SALTS OF NEODYMIUM, PRASEODYMIUM, AND ERBIUM. 79 



NEODYMIUM CHLORIDE IN MIXTURES OF METHYL ALCOHOL AND WATER. 

 (See Plates 65, 66, and 76 B.) 



Since, as we have just seen, the absorption spectrum of neodymium 

 chloride in aqueous solution is so different from that of the alcoholic solu- 

 tions, it was thought to be of some interest to see how the change from the 

 one to the other would take place if one of the solvents was made to dis- 

 place the other gradually. A series of solutions was accordingly made up, 

 the concentration of the dissolved salt being constant and equal to 0.5 

 normal, but the character of the solvent varying as follows : The percent- 

 ages of water in the seven solutions were 0, 16.6, 33.3, 50, 66.6, 83.3, and 

 100; the corresponding percentages of methyl alcohol were 100, 83.3, 66.6, 

 50, 33.3, 16.6, and 0. Two spectrograms were made, namely, A, Plate 

 65, where the depth of the cell was 1.5 cm., and B, where the cell had a 

 depth of only 5 mm. A was made in order to show clearly the change 

 taking place in the narrower and fainter bands, while B was intended to 

 show the change of structure of the more intense bands, such as. the green 

 and yellow ones. The strip which is adjacent to the numbered scale belongs 

 to the solution in pure water, while the one nearest the narrow, comparison 

 spark spectrum belongs to the solution in pure methyl alcohol. 



Plate 65 shows that, beginning with the strip nearest the scale, the first 

 six spectra are very nearly identical. From the sixth to the seventh there 

 is an abrupt change, which at first sight consists in a shift of all the bands 

 towards the red, but which on closer examination is seen to consist in a 

 disappearance of one spectrum and the appearance of the other. Since the 

 first strip is the spectrum of the solution in pure water, it follows, since the 

 sixth is nearly identical with the first, that as large a percentage of alcohol 

 in the solvent as 83 per cent does not change the absorption spectrum 

 materially; the chief change taking place when the percentage of alcohol 

 is varied from 83 per cent to 100 per cent. 



It is to be noted that the apparent shift of the bands towards the red 

 is in reality not quite as great as it appears at first sight from Plate 65, 

 owing to the fact that the film accidentally shifted slightly towards the 

 red between the sixth and seventh exposures. The amount of this me- 

 chanical shift is easily seen, however, by comparing the spark lines in the 

 ultra-violet. A measurement of the shift shows it to be approximately 3 

 Angstrom units, and the same for both A and B, while the "apparent" 

 shift of the absorption line at X 4275 in aqueous solution is actually 15 

 Angstrom units, its position in the alcoholic solution being A. 4290. 



The slight changes taking place with some of the bands throughout the 

 spectrograms of Plate 65 are perhaps sufficiently clear in the reproductions. 

 However, as a good deal of the detail shown by the negatives is lost, even 

 in the most perfect processes of reproduction, we give here a description 

 of the changes taking place in two of the bands as seen on the original 

 negative. We select the bands at ^ 4275 and A 4760 from the negative for 

 A, Plate 65. 



In the aqueous solution the X 4275 band is very intense and narrow, 

 its whole width being less than 5 Angstrom units. The edges are only 

 very slightly shaded. In the alcoholic solution the position of the center 



