288 Hartley — The Action of Heat on the Absorption Spectra and 



The Spectrum of Didyniium Chloride. 



The solution made in 1875 had merely the cerium separated from the other 

 rare earths by the process of Mosander. The didymium hydroxide was pre- 

 cipitated and dissolved in hydrochloric acid, after which it was evaporated and 

 crystallized. The earths, such as erbia, samaria, yttria, and the like, were not 

 removed, but there was an imperfect separation of lanthana. 



Measurements were made on each side of the broad bands both at 20" and at 

 100°. Accordingly they are not exactly comparable with those made by Lecoq 

 de Boisbaudran and others, who adopted a reading at the maximum of absorption 

 or at the centre of each band, because, from the absorption developing more 

 strongly on one than the other, the strength of the solution alters the position 

 of the mean reading. There is this also to be said, that solutions of different 

 strengths yield different spectra ; and it was also found that solutions containing 

 the same quantity of didymium hydroxide converted into different salts yielded 

 bands differing in intensity or position. This is partly due to differences in 

 solubility, the suljjhate, for instance, being much less soluble than the chloride, and 

 in each case saturated solutions were used. But even when the solutions were not 

 saturated, but diluted so as to contain the same quantity of base in equal volumes 

 of liquid, the chloride differed from the sulphate, the nitrate, and the acetate. 



Didymium Chloride^ DiCl2'2H20. — Thick wedge-cell. The absorption bands 

 are numbered. 



Band I. (lied). A. x 



At 20% . . . 676 to 667 



At 100°, . . . 685 to 667 



There is an erbium band at 6671 and another at 6839. 



Band II. (Orange). 



At 20° 590 to 577 



At 100°, . . . . 592 to 577 

 Bands of both praseo- and neodymium come together here. Pr. 5963 and 5886. 

 Neo. 5824 and 5748. 



Band III. (Green). 



At 20°, . . . . 540 to 529, extending to 526 

 At 100°, . . . 540 to 532, ,, 528 



Neodymium occurs at 5313, and erbium at 5364. 



Band IV. (Green). 



At 20°, . .521 to 519 



At 100°, . .522 to 519 



Thal^n gives a line of erbium 522 5 to 518. 



