( 59Ö ) 



general their intensity increases. Tlie bands whioh decrease in inten- 

 sity or wiiicli vanisli altogether, are exceedingly few in number. 

 The measurements on anomalous dispersion in the neighbourhood 

 of some bands of tysonite had proved that this increase of inten- 

 sity is not only the consequence of the bands becoming narrower, 

 but also of a modification which, according to the theory of electrons 

 on the supposition of quasi-elastic forces, indicates the increase of 

 the dielectric constant in every band, and implies that the number 

 of electrons which determine such a band, has increased. 



Passing to the temperature of Hquid hydrogen (T = 20)°, we saw 

 some bands continue to increase in intensity, but also others which 

 showed an increasing absorption with fall of temperature down 

 to that of liquid air, decrease both in intensity and in breadth. There 

 are even bands having appeared in liquid air, which become almost 

 invisible in liquid hydrogen. An example of such a change with 

 the temperature is furnished by the bands 523.5 and 479.1 of tysonite. 



The measurements of the anomalous dispersion in the neighbourhood 

 of these bands had shown that the electrons belonging to these bands 

 are about twice or three times as numerous at the temperature of 

 liquid air as a*^ the ordinary temperature. In liquid hydrogen the 

 number has already become very small, and at the temperature of 

 solid hydrogen (14°) hardly any electrons of this kind take part in 

 the motion. Fig. 1, PI. II, which represents the compensator fringes ') 

 in the neighbourhood of bajid 523,5 of tysonite at ditferent tempe- 

 ratures and with ditferent thickness, allows us to measure the distur- 

 bance in the fringe with regard to height and breadth. Figs. 2 and 

 3, which we treat in ^ 8 and -', and which represent the magneto- 

 optic phenomena, may elucidate this. 



§ 4. Mdüiniam of hitenslti/ of ever// hdiul for a definite teuipem- 

 tare. It follows from the foregoing that several bands pass through 

 a maximum of intensity with decrease of the temperature. In general 

 the place of this maximum is ditferent for ditferent bands. When in 

 the experiment with tysonite described in § 3 we wait till the last 

 traces of hydrogen evaporate from the crystal, immediately after 

 wheu the temperature of the crystal rises, the band 523,5 is seen to 

 greatly increase in intensity. Without doubt the maximum for this 

 band lies at a temperature not far above the boiling point of hydrogen. 

 All the crystals of xenotime, tysonite, parisite, apatite, monazite, didy- 

 mium sulphate, praseodymium sulphate, neodymiuin sulphate, exhibit 



b Jean Becquerel, Radium IV no. 9 p. 328. 



