290 Hartiey—TZhe Action of Heat on the Absorption Spectra and 
Didymium Potassium Nitrate—Thick wedge-cell, sunlight. 
From 380° to 60°. 
Spectrum begins with a weak band at d 594. 
A feeble absorption extends from 594 to 588. 
Absorption Band— 
588 to 581 
Absorption Band— 
533 to 530 
Absorption Band— 
523 to 521 
Narrow Adsorption Bands or lines— 
488 
478 
Absorption Band— 
454 to 449 
At 100°, 
Spectrum begins with a dark band at » 600. 
Absorption Band— 
600 to 581. Feeble, narrow. 
Absorption Band or line— 
543 
Absorption Band— 
534 to 530 
Absorption Band— 
528 to 521 
Absorption Bands or lines— 
488 
478 
Absorption Band— 
454 to 449 
The measurement of the decided black band in the red was at 588 on the side 
of least refrangibility, and a faint shade extended farther in this direction to 594. 
On heating the solution the band thickened out to 600. In the thin wedge-cell 
the following measurements were the only ones possible :— 
At 20°. : 0 : 588 to 581 
At 100°, : : : 588 to 581 
Band. Narrow Band. 
53838 to 530 455 
533 to 530 455 
The narrow band at 455 was described in the note-book as a faint shade. 
It will be seen that the principal band, which widens out from 630 to 581 when 
the solution is heated in the thick cell, here remains quite unchanged at 100°. 
The measurement at 543 is a feeble band like a line, not visible in the cold solution. 
It may be mentioned that, at 588 and 581, the bands belonging to praseodym 
and neodym meet. The bands at 488 and 454 to 449 are probably due to 
erbium. 
Didymium Acetate.-—Thin wedge-cell. 
At 20°. 
Rays visible at 706. 
Absorption Bands— 
J88y 6 56 0 o  (étsha) to O63 
Orange, . . . 588 to 572 
533 to 529 
FERRY {509 to 519 
Bio, ie to 488 
481 to 478 
Edge of a band at 476. 
Edge of a band, 466. 
Broad Absorption Band— 
Blue,*. . . . 458 to 448 
A dark line, 456. 
At 100°. 
Absorption Bands— 
704 to 627 
598 to 577 
Beyond this all was obscured. 
* Probably an erbium band, 
