 6, 9, 13, 18, and 24 mm. It will be 

 noticed that the concentrations for B, as well as the depths of cell, are 

 identical with those used in making the negative for Plate 3, and, hence, 

 a comparison of the two sets of spectra shows at a glance the relative 

 absorbing power of cobalt bromide and cobalt chloride solutions. The 

 latter exerts a considerably greater absorbing power throughout the 

 spectrum. 



The exposure to the Nernst filament and spark for both A and B lasted 

 respectively 1^ and 3 minutes, the width of slit being 0.01 cm. as usual. 

 The spectrogram shows that the absorption in the extreme ultra-violet 

 remains constant, the limit of transmission in A being at A 2700, in B at 

 X 2470. The band in the green widens with dilution throughout, and 

 quite uniformly, differing in this respect from the chloride, where it will 

 be recalled that the band at first remained of constant width, and then 

 began to widen with dilution. No exposure to the red end of the spectrum 

 was made at all; the solutions transmitting freely light of all wave-lengths 

 between A 5700 and A 7600. 



COBALT BROMIDE WITH CALCIUM BROMIDE. (See Plate 12.) 



The concentration of cobalt bromide throughout was constant and 

 equal to 0.260. The concentrations of calcium bromide, beginning with 

 the solution corresponding to the strip adjacent to the numbered scale, 

 were 3.73, 3.56, 3.39, 3.22, 3.05, 2.88, 2.72, 2.54, 2.37, 2.20, 1.87, 1.53, 

 1.19, 0.85, 0.51, 0.00. The differences in concentration were, accord- 

 ingly, 0.17, 0.17, 0.17, 0.17, 0.17, 0.17, 0.17, 0.17, 0.17, 0.34, 0.34, 0.34, 

 0.34, 0.34, 0.51. 



The solution containing the greatest amount of calcium bromide was 

 purplish-blue in color, from which the color changed to the reddish-pink 

 of cobalt bromide in the solution containing no calcium salt. 



The cell was adjusted to a depth of 1.1 cm. for each of the solutions; 

 the exposures to the Nernst lamp and spark being, respectively, 1 and 

 3 minutes. The solution containing no calcium bromide transmits all 

 wave-lengths from A 2400 to A 7600, with the exception of the region 

 A 5000 to /I 5300 which is absorbed. With addition of calcium bromide the 

 band in the extreme ultra-violet widens rapidly, so that in the solution 

 pertaining to the tenth strip, counting from the scale, all radiations of 



