INTRODUCTORY. 5 



kept constant, while in the latter case the band would remain constant in 

 width and position under the same conditions. If the band narrows with 

 decrease in concentration when the number of undissociated molecules is 

 kept constant, dissociation alone can not possibly explain the facts. An 

 example of this case is furnished by the ultra-violet absorption of most 

 copper salts in aqueous solution. 



The case often met with in the present work where a band narrows 

 with dilution when the product of concentration and depth of layer is 

 kept constant, but widens when the number of undissociated molecules 

 is kept constant, deserves further consideration, as dissociation may or 

 may not be able to account for it, depending on the nature of the change 

 in the band. This was discussed by E. Miiller, who showed that by meas- 

 urement of the extinction coefficient at various concentrations, using a 

 spectrophotometer, we may determine whether Beer's law holds for each one 

 of the three absorbers in a solution of an electrolyte, even when these have 

 quite different powers of absorption. Work on this point is now in prog- 

 ress for the solutions showing the effect just spoken of, and the results will 

 be published shortly. 



Another interesting method of studying absorption spectra is to keep 

 both concentration and depth of layer constant, and to vary the tem- 

 perature. Much w y ork of this kind has already been done by Hartley and 

 others, and some work is now in progress in this laboratory. As is well 

 known, changing the temperature has only a small effect on the dissocia- 

 tion, hence we may say that when the temperature is varied the disso- 

 ciation remains roughly constant. The spectrum of some solutions, how- 

 ever, undergoes very great changes; for example, solutions of cobalt 

 chloride which are red at room temperatures become blue when the 

 temperature is elevated sufficiently; and according to Donnan and Bassett 

 solutions of the same salt in ethyl alcohol, which at ordinary temperatures 

 are blue, turn red when cooled down to 75 C. This effect, which involves 

 the appearance or disappearance of a complicated set of absorption bands 

 in the red, can evidently not be accounted for by dissociation, since it takes 

 place when the dissociation is known to change but very slightly. 



In the present work considerable attention has been given to solutions 

 in non-aqueous solvents, such as methyl alcohol, ethyl alcohol, and ace- 

 tone, as well as to solutions in mixtures of these solvents with water. 

 In this part of the work great care has been taken to have both the salts 

 and solvents as free from water as possible, and this is of fundamental 

 importance, as is shown by the spectrograms of solutions of neodymium 

 chloride in mixtures of alcohol and water, where it appears that the change 

 in the spectrum produced by 1 per cent or less of water may easily be 

 detected. 



The question of the effect of the solvent comes up in this connection, 

 and this in turn involves the more general question as to the condition 

 of a molecule of a dissolved substance in any solvent whatsoever. Does 

 it form some kind of a compound with the solvent, or does it move about 

 freely in the solvent without materially changing its nature? Kundt's 



