AT HIGH PEESSURES BY OPTICAL METHODS. 
139 
larger extent the equilil^riuni can he exceeded when no crystal germs of tlie 
modification, stable at that temperature and pressure, are present. 
The curve P 5 runs nearer and nearer to P;, as the temperature of the transition- 
point rises, which means that the unstable region grows gradually smaller at high 
temperatures, and probably would disappear at about 1,700 kg./cm.^ 
As seen from both series of measurements (Table III.), which were made at a 
different rate of cooling, the transition-point is exceeded to a liigher degree at high 
temperatures than at low. The unstable region in the case of the transition from 
the isotropic to the anisotropic modification would thus 1)ecome larger with increasing 
temperature, which is just opposite to what takes place in the case of the inverse 
transition, as we have seen, and is not what one should expect. The extent to which 
the equilibrium curve may be crossed on cooling at constant pressure seems also to 
depend to a certain amount on the rate at which the cooling takes place, but this 
circumstance does not furnish an explanation for the general trend of the curve P^, 
as this remains similar in direction in the difierent series of measurements. The 
conclusion to be drawn, in the present state of knowledge, would thus be that 
pressure influences this transition from the isotropic state to the anisotropic in such 
a way as to facilitate a retai'dation of the transition. 
The distance between the curves Pg and P 4 remains quite constant, and this shows 
that the conditions of transition of the one form of the substance to the other form 
are not influenced by pressure as far as regards the degree to which the equilibrium- 
curve is crossed, just as the conditions of crystallization are not changed by pressure, 
as far as regards the degree of super-cooling, as pointed out on p. 136. This may be 
considered a proof that tlie curve for the velocity of crystallization and the curve 
for the velocity of transition of polymorphic crystal forms of a substance are similar 
in shape, a similarity which might be expected. So far a curve of velocity of 
transformation has only once been actually measured, that is in the case of the 
transformation of a “ monotropic,” unstable modification to the stable one (Benzo- 
phenone III. to I.).* * * § 
This group of four curves, limiting the areas in the diagram of state where a false 
equilibrium exists between the two crystalline forms of carbon tetrabromide or where 
spontaneous formation does not occur, are quite analogous to those found by 
Tammann in his latest researches on phenol,! silver iodide,;}; and ice.§ In these cases 
the equilibrium curve is found to split up at low temperatures into four different 
curves, corresponding to the above. The distance between the curves is at low 
temperatures very large, and the transformation at last entirely ceases to take place 
when the temperature is sufliciently lowered. The transition in the case of carbon 
* G. Tammann, ‘Zeitschr. f. Phys. Chem.,’ 29, pp. 58, 67 (1899). 
t G. Tammann, ‘Zeitschr. f. Phys. Chem.,’ 75, pp. 75 (1911). 
I G. Tammann, ‘ Zeitschr. f. Phys. Chem.,’ 75, p. 733 (1911). 
§ G. Tammann, ‘Zeitschr. f. Phys. Chem.,’ 72, p. 602 (1910). 
