490 JOHN JOHNSTON AND PAUL NIGGLI 
affected by the presence of impurities if the system is homogeneous, 
and by uniform pressure, this effect being altogether analogous to 
that observed with melting-points, and subject to the same ther- 
modynamical formulation. So far as we are aware, no experi- 
mental study has been made of the influence of uniform pressure 
on the transition point of any system solid-solid of direct geological 
interest;* but if the appropriate data were known, this effect could 
be calculated with sufficient exactness from the equation 
AT Vissi) 
AP, 41-300 
whereby AT7,, the change of transition point produced by a change 
of pressure, AP, (reckoned in atmospheres), is expressed in terms 
of Q, the heat of transformation in calories per gram, 7, the tem- 
perature of transformation at 1 atm., and V, and Vg, the respective 
volumes (in c.c.) of 1 gram of substance before and after the transi- 
tion at J. Unfortunately, however, the requisite data, especially 
the values of Q, are known in very few instances, and are altogether 
lacking for the inversions of geologic significance. It is perfectly 
obvious, therefore, that one may just as well guess the final result 
as calculate it from the formula, using assumed values of the con- 
stants involved. 
In passing, it may be observed that there is no general parallel- 
ism between Q and the volume change AV=(Vg—V.). Thus 
potassium bichromate has an inversion point at about 240°, accom- 
panied by a large volume change but by only an inappreciable 
heat effect.2. On the other hand, during the inversion of cuprous 
sulphide at 79° considerable heat is evolved, and yet AV is only 
about 0.0001 c.c. per gram.3 
™ The effect of pressure on the equilibrium between the various solid forms of 
water has been investigated by Bridgman (Proc. Am. Acad., XLVII [1012], 441; 
Z. anorg. Chem., LX XVII [1912], 377); the case of sulphur, which was studied by 
Tammann (Krystallisieren und Schmelzen), is discussed in most textbooks. Some 
observations have also been made on the effect of pressure on liquid-crystalline trans- 
formations, which, however, resemble melting-points much more than they resemble 
any transitions of geologic interest. 
2 Mitscherlich, Pogg. Ann., XXVIII (1833), 120. 
3 Hittorf, ibid.. LXXXIV (1851), 1; Tammann, Krystallisieren und Schmelzen 
(Leipzig, 1903), p. 40. 
