SALTS OF NEODYMIUM, PRASEODYMIUM, AND ERBIUM. 81 



In order to study the change that takes place between the sixth and 

 seventh strips of the spectrograms of Plate 65 more carefully, a series of 

 alcoholic solutions was prepared containing the following percentages of 

 water, 0, 2.6, 5.3, 8, 10.6, 13.3, and 16. The concentration of the neo- 

 dymium chloride was constant and equal to 0.5 normal. Two spectrograms 

 were made, one with a depth of absorbing layer of 1.5 cm. in order to show 

 the fainter bands, and the other with the depth of the cell only 5 mm. in 

 order to show as much as possible of the structure of the larger bands. 

 The first spectrogram is reproduced as Plate 66 A and the second as Plate 

 66 B. The strips corresponding to the pure alcohol solutions are adjacent 

 to the numbered scale, the spectrum of the solution containing 16 per cent 

 water being next to the comparison spark spectrum. 



Although we found on considering Plate 65 that some slight change in 

 the spectrum takes place when the percentage of alcohol is changed from 

 to 83 per cent, yet this change is so small, and the bands due to the 

 aqueous solution are so strong, that we may regard the spectrum of a 

 solution containing 16 per cent of water as practically that of the aqueous 

 solution. Accordingly, the spectrograms on Plate 66 may be taken to show 

 very nearly the whole change which takes place when the solvent of neodym- 

 ium chloride is gradually changed from pure water to pure methyl alcohol. 



In A the ultra-violet band is rather too intense to allow its structure 

 to be seen. Accordingly, we see the whole band remain sensibly unchanged 

 as the water is varied from 16 per cent to 8 per cent, and then shift towards 

 the red with increasing rapidity as the water is reduced to zero, the whole 

 apparent shift amounting to about 20 Angstrom units. On the negative 

 the intense band at ^ 3465 may, however, be clearly seen, and its intensity 

 decreases very slowly from the first to the third strips, counting from the 

 narrow, comparison spark spectrum. In the fourth strip its intensity is 

 about half of what it was in the first strip, and from this it decreases rapidly, 

 vanishing entirely in the strip nearest the scale. 



In B the structure of this band is seen very distinctly, and we find that 

 the bands characteristic of the aqueous solution gradually decrease in 

 intensity, especially from the third to the sixth strips, while the wider 

 bands, characteristic of the alcoholic solutions, increase in intensity, the 

 two sets existing together. The change in the band at A 4275 is the one 

 that shows the best, because here the two bands belonging to the aque- 

 ous and alcoholic solutions, respectively, are both intense and narrow and 

 clearly separated from one another. 



The alcoholic band is clearly visible in the first strip, and it increases 

 continuously in intensity as the amount of water is decreased, but more 

 rapidly from the fourth to the seventh strips than from the first to the 

 fourth. Its position also shifts somewhat towards the red from the first 

 to the fourth strips, the wave-lengths of its center for the two strips being, 

 respectively, ^ 4287 and ^ 4292. Accompanying this shift is a change in 

 its character, which may be gathered from the following statements : In 

 the first strip it has the appearance of an unsymmetrical band, the 

 maximum intensity being nearer the violet. In the third strip it extends 

 from ^ 4280 to X 4295, and has about the same intensity throughout. In 

 6 



