,go8.] NEODYMIUM AND PRASEODYMIUM. 295 



rise in temperature ; that is, a rise in temperature produces very 

 much the same effect as increase in concentration. This seems to 

 us pretty conclusive evidence against the theories which 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 be- 

 comes 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 c^ at temperature ^^ 

 are exactly the same as those in a solution of concentration c. at a 

 temperature to ; since under the changed conditions it may happen 

 that the particular solvates, which were most stable when the con- 

 ditions were q and t^, may be less stable than solvates of nearly 

 the same composition at c^, t^. 



For this reason and also because our work on neodymium and 

 praseodymium salts in mixed solvents seems almost conclusive evi- 

 dence in favor of the existence of solvates, we Jmve used the solvate 

 theory as a working hypothesis throughout this work. That it is 

 not far from being correct is shown by the fact that all the phenom- 

 ena observed in the great number, about 1,200, of solutions studied, 

 are accounted for without anything but the simplest assumptions in 

 regard to the behavior of the solvates in question. 



The most interesting and important results were obtained from 

 the study of the salts of neodymium and praseodymium, especially 

 those of the former. These substances have not only very many 

 absorption bands, but they are remarkably narrow and sharp, and, 

 hence, peculiarly suitable for spectrographic study. The chief ex- 

 perimental results were the following: 



I. The absorption spectrum of aqueous solutions of the chloride 

 and bromide of neodvmium changes verv little with change in con- 



