( 596 ) 



similar phenomena. Tlje green line 523,5 of neodjmium which is 

 exceedingly line and sharp at 7'= 20^, has almost vanished at 7'= 14°, 



We have further examined the influence of the fall from 7'= 91° 

 to T = 58"^ by immersing the crystals in liquid oxygen boiling at 

 the airpump. The change in this region is only slight. This confirms 

 the conclusion drawn from what was observed in heating from 

 7^= 20° upwards that the maximum must lie near this latter tem- 

 perature and at all events far below 7'= 58°. 



Naturally the question obtrudes itself whether those few bands, 

 whose intensity diminishes between the ordinary temperature and 

 that of liquid air, do not also pass through a maximum either between 

 r=290° and 7'=:95^ or at a temperature above T= 290°. It 

 will be difïicult to decide the question, because in consequence of 

 the broadening and overlapping of the bands the ciiange of each of 

 these bands in itself escapes observation. 



§ 5. Change in iindtli. In the previous experiments') it had been 

 found generally valid for all bands measured down to the tempe- 

 rature of liquid air, that the width of the bands was proportional 

 to the square root of the absolute temperature. This is the law 

 which for the case of a gas may be deduced from the formulae 

 formerly developed by Lorentz '') 



When we pass io the temperature of liquid hydrogen this law 

 appears to be no longer valid for some bands, whereas for others 

 tlte order of magnitude of the change seems to remain the same. 

 In the figures 1, PI. II obtained by the method of the compensator 

 fringes, it is xery clearly to be seen, that 523.5 of tysonite is not half 

 as broad at 7'= 20° as at 7'= 85°, as the law of the y/T would 

 require. And it was this very band which had served to show 

 experimentally, that this law held down to 7'= 85° with a high 

 degree of approximation. 



The question whether there is a minimum of iridth, could not 

 be solved yet. At first sight some bands do not seem to contract 

 any further between 7'^ 20"^ and 7'== 14", two of xenotime seem 

 even to get wider. 



With regard to the totality of the phenomena of change of width 

 in liquid and solid hydrogen we may further observe that in these 

 even more than in liquid air^) the spectra manifest a pronounced 



1) Jean Begquehel, Radium IV no. 9 p. 328. 



^) H. A. Lorentz. Kon. Akad. v. W. VI p 5n(i and p. 555 (1898). 

 ■') 'I hal tysonite and xenotime have this tendency has been observed by Jean 

 Becquerel, Radium 1. c. 



