14(3 
DR. WALTER WAHL; PHYSICO-CHEMICAL DETERMINATIONS 
ScHAUM,* * * § who both considered the difference due to a different relative position of the 
transition-point with regard to the melting-point. In the case of enantiotropy the 
transition-point is situated below the melting-point, and the modification which is 
stable at temperatures below the transition-point cannot be melted ; but if we suppose 
the transition-point of a modification to lie above its melting-point such a transition- 
point could not be observed, and we would liave a case of monotropy. As pressure 
changes the melting-point and also the transition-point, it has been generally expected 
that it might be possible to change an “ enantiotropic ” body into a “ monotropic” by 
pressure, and ince versd. The pressures at wliich this ought to be possible were, in 
the case of sulphur, calculated by RoozEBOOM.t Tammann then found that sulphur 
really behaves in this way,| and determined the triple-point temperature and 
pressure, above which the monoclinic sulphur changes from an enantiotropic condition 
to a monotropic, as 153°’5 C. and 1,470 kg./cm.^ This experimental result in the 
case of sulphur has generally been thought to be a verification of the theoretic 
explanation given by Ostwald and Sciiaum,§ and it lias been expected that the 
change of an unstalile, monotropic modification to a stable modification also would 
take place at high pressures. 
lIoozEBOOM, in his systematical treatise of the application of the phase-rule, has 
pointed out tliat the explanation given by Ostwald and Schaum is a possible one, 
but not necessarily the true one.|| The experimental data given in this paper show 
that carbontetrabromide at all pressures remains an “enantiotropic” body, of wliich 
the modification, stable below the transition-point, cannot be caused to melt, and 
that a/3-blbrompropionic acid remains at all pressures a “ monotropic ” body, the 
unstable modification remaining unstable at all pressures. It remains, therefore, an 
open question whether the explanation given by Ostwald and Schaum for the non¬ 
existence of a definite transition-point, in the case of monotropic bodies, is the true 
one or not. And the deduction from this theory, that a monotropic body might be 
changed into an enantiotropic by pressure, which, if it were true, would at the same 
time prove the correctness of the theory, is opposed liy the liehaviour of the melting- 
point curves of a/3-bibronipropionic acid. 
The determination of the melting-point curves of the two modifications of a^-bibrom- 
proplonic acid is also of interest in another respect. Generally it must be expected 
* R. Schaum, ‘ Habilitationschrift,’ Marburg, 1897, p. 27. 
t Backhuis Roozeboom, ‘ Rec. Tr. Chim. Pays-Bas,’ 6, p. 314 (1887), and ‘ Zeitschr. f. Phys. Chem.,’ 
2, p. 475 (1888). 
I G. Tammanx, ‘Wikd. Ann.,’ 68, p. 675 (1899); ‘Ann. d. Phys.,’ 3, p. 178 (1900); and “ Kristal- 
lisieren u. Schmelzen,” pp. 269-275. 
§ Some other cases where Tammann claims to have found a trij^le-point at higher pressures (dimethyl- 
ether of oxalic acid, ortho-kresol) will be dealt with in a further communication, and in the case of acetic 
acid the curves given by Tammann do not intersect at the triple-point in the way required by theory. 
These substances, therefore, cannot be quoted as evidence in this case. 
|[ Backhuis Roozeboom, ‘Die Heterogenen Gleichgewichte,’ L, p. 160. 
