Chemical Constitution of Saline Solutions. 287 
The measurements of the spectra of chromium oxalate, and the blue and red 
salts derived from it were made by Lapraik, and are interesting since we know 
that these latter arise from the formation of a distinct chromoxalic acid as first 
propounded by Malaguti.* 
Both Clarke and Werner established the formula for the acid, H,Cr.(C,0,), by 
determining the constitution of the blue salts, but the acid itself was not isolated. 
Werner found the red salts to be represented by M,Cr.(C,O,),. From seven 
different reactions I deduced the existence of chromoxalic acid, and showed the 
relationship of the red to the blue salts.+ But Lapraik showed from the spectra 
that there were two compounds formed by dissolving chromium hydroxide in oxalic 
acid, and he concluded they were two different chromoxalic acids corresponding 
to the red and the blue potassium salts. 
PART IV. 
Spectra of the Third Kind. 
Of spectra interrupted by black bands or lines, we have five examples in the 
salts of uranium, of didymium (neodym and praseodym), samarium, and erbium. 
Potassium permanganate, also in a very dilute solution, shows seven bands in the 
blue and green, but their edges are not well defined. 
The result of a large number of measurements of the bands seen in the spectra 
of uranium salts showed that no remarkable alteration occurred when they were 
heated. The measurements on each side of the different absorption bands remained 
identical or underwent a slight alteration when the temperature rose from 16° to 
100°, showing that the bands were widened. Messrs. Morton and Carrington 
Bolton also arrived at the conclusion that heat did not affect their spectra.§ 
The spectra of salts of the rare earths are altered when their solutions are 
heated ; and the extent and nature of the alterations will be seen from the measure- 
ments which are given. 
Since this work was executed, the separation of compounds of neodym, 
praseodym, samarium, and thulium has been accomplished, and therefore the 
didymium salts must be regarded more or less as mixtures. 
* Comptes Rendus, vol. 16, p. 456; F. W. Clarke, Ber., vol. 14, p. 86; and E. Werner, C. S. Trans., 
vol, 51, p. 388, and vol. 53, p. 404. 
{ For instance, it was shown by me that potassio-calcium chromoxalate (Proc. Royal Soc., vol. xxi., 
p. 499) could be prepared by boiling calcium oxalate with Croft’s salt, the red potassium chromoxalate. 
This is a very striking reaction. Proc. Chem. Soc., p. 4, 1887. 
{ Chem. News, vol. 67, p. 255, 1898. 
§ Chem. News, vol. 27, pp. 47 to 270, 1872. 
2R 2 
