( 601 ) 



electrons. Therefore we meet with disturbances in the magnetic 

 rotation which are different for the different bands, and whose 

 effects are superposed. Thus two bands i)laced side by side, one of 

 positive and the otlier of negative electrons, may give rise to distur- 

 bances in opposite direction in the dispersion of magnetic rotation. 

 It is periia|)S to this that we must look for the explanation of what 

 is observed in band 577 of tysonite, which is clearly double in 

 liquid hydrogen. 



In general we may say that with I'egard to the theory of the 

 magnetic rotation for absorption bands, the conclusions drawn from 

 experiments at the ordinary temperatm-e do not lead to a definite 

 result. For at the ordinary temperature it is uncertain whether we 

 have really to deal with a simple band. On the otlier hand at the 

 low temperatures, at which the bands become narrow, and their 

 change in the magnetic field may be closely followed, it is easy to 

 find the true explanation of the different types of disturbances in the 

 magnetic dispersion of rotation for the bands in the different cases. 



§ 12. Magnetic rotatory power of the paramagnetic crystals. 

 One of us (J. B.)^) had previously shown that the negative magnetic 

 rotatory power of the crystals of tysonite and parisite increases con- 

 siderably with decrease of temperature. The rotatory power is about 

 inversely proportional to the absolute temperature. If this is brought 

 into connection with the law of Curie that the paramagnetic sus- 

 ceptibility is inversely proi)ortional to T, it appears that the negative 

 rotation of these crystals is probably a consequence of the increase 

 of the paramagnetic polarisation of the crystal. 



If these crystals are placed in liquid hydrogen we find that the 

 increase continues in the same way with decrease of temperature, 

 and the rotatory power rises to exceedingly high values. The exact 

 numbers will be given later, but in round numbers the rotation of 

 the plane of polarisation of the blue light amounts to 150'^ for a 

 plate of tysonite of i mm. in a field of 10000 Gauss at the boiling 

 point of hydrogen. Xenotime, which gives a very slight rotation at 

 the ordinary temperature, shows a considerable rotatory power in 

 liquid hydrogen. 



§ 1.^. Connection between the phenomena of the asymmetry of the 

 left- and right-handed polarized components by the magnetic field at 

 very low temperatures, and the electronic theory. 



In connection with § 4 the phenomena taken togethei- give rise 



ij Jean Becquerel, Radium. Tom. V, N". 1, p. 5, 1908. 



41* 



