SALTS OF NEODYMIUM, PRASEODYMIUM, AND ERBIUM. 83 



Throughout this description we have laid great stress on the fact that 

 on Plate 66 the two sets of bands coexist, the bands due to the aqueous 

 solution decreasing, while those belonging to the alcoholic solution increase 

 in intensity with decrease in the percentage of water; we have also called 

 attention to the fact that the two sets of bands have about half their full 

 intensity in a solution containing about 8 per cent of water. This was for 

 a 0.5 normal solution. 



The next question which suggested itself was whether the composition 

 of the solvent, in order to give the two sets of bands with about half their 

 normal intensity, is independent of the concentration of the dissolved 

 substance. If this be independent of the concentration, then we should 

 have to conclude that the determining factor is the nature of the solvent; 

 while if it depends upon the concentration, the ratio between the amount 

 of dissolved substance and one or other of the solvents would perhaps be 

 the important thing. To answer this question a set of solutions was made 

 up, keeping the solvent exactly the same as it was for the solutions used 

 in making the negatives for Plate 66, but making the concentration of 

 neodymium chloride 0.25 normal instead of 0.5 normal. The resulting 

 spectrogram is shown in Plate 76 B. In order to have this spectrogram 

 directly comparable with B, Plate 66, the depth of cell was kept at 1.0 

 cm. throughout. 



A study of this negative shows that the two sets of bands have about 

 half their normal intensity in the third strip, counting from the numbered 

 scale, corresponding to 5.3 per cent of water. In the fourth strip the bands 

 characteristic of the alcoholic solutions are very weak compared with the 

 bands belonging to the aqueous solution, while in the second strip the 

 opposite is the case. It is plain, therefore, that the composition of the 

 solvent, in order that the two sets of bands may show with about half their 

 normal intensity, depends upon the concentration, and it also seems very 

 probable that, provided the ratio of water to dissolved substance is kept 

 constant, the two sets of bands will not vary much in relative intensity. 

 A simple calculation shows that in the solutions which produced the bands 

 with about half their normal intensity, there were present approximately 

 10 molecules of water to 1 molecule of neodymium chloride. 



NEODYMIUM CHLORIDE IN ETHYL ALCOHOL WITH WATER. (See Plate 67 A.) 



The concentration of neodymium chloride was constant and equal to 

 0.5 normal. The percentages of water, beginning with the solution whose 

 spectrum is adjacent to the numbered scale, were 0, 5.3, 10.6, 16, 21.3, 26.6, 

 and 32. The depth of the cell throughout was 0.5 cm. 



This spectrogram shows exactly the same kind of change that we have 

 considered rather fully under Plate 66. The increments of water added 

 were twice as large here as in the case of methyl alcohol, and hence the 

 change takes place more rapidly as we pass from strip to strip, beginning 

 with the one next to the numbered scale. It is seen that in the second 

 strip the bands characteristic of the alcohol solution are very much more 

 prominent than those belonging to the* water solution, while in the third 

 strip the reverse is true. This points to the fact that here too the com- 



