( 597 ) 



tendency to assnine the cliaracter of gas spectra when the teinperatnie 

 decreases. Some absorption lines of praseodymium and neodymium 

 sulphate, cleared of l)road bands that covered them, are even finer 

 than the D-Wnes. 



§ B. The a/)proach to tt limit of the (Jonbk refraction of cri/sta/s 

 in the non absorbed parts 0/ the spectrum. If we watch the bands, 

 by the aid of which the double refraction is investigated, with change 

 of temperature, w^e observe the following. If the ciystal is heated above 

 the ordinary temperature, they are greatly displaced. When the 

 temperature is lowered to that of liquid air they move in the 

 opposite direction. For a crystal of tysonite we have also examined 

 them with further cooling with liquid hydrogen. In spite of the great 

 difTerence of temperature the displacement is then hardly perceptible. 

 This may point to the fact that the difference of the expansion of 

 the crystals in the different directions a|)proaches a limit at very 

 low temperatures. 



§ 7. Connection of the chan<je of the absorption bands occur rin(j 

 at very loiu temperatures with the electronic theory. Already in § 3 

 we pointed out the connection of the change of the bands with that 

 of the number of the electrons which are concerned with a certain 

 band according to the electronic theoi-y coupled with the assumption of 

 quasi-elastic forces. The experimental problems raised by § 3 and § 4 

 may be defined as follows in the language of this theory ^) : to 

 determine as functions of T on one side the number and on the 

 other side the damping coefficient (proportional to the width of 

 the band) of the electrons which belong to a certain band. We 

 might make use of the position of the maxima to find mutually 

 related bands, in the first place in the different s|)ectra of one crystal. An 

 investigation into the connection between what we already know about 

 these functions and what the change of the electrical i-esistance of the 

 metals leads us to expect about the action of forces exercised by the pon- 

 derable substance on the electrons natm-ally suggests itself.^) At very low 

 temperatures we shall no longer be justified in considering the electrons 

 as a |)erfect gas, but we shall rather have to compare them to a vapour 

 which precipitates on parts of the atoms (dynamides (Lenakd)), and soli- 

 difies at still lower temperature '). When we approach these centres the 

 paths of the electrons are subjecteil to changes which modify the free 



M Gf Jean Becquerel. P»adium 1. c 

 -1 Irl. Kameruncih Onnes. Loc. cit. 

 ^) A metal would become liansparenl at very low temperature. 



