288 Harriey—The Action of Heat on the Absorption Spectra and 
The Spectrum of Didymium 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 
erystallized. 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 sulphate, 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, DiCl,;2H,0.—Thick wedge-cell. The absorption bands 
are numbered, 
Banyp I. (Red). N d 
INGEN oe | oo GS to GY 
MUO, 5 0 oo o GH to GB 
There is an erbium band at 6671 and another at 6839. 
Banp II. (Orange). 
ING AW, : 3 . 590 to 577 
MOO, o oo o o RD % BY 
Bands of both praseo- and neodymium come together here. Pr. 5963 and 5886. 
Neo. 5824 and 5748. 
Banp III. (Green). 
INS AV). : ; . 540 to 529, extending to 526 
At100, . . . . 540 to 582, 1» 528 
Neodymium occurs at 5318, and erbium at 5364. 
Banp IV. (Green). 
Ati 2 0S Sauee : : . 521 to 519 
IMs WOO, : . . 522 to 519 
Thalén gives a line of erbium 522°5 to 518. 
