( 592 ) 



h.hI in order ^ij,-^ii,. ^ O at — 252° by 0,0000013 to that 



IK' 



((a = 0,0086614. 



The ürst vahie wliicli has l)eeii derived witliout extrapolation and 

 which is therefore the most reliable, appears to agree perfectly with 

 the one derived by me from the isothermals in § 1. 



With regard to the method of derivation followed here we may 

 remark that it allows of a fairly large accuracy. Thoiigli the certainty 

 of the determinations of temperature on which it is based may be 

 doubted to the absolute value, yet the only difference which comes 

 into account here is known with sufficient certainty. The calcu- 

 lation mentione<l above therefore iiot only gives an explanation of 

 the too large differences found l>y Travkrs, Skntkr and Jaoqukrod, 

 but is also a welcome control for the coefficient of [pressure variation 

 of helium found in section 1. 



Physics. — " The absorption .spectra of the compounds of the rare 

 earths at the temperatures obtainable with liquid lujdroqen, 

 and their change by the magnetic field"', by .Ikan Bkcquerel 

 and H. Kamerlingh Onnes. Communication N". 103 from the 

 Physical Laboratory at Leiden. 



§ 1. Introduction. The investigations of one of us (J. B.) ') proved 

 that the absorption spectra of the compounds of the rare earths, 

 cooled down to the temperature of liquid air, may serve to acquire 

 new data for the nature, the number, and the motion of the electrons 

 which play a part in the formation of these spectra. So it seemed 

 to us of great importance to continue these investigations at the 

 temperatures obtainable with liquid hydrogen, which are so many 

 times lower and seem particularly adapted ") to reveal the forces which 

 the ponderable substance exerts on the electrons. For this purpose 

 the apparatus used at Paris foi- the observation of the spectra were 

 conveyed to the cryogenic laboratory at Leiden, so that we were 

 enabled to obtain some three hundred of spectrograms which re- 

 present the observed phenomena. The study of these photographs 

 will take a long time ; we shall therefore confine ourselves on this 

 occasion to the communication of some facts which immediately draw 

 the attention. 



1) Jean Becquf.rel, Radium IV. 9, p. 328 and IV, 11, p. 385 (1907). 

 2j H. Kamerlingh Onnes, The impoilance of accurate measurements at very low 

 temperatures. Comm. of the phys. lab. of Leiden Suppl. ho. 9, p. 25 sqq. (1904). 



