AT HIC4H PKESSURES BY OPTICAL METHODS. 
145 
case the pressures at which rapid growth and the formation of growth-structures 
takes place were also determined, and are given as Py in the same talde. Several 
points on the curve, where suddenly rapid melting sets in, were also determined, and 
are given as pressures, P^. An interesting feature of this melting-point curve, P 5 , is 
that it at higher pressure l^ends slightly towards the temperature axis, which the 
curve of the pure substance does not do. This behaviour of the curve will be discussed 
at a later stage with regard to its hearing upon the problem of the influence of 
impurities upon the form of the melting-point curve. 
Table VII. 
Temperatures 
(corrected). 
Pressures, P4. 
Pressures, P5 
(corrected). 
Pressures, P^. 
° C. 
(51 
1 kg./cm.^ 
- ) 
53 
(al30Ut 300 kg./cm.2) 
450 „ 
900 kg./cm.- 
55 
575 
1,000 
58-6 
650 kg./cra.2 
80t» 
1,200 „ 
61-1 
800 
980 
1,350 
65-24 
1,050 
1,200 „ 
1,550 
70-14 
— 
1,420 
As seen from the diagram, fig. 7, and from the Tables V. and VI., the melting-point 
of the stable modification is raised one degree by a pressure of 51'28 kg./cm.^, and the 
melting-point of the unstable modification one degree by a pressure of 53‘48 kg./cm.^. 
The melting-point curves will thus run further apart as the pressure is increased, and 
the, at ordinary 2 ^ressure, unstable modification will at all pressures melt at a loiver 
temperature than the modifiication melting at 64° C. and remain unstable, 
7. Discussio'ri of Residts. 
The fact that the melting-point curves of a/ 8 -hibrompropionic acid do not intersect 
at high pressures, taken together with the somewhat similar relation between the 
melting-point curve and the transition-point curve of carbon tetrabromide, as given 
in the diagram, fig. 6 , are of interest with regard to the theoretical explanations 
given as the cause of the occurrence of two different classes of polymorphic substances, 
the “ enantiotropic” and the “ monotropic.” These terms were first used by Lehmann, 
who called those polymorphic substances which pass reversildy into each other at a 
given transition-point “ enantiotropic,” and those which have one stable and one or 
several unstable modifications “ monotropic.” Later an explanation for this different 
behaviour of the two classes was given almost simultaneously by Ostwald* and by 
* W. OsTWALD, ‘ Zeitschr, f. Phys. Chem.,’ 22, p. 312 (1897). 
VOL. CCXII.—A. 
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