SUMMARY AND CONCLUSIONS. 101 



The other theories which aim to account for the deviations are of two 

 kinds, viz: 



(1) Those that assume that the increased absorption in concentrated 

 solutions is due to the formation of aggregates of the molecules of the 

 dissolved substance, or of the molecules and the ions into which they 

 break down on dissociation. 



(2) Those that assume that the deviation is due to the formation of 

 solvates, that is, combinations of the parts of the dissolved substance with 

 the molecules of the solvent. 



It has been shown by Hartley and other workers who have studied the 

 change in the absorption with change in temperature, that the bands which 

 widen with increase in concentration (conditions for Beer's law assumed 

 to obtain) also widen with rise in temperature; that is, a rise in tempera- 

 ture produces very much the same effect as increase in concentration. This 

 seems to us pretty conclusive evidence against the theories that are based 

 on the formation of aggregates, for it is well known that the change in the 

 aggregates produced by rise in temperature is not the same as that produced 

 by increase in concentration, but exactly the opposite. 



The theories which assume the formation of solvates are not open to 

 this objection, because it is well known that the change in the solvates 

 produced by rise in temperature is in general the same as that produced 

 by increase in concentration. As a solution becomes more concentrated 

 the solvates become simpler and simpler, that is, fewer molecules of the 

 solvent are combined with each part of the dissolved substance. Rise in 

 temperature also breaks down complex solvates into simpler ones. Of 

 course, it does not follow that the solvates of a solution of concentration 

 GI at temperature t 1 are exactly the same as those in a solution of concen- 

 tration c 2 at a temperature t. 2 ; since under the changed conditions it may 

 happen that the particular solvates which were most stable when the 

 conditions were c l and t l may be less stable than solvates of nearty the 

 same composition at c 2 , t z . 



For this reason, and also because our work on neodymium and praseo- 

 dymium salts in mixed solvates seems almost conclusive evidence in favor of 

 the existence of solvates, we have used the solvate theory as a working 

 hypothesis throughout this investigation. That it is not far from being 

 correct is shown by the fact that all the phenomena observed in the great 

 number of solutions studied are accounted for without anything but the 

 simplest assumptions in regard to the behavior of the solvates in question. 



We shall now summarize briefly the main points brought out in the 

 present work. 



Solutions of cobalt salts have, in general, three regions of absorption 

 in that part of the spectrum which can be photographed without resorting 

 to other means than the commercial dry plate. One is in the extreme 

 ultra-violet, and we concluded that it is due to the molecules of the dis- 

 solved substance. Their absorption is influenced to some slight extent by 

 solvation, but differently for the different salts. That no part of this 

 absorption is due to the cobalt ions is shown by the fact that solutions of 

 cobalt sulphate are perfectly transparent beyond X 2200, although they are 

 dissociated to a very considerable extent. 



