SALTS OF COBALT. 35 



GENERAL SUMMARY OF RESULTS WITH COBALT SALTS. 



We shall consider the aqueous solutions first, since they are the sim- 

 plest from the standpoint of the absorption spectra. All of the solutions 

 studied, with two exceptions, at room temperatures and with such depths 

 of absorbing layer as were employed, show only two regions of absorption, 

 one in the ultra-violet and one in the green. The exceptions are the con- 

 centrated solutions of the sulphocyanate, and the solutions of cobalt 

 bromide to which large amounts of calcium bromide had been added; 

 both of which show some absorption in the red. 



Let us first consider the absorption in the ultra-violet. Solutions of 

 all the salts studied, except the sulphate, have a region of so-called one- 

 sided absorption, which cuts off more or less of the ultra-violet end of the 

 spectrum, depending upon the salt used. In all cases the band narrows 

 with dilution when the conditions for Beer's law obtain, but tends to remain 

 approximately of constant width when molecules are kept constant. In 

 the bromide and nitrate the band is constant with molecules constant. 

 This indicates that the absorber which is responsible for this band is in 

 every case the undissociated molecule, and to account for the deviations 

 from constancy in the band when the number of absorbers is kept constant 

 we may assume that the molecules in concentrated solutions associate to 

 some extent, and that their absorbing power is thereby increased; or we may 

 assume that with increasing dilution they become more and more hydrated, 

 and that this decreases their absorbing power. A choice between these 

 two explanations can not be made without a further study of the subject. 



In addition to the one-sided ultra-violet band, cobalt chloride has a 

 band at A 3300, which disappears rapidly with dilution even when mole- 

 cules remain constant. This band seems to increase in intensity very 

 rapidly with rise in temperature. We can not very reasonably ascribe 

 this band to the undissociated molecules as such, since it not only narrows 

 rapidly but entirely disappears when these are kept constant. To explain 

 it we must hence look to either association or hydration. It may be re- 

 marked here that by association we do not mean simply a grouping together 

 of similar particles, but also a grouping together of such parts as mole- 

 cules and ions, or aggregates of molecules and ions, etc. The term associa- 

 tion, therefore, includes the complex anions assumed to exist by Donnan 

 and Bassett. Both association and hydration are known to diminish with 

 rise in temperature (except Donnan and Bassett's complex anions); with 

 increasing concentration at a given temperature association is known to 

 increase while hydration decreases. In a fairly dilute solution the amount 

 of association is perhaps negligible. If we then assume that the A 3300 

 band is due to some aggregate, this would explain its disappearance on 

 dilution, since this process destroys the aggregate. But raising the tempera- 

 ture also destroys the aggregate without, however, causing the absorption 

 band to disappear. The fact is that it becomes more intense as the tem- 

 perature rises. It seems, therefore, rather difficult to assume that it is due to 

 aggregates, at least to aggregates which are not abnormal in their behavior. 



If we assume that the band is due to some relatively simple hydrate 

 the facts are at once accounted for, since with rise in temperature com- 



