78 ABSORPTION SPECTRA OF SOLUTIONS. 



The group in the green is made up of six bands as follows : A 5125, hazy 

 and rather wide, moderately intense; A 5180, also hazy but much fainter; 

 A 5220, moderately intense and narrow; A 5245, intense, and with faint 

 companion towards the red; X 5290, narrow and moderately intense; shad- 

 ing as far as A 5330, with indications of a faint band at A 5315. 



The yellow group is made up of seven bands having the following 

 characteristics : X 5725, moderately intense with hazy edges ; A 5765, 

 narrower, but not quite as intense as A 5725; A 5800, fairly narrow, strong; 

 A 5835, very intense; A 5860, hazy and moderately intense, not clearly 

 separated from A 5835, shading to A 5970, with two faint bands super- 

 imposed upon it, one at A 5895 and the other at A 5925. 



No trace of bands is to be seen in the orange, but in the red there is 

 a fairly narrow but faint band at A 6860. The spectrum ends at A 7355 

 in a deep, rather narrow band. It is evident that the spectrum of neo- 

 dymium chloride when dissolved in methyl alcohol is quite different from 

 its spectrum in aqueous solution, but this point will be taken up more 

 fully in the discussion of Plates 65 and 66. 



NEODYMIUM CHLORIDE IN ETHYL ALCOHOL BEER'S LAW. (See Plate 64.) 



The concentrations of the solutions used in making the negative for A, 

 beginning w r ith the one whose spectrum is adjacent to the numbered scale, 

 were 0.50, 0.40, 0.315, 0.25, 0.20, 0.16, and 0.125; the corresponding 

 depths of absorbing layer being 6, 7.5, 9.5, 12, 15, 19, and 24 mm. For 

 B, the concentrations were 0.20, 0.16, 0.13, 0.10, 0.08, 0.06, and 0.05; the 

 depths of cell were the same as used in A. The concentrations and depths 

 of cell were, therefore, exactly the same as those in methyl alcohol, so 

 that Plates 63 and 64 are directly comparable. A very careful comparison 

 of the two plates reveals the remarkable fact that the two spectra are 

 identical; the very slight differences noted being perhaps due to slight 

 differences in development of the negatives. 



In view of the great difference between either one of these spectra 

 and that of the aqueous solution, this similarity is rather surprising, and it 

 led us to think that perhaps in these alcoholic solutions we were getting the 

 absorption of the neodymium chloride molecules themselves, while in the 

 aqueous solution we get the absorption of some compound of the molecules 

 with water. But this was answered in the negative by the spectrum of anhy- 

 drous neodymium chloride (Plate 68), which is very different from that of any 

 of the solutions. The spectrum of the alcoholic solutions is, therefore, not 

 that of the NdCl 3 molecule per se, but must be that of some solvate of it or of 

 the neodymium ion. But that solvates with methyl alcohol and ethyl alco- 

 hol should affect the frequencies of the vibrators in the metallic atom so 

 very nearly the same seems a little surprising, to say the least, especially as 

 solutions of the nitrate in the two solvents give somewhat different spectra, 

 as will be fully discussed when we come to consider Plates 73 and 74. 



The very slight differences between the bands shown by Plates 64 and 

 63 seemed to indicate that they were a little more hazy in the ethyl alcohol 

 solutions, but the development of the negatives for Plate 64 was not car- 

 ried quite as far as was the case with those for Plate 63, and this would 

 tend to produce just the land of difference that was noted. 



