NEODYMIUM SALTS. 83 



A question which was raised by a study of some of the other spectro- 

 grams was whether by the addition of hydrochloric acid to the acetate 

 different chemical compounds were formed. In the above spectrogram only 

 two sets of bands appear, and it thus seems very probable that when hydro- 

 chloric acid is added to the acetate more than two compounds exist, for 

 there are more than two sets of bands. Indeed, it seems probable that 

 there is a whole series of compounds or systems formed between the acetate on 

 the one hand and the chloride on the other. 



Plate 45, B, represents the effect of adding hydrochloric acid to an 

 aqueous solution of neodymium citrate. The absorption spectrum of the 

 citrate is very similar to that of the acetate, and the changes in the absorp- 

 tion spectra are very similar to those that take place when mixtures of the 

 acetate and chloride are dissolved in water. In other words, there are 

 bands here: " citrate " bands which are very similar to the " acetate " bands. 

 As hydrochloric acid is added the characteristic " citrate " bands gradually 

 decrease in intensity while the " chloride " bands increase in intensity. 

 There is no evidence here of more than two chemical compounds. 



SUMMARY. 



No salts show the complexity of absorption spectra better than those 

 of neodymium and erbium. Some of the bands are wide and diffuse, some 

 narrow and strong in fact bands of very great diversity of appearance are 

 present. In any given solvent the absorption spectra of the various salts 

 are very similar and in many cases practically identical. But when the 

 solutions are very concentrated or when the salts themselves are investi- 

 gated, it is found that the absorption spectra are entirely different for each 

 salt. The fact that the absorption of different salts in the same solvent is 

 very similar is a strong argument that the solvent plays a very important 

 role in the absorption of light. This view is very much strengthened when 

 it is found that the absorption in different solvents is different, and that in 

 mixtures of solvents both solvent bands coexist. 



During the work on the absorption of uranyl nitrate to which sulphuric 

 acid was added, the very fine banded absorption spectra of nitric oxide 

 were obtained. From the conditions of the experiment it seems very prob- 

 able that this nitric oxide was in solution. Granting that this is the case, 

 the experiment shows that the solvent itself under some conditions may 

 not have any effect upon the absorption spectra. It seems reasonable to 

 suppose that it is when chemical combination between solvent and solute 

 takes place that the absorption of light is greatly modified. 



In the case of neodymium bands it is very difficult to change the 

 wave-length of the bands. Becquerel and others show that there are small 

 changes as the neodymium is cooled to very low temperatures. The ab- 

 sorption spectra of salts at 600 indicate but slight changes in wave-length 

 compared with the salts at 0. For some bands the shift is about 10 Ang- 

 strom units. Our work shows practically no shift in the absorption bands 

 of pure aqueous solutions between and 90 C. However, when calcium 

 or aluminium is present, in some cases there are shifts. 



