PHYSICS: P. W. BRIDGMAN 
513 
THE EFFECT OF PRESSURE ON POLYMORPHIC TRANSITIONS 
OF SOLIDS 
By P. W. Bridgman 
JEFFERSON PHYSICAL LABORATORY. HARVARD UNIVERSITY 
Presented to the Academy, August 7, 1915 
This note presents in a compact form by means of diagrams many 
of the essential facts concerning the effect of high hydrostatic pressure 
on the polymorphic transitions of 30 substances. Five of these dia- 
grams have been previously published/ eleven are to be published with 
greater detail in a forthcoming number of the Proceedings of the American 
Academy, and the rest await detailed publication. The experimental 
methods have been fully described previously. The investigation has 
been assisted in great measure by generous grants from the Bache Fund 
of the National Academy of Sciences and from the Rumford Fund of 
the American Academy of Arts and Sciences. 
The diagrams show the transition curves on the pressure-temperature 
plane of the several soHd phases, which are indicated by Roman numerals. 
The liquid phase, where it occurs, is denoted by an L. Notice that 
the temperature scale is changed in the diagrams for KHS04and H2O. 
NH4NO3 has one transition line not shown, of the ice type, beginning 
at —16°, and RbNOs has one at 219°, probably of normal type. The 
arrows on the transition lines indicate the directions in which the differ- 
ence of volume of the two phases decreases numerically. An a, (3 or Cp 
placed on one side of a curve indicates that the phase on that side of 
the curve has the larger compressibility, thermal expansion, or specific 
heat. In the detailed presentation of data, the numerical values of all 
these factors, as well as of the latent heat of transition and change of 
energy are given. 
The number of substances is perhaps sufficient to justify an enumera- 
tion of the relative frequency of different types of behavior. The first 
impression is one of bewildering complexity, it is obvious that the 
phenomena of polymorphism, even under high pressures, do not tend to 
any simple type. The apparent complexity is rather increased when 
one considers the diagrams of chemically related substances, such as 
the six nitrates, the four iodides, the two sulfocyanides, and acid sul- 
fates. It is possible, however, to detect traces of regularity among the 
nitrates and sulfocyanides by putting into correspondence the phases 
which belong to the same crystalHne system. 
The variety of shapes possible for the individual curves is in striking 
contrast with the case for melting curves. Every rising melting curve 
