SALTS OF COBALT. 15 



COBALT CHLORIDE IN WATER NUMBER OF IONS IN THE PATH OF THE BEAM 

 OF LIGHT CONSTANT. (See Plate 39 B.) 



The concentrations were 2.00, 0.99, 0.546, 0.318, 0.205, 0.140, and 

 0.103, the corresponding depths of absorbing layer being 3, 4, 6, 9, 13, 

 18, and 24 mm., respectively. 



Very little need be said about this spectrogram, since it shows that 

 the absorption decreases rapidly with dilution, as was to be expected from 

 the result when the product of concentration and depth of absorbing layer 

 was kept constant. This spectrogram also shows the existence of the band 

 at X 3300, as well as indications of the decrease of transparency at X 2900 

 with dilution from half-normal to tenth-normal. That an increase in 

 absorption in this region is indicated in this spectrogram is significant, 

 inasmuch as it shows that dissociation is not able to account for it. For 

 the reason stated in the description of Plate 2, no exposure was made 

 to the red end of the spectrum for this set of solutions. 



COBALT CHLORIDE IN WATER MOLECULES CONSTANT. (See Plate 3.) 



In both sets the strip corresponding to the most concentrated solution 

 is adjacent to the numbered scale. The solutions were made up of such 

 concentrations that the number of undissociated molecules of cobalt 

 chloride in the path of the light should be constant for each set. The 

 concentrations for set A were 2.50, 2.04, 1.53, 1.14, 0.885, 0.695, and 

 0.570; the corresponding depths of absorbing layer were 3, 4, 6, 9, 13, 

 18, and 24 mm., respectively. The concentrations for set B were 0.70, 

 0.575, 0.434, 0.329, 0.253, 0.197, 0.158. The data used in calculating the 

 concentrations were taken from the table on page 81 of "Hydrates in 

 Aqueous Solution." 



In making the exposures for set A only the light from the Nernst 

 lamp was used, as a preliminary test showed that none of the solutions 

 transmitted any light of shorter wave-length than X 3850. Owing to the 

 general absorption of concentrated solutions of cobalt chloride, it was 

 necessary to increase the time of exposure, and hence for set A the light 

 from the Nernst lamp was allowed to act for a period of 3 minutes through 

 each of the solutions. 



The exposures for set B were 1 and 3 minutes, respectively, for the 

 Nernst lamp and the spark. No exposures were made to the red end of 

 the spectrum, since all the solutions showed uniform transmission to be- 

 yond X 7600. 



A shows that the band having its center at X 3300 narrows with dilu- 

 tion, even when the number of molecules in the path of the beam of light 

 is kept constant; thus demonstrating that the change in this band can 

 not be accounted for by dissociation. The limits of transmission as given 

 by the negative for the seven solutions beginning with the most concen- 

 trated are X 4130, X 4070, X 3970, X 3930, X 3900, X 3870, X 3850, which 

 shows that the band narrows more rapidly at first. In B this band may be 

 seen in the three strips nearest to the numbered scale, corresponding to 

 concentrations 0.70, 0.575, and 0.434, but it has practically disappeared 

 in the fourth strip, corresponding to concentration 0.329. The maximum 



