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, H6Cr2( 0204)5 by 

 determining- the constitution of the blue salts, but the acid itself was not isolated. 

 Werner found the red salts to be represented by M20r2(C204)4. From seven 

 different reactions I deduced the existence of chromoxalic acid, and showed the 

 relationship of the red to the blue salts. f But Lapraik showed from tlie 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. J 



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 sliowed 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, samai'ium, and thulium has been accomplished, and therefore the 

 didymium salts must be regarded more or less as mixtures. 



* Comptes Eendus, vol. 16, p. 456; F. W. Clarke, Ben, vol. 14, p. 36 ; and E. Werner, C. S. Trans., 

 vol. 51, p. 383, and vol. 53, p. 404. 



f For instance, it was shown by me that potassio-oalcium chromoxalate (Proc. Eoyal 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. 



X Chem. News, vol. 67, p. 255, 1893. 



§ Chem. News, vol. 27, pp. 47 to 270, 1872. 



2K2 



