( 76 ) 
same spot, where I found a fen stratum of 8 metres thickness, on 
March 20—21 1891 at the time of the IJzERMAN-expedition. 
At the same time I tender my hearty thanks to my friend, General 
G. M. Bleckmann, for his kind assistance, by which my request was 
transmitted to Mr. J. G. Larive through the valuable intervention to 
the Resident of Sumatra’s East coast. 
Leiden, May 29^ 1909. 
Physics. — u 0n Phosphorescence at very low temperatures.” By 
Henri and Jean Becquerel, and H. Kamerlingh Onnes. Com¬ 
munication N u . lJ0 a from the Physical Laboratory, Leiden. 
(Communicated in the meeting of April 23, 1909). 
$ 1. Introduction. It is known that in the case of the salts of the 
rare earths both in their crystalline form and in solution, absorption 
spectra with particularly clear and line lines can be obtained by 
the application of very low temperatures, and that the investigation 
of the optical and magneto-optical properties of these substances at 
the temperatures which can be reached by means of liquid air and 
liquid hydrogen opens up a new method for the study of selective 
absorption *). 
A problem, not less important than this,, is that of the emission 
ol light at temperatures below that of beginning incandescence. For 
certain substances, such as the uranylsalts 2 ), there exists such an inti¬ 
mate connection between the bands in the emission . spectra of their 
phosphorescence and the bands of their absorption spectra, that their 
distribution in each spectrum follows the same law, so that the bands 
m the one spectrum form the continuation of those in the other, 
while they have even two bands in common. Any peculiarity in the 
occurrence of one of the bands in any of the compounds periodically 
lecurs in all the other bands, as well in the emission as in the 
absorption spectrum. 
Thus, when the absorption spectrum of any of the uranyl compounds 
9 Jean Becquerel Gompt. Rend. 1906, 1907, 1908, 1909. le Radium t. IV. 
•• «, P_ 107, p. 328, p. 383. t. V p. 5; Physik. Zeitschr. t. VIII p. 632, P-929- 
Jean Becquerel and H. Kamerlingh Onnes, These Proc. Febr. 1908. p. 592. 
.omm. fr. the physic. Labor, at. Leiden n». 103. Gompt. Rend. t. CXLVI p. 62o. 
e Radium t. V p. 227. 
2 ) Edmond Becquerel Mem. de l’Ac. d. Sciences t. XL. Ann de Ch. et de 
ys. ue ser t. X p. 5. Henri Becquerel Gompt. Rend. t. GI 1385 p. 1252. 
