212 



By the great difference in index of refraction of the isotropic and 

 of the crystalline liquid, phenomena of refraction occur at the limit, 

 and therefore the image of the NERNST-burner is broadened, deformed 

 or shifted. A temporary weakening of the thermo-current, therefore 

 a drop of the extinction-curve will be the consequence of this. 



In our opinion the slow rise of the extinction-curve when the 

 liquid-crystalline phase has proceeded from the isotropic-liquid phase 

 (the continuation of the bag-shaped drop) can be explained by 

 the fact that at the sudden turbidily a very disorientated state 

 appears, on which only very slowly the directing influence of the 

 glass makes itself felt. 



Sometimes the extinction-curve of p-azoxy-anisol showed a peculiar 

 drop at the transition from liquid-crystalline into solid. In fig. 1 

 this drop is represented. It occurred especially when the preparation 

 had been examined in a verj thin layer and was then strongly 

 undercooled. At macroscopic examination it appeared to us 

 that under these conditions greenish-yellow crystals were formed, 

 which we soon could identify with the meta-stable solid phase 

 already described by Lehmamn in 1890. ^) 



Finally from our curves the dependence on the temperature of 

 the extinction can be read. If we define ourselves to p-azoxy-anisol 

 (fig. 1) then a strong dependence on the temperature may be stated 

 in the ex-solid condition, in such a way that at rising temperature 

 the extinction decreases. Also the reverse effect, increase of the 

 extinction at falling temperature, could be established with certainty 

 for the ex-solid condition. In the ex-liquid condition the dependence 

 on the temperature is very much less evident, without any doubt 

 it exists however in the same sense as in the case of the ex-solid 

 state. Already in 1902 Schenck has performed measurings on this 

 question with the aid of the spectrophotometer of Glan, and has 

 only reached a negative result. But he examined the extinction for 

 yellow light, whereas our method gives the extinction for a mixture 

 of rays in which ultra-red dominates. 



So we thought it useful to examine the dependence on the tem- 

 perature also in another range of wave-lengths. We chose as such 

 the photographically active rays. The image of the NERNST-burner 

 was for that purpose formed instead of on the vertical slit of the 

 thermopile on the horizontal slit of a photographical registration- 



1) Perhaps it is well to remark that we succeeded in establishing the reversible 

 melting-point of this greenish-yellow phase at 108°. VoRLawDER as well as Schenck 

 doubt the validity of an analogeous result of Lehmann. 



