xgoS.] NEODYMIUM AND PRASEODYMIUM. 285 



solvents was made to displace the other gradually. A series of solu- 

 tions 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 percentages of water in the seven 

 solutions were o, i6f, 33 J, 50, 66|, 83^ and 100; the corresponding 

 percentages of methyl alcohol were 100, 83^, 66|, 50, 33^, i6f 

 and o. Two spectrograms were made, namely a, plate 3, 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 num- 

 bered scale belongs to the solution in pure water, while the one 

 nearest the narrow comparison spark spectrum belongs to the solu- 

 tion in pure methyl alcohol. 



Plate 3 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 examina- 

 tion 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 3, owing to the fact that the film accidently shifted 

 slightly towards the red between the sixth and seventh exposures. 

 The amount of this mechanical 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 absorp- 

 tion line at A 4275 in aqueous solution is actually 15 Angstrom 

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



PROC. AMER. PHIL. .SOC. , XLVII. 189 S, PRINTED SEPTEMBER 25, I908. 



