140 
DR. WALTER WAHL: PHYSICO-CHEMICAL DETERMINATIONS 
tetralH'omide is, on the other liand, one which takes place more readily than in most 
cases of enantiotropic transformation which are known, and the possibility of 
studying the same phenomena of false equilibrium ” in a substance like this 
indicates the sensitiveness of the method. 
As seen from the Tables I. and II., 16 kg./cm.^ suffice to raise the melting-point of 
CBiq 1°, whereas the transition-point is only raised O'5 degree by this pressure. The 
melting-point curve and the transformation-point curve will, therefore, not intersect 
at high pressure to form a triple-point. Tlie consequence of this direction of the 
melting-point curve and the transition-point curve is that the anisotropic mono¬ 
clinic modification ofi carlmi tetrabromide cannot he cmised to melt, even at any 
pressure. The theoretical triple-point deduced from these curves would lie at low 
temperature and high negative pressures (0° and —1500 kg./cm.^). A case like this 
has not been experimentally met with earlier, but the possibility of the occurrence of 
such a case has been theoretically foreseen by Backhuis Boozeboom in his treatise 
on the applications of the phase-rule.^'' 
6. I'he Diagram of State of rxd>-hil>rompropionic Acid, CHsBr.CHBr.COOH. 
Among “ monotrope ’’-polymorphic substances bibrompropionic acid is probably 
unique in this respect, that it melts twice when slowly heated. The unstable 
modification of lower melting-point is nearly always obtained when the fused 
substance is allowed to cool, and on heating again it melts at 51° C., but at a few 
degrees higher the stable modification crystallizes out of the “ melt ” and then on 
further heating melts at 64° C. This behaviour was described in detail by Tollens,! 
who first prepared this substance, and it has later been studied by Lehmann.| The 
crystals of both these modifications have been measured l)y Zepharowich,§ who 
found that both crystallize in the monoclinic system. 
This behaviour of a/3-bibrompropionic acid, together with the comparative!}^ great 
stability of the “ unstable ” modification, made it probable that it, in this case, would 
be possible to determine tiie melting-point curve of two difterent modifications of a 
polymorphic substance, which would he of interest, as hitherto only melting-point 
curves of stable modifications have Ijeen measured.|| Before giving the results of 
* H. W. Backhuis Roozeboom, ‘ Die Heterogeneii Gleiehgewichte 1.,’ p. 185, fig. tl (Braunschweig, 
1901). 
t Tollens, ‘Liebig’s Ann.,’ 167, pp. 222, 337. 
I 0. Lehmann, ‘ Molekularphysik,’ L, p. 690. 
§ Zepharowich, ‘ Jahresber.,’ 1878, p. 693. 
II Tammann’s statements about melting-point curves of alleged unstable modifications of j;)-xylol, 
carbon tetrachloride, acetic acid, diethylamin, trimethylcarbinol, and ethylendibromide (‘ WiED. Ann.,’ 
66 , p. 473, and 68, pp. 553, 629) have been withdrawn by him in his later publication of the measurements 
in “ Kristallisieren u. Schmelzen.” 
