292 Hartitey— The Action of Heat on the Absorption Spectra and 
quantities of these salts which Mr. Hugh Ramage was so good as to lend me, I here- 
with submit photographs taken with the 4-prism spectrograph. The dispersion of 
this instrument is considerable as may be understood by the measurements between 
the D' and D? lines being 0:045 and 0:05 mm. on different occasions ; in wave- 
length the difference is only four tenth-metres, so that, at this part of the spectrum, 
0-01 mm. of linear measurement is equivalent to a difference of ———__ th of 
a millimetre in wave-length. 
Neodym-ammoniuwm Nitrate.—Solution was made by dissolving 2°8 grs. in 2°2 ¢.¢. 
of water. It was placed in a thin wedge-shaped cell to be photographed. 
Praseodym-ammonium Nitrate.—Solution was made by dissolving 1:9 grs. of the 
salt in 2°3 c.c. of water. It was photographed in a similar cell. 
The position of the solar lines and of the absorption bands in cold and hot 
solutions is shown; and it will be observed how the bands in the hot solutions are 
widened out; also how the absorption increases at the end in the ultra-violet near 
the lines N and O, PI. XXII. I beg to express my thanks to Mr. Hugh Ramage 
for the assistance he gave me, by the loan of these salts, and in other ways when 
photographing their spectra. 
In a recently published paper,* Muthmann and Stiitzel remark upon the great 
difference between the spectra of the nitrate, chloride, and carbonate of neodym. 
This last compound differs from the others, and possesses a somewhat intense blue 
colour. The intensity of the absorption was greatly increased; the band in the 
violet, \ 432-434 was no longer visible, and that in the green was stronger than in 
the nitrate. There was a marked alteration in the wave-lengths of the bands, 
which showed a shifting towards the red end of the spectrum, the maximum of 
absorption in the bands being altered to the extent of 7-5 millionths of a 
millimetre of wave-length in the yellow and green. The neodym carbonate 
showed, in addition, a fine well-marked dark line in the oranget at \ 600°5, which 
disappeared when the carbonate was converted into salts with mineral acids, but 
in the acetate, though much weaker, it was visible at \ 5970. The relative 
intensities of the bands in praseodym carbonate, also the order in which the lines 
disappear upon dilution, are quite different from the nitrate and chloride. For 
instance, while in the nitrate solution the blue line is the most persistent, that in 
the yellow about 4596°5 is the last to disappear from the carbonate. As I have 
explained with regard to the absorption bands in organic compounds, the most 
persistent bands are those corresponding with the greatest amplitude of vibration. 
The authors connect these alterations in the spectra with the state of electro- 
lytic dissociation of the solutions, but meet with a difficulty in accounting for the 
* “ Beitrige zur Spectralanalyse von Neodym und Praseodym,”’ Berichte der Deutsch. Chem. Ges., vol. 
32, p. 2653, 1899. 
+ Compare the didymium potassium nitrate and didymium acetate spectra already described. 
