14 ABSORPTION SPECTRA OF SOLUTIONS. 



tion is recorded, and where the intensity of the spark was constant or 

 very nearly so throughout. The increased intensity of the seventh strip 

 of Plate 2 B is due to the fact that before making the exposure the poten- 

 tial was readjusted to its original value. 



The absorption in the extreme ultra-violet decreases regularly with 

 dilution, the strip corresponding to the most concentrated solution in set B 

 transmitting as far as A 2650, while that corresponding to the most dilute 

 solution shows transmission a little below ^ 2500. 



In set A the band in the green narrows quite rapidly at first, then 

 more slowly with dilution. In set B it remains very nearly constant, 

 especially in the strips corresponding to the more dilute solutions. 



The facts brought out by the spectrogram may be summed up briefly 

 as follows: When the product of concentration and depth of absorbing 

 layer is kept constant, the absorption band in the green remains constant, 

 except in very concentrated solutions, where it narrows somewhat with 

 dilution; the one-sided, extreme, ultra-violet absorption decreases with 

 dilution, while the band having its center near ^ 3300 narrows very rapidly 

 with dilution, disappearing entirely when a concentration of about quar- 

 ter-normal is used in a layer about 1 cm. in depth. In the region A 2800 

 to A 3000 there is remarkable transparency in solutions having a concen- 

 tration about half-normal. This transparency decreases somewhat with 

 dilution to about one-tenth normal. 



A comparison between this plate and Plate 2 in the work of Jones and 

 Uhler already referred to, reveals a remarkable difference in the ultra- 

 violet. The four strips nearest the comparison spectrum of their plate 

 correspond to concentrations of more than 2.0 normal, and with a depth 

 of absorbing layer of 0.67 cm., and yet the absorption in the region 

 ^ 3000 to A 3500 is not very great. It might at first sight seem probable that 

 the absorption band in this region was produced by some impurity in the 

 cobalt chloride used. But this is improbable, since the salt from which 

 were obtained the solutions used by Jones and Uhler and those used in 

 the present work came from the same sample of material. Besides, Plate 

 23, in "Hydrates in Aqueous Solution," shows that the solution of the 

 cobalt chloride used by Jones and Uhler, when dissolved in methyl alco- 

 hol, exerts strong absorption in the ultra-violet, while Plate 7 of the pres- 

 ent work indicates only faint absorption in this region, the concentrations, 

 depth of cell, etc., used in the two cases, not differing materially; hence, it 

 is evident that it is not a question of the presence of an impurity. The 

 probable explanation is that the discrepancy is due to a difference of tem- 

 perature. 



It is well known that the absorption of cobalt salts in solution is 

 greatly affected by even slight changes in temperature, and if the X 3300 

 band is especially sensitive, such variations of temperature as occur in 

 the laboratory on different days might be sufficient to account for the 

 changes in the spectrum observed. No data are at hand, however, giving 

 the change of absorption in the ultra-violet produced by change of tempera- 

 ture, and hence this point will have to remain unsettled until the work now 

 in progress shall have supplied the data for the absorption band in question. 



