POLYMORPHIC TRANSFORMATIONS OF SOLIDS. 117 



There are three different methods of attack on the values for the 

 difference of compressibility, expansion, and specific heat for this 

 substance. In the first place, a direct measurement of the difference 

 of thermal expansion of the liquid and the solid I was possible at low 

 pressures. The phenol was so pure that the rounding of the corners 

 was very slight, and this value ought to be fairly good. Then direct 

 measurements were made of the difference of compressibility of I 

 and II along the transition line. These values are fairly good, espe- 

 cially at the lower temperature. And finally, at the triple point, we 

 have theoretically the means to calculate all the differences between 

 the two phases. But actually the various derivatives are not known 

 accurately enough to permit this calculation at the triple point. For 

 instance, a rigorous solution of the six equations at this point demands 

 that I is less compressible than II, whereas direct experiment shows 

 that it is more compressible. The values follow. At 77 kgm. the 

 liquid is more compressible, more expansible and has a greater specific 

 heat than I, as follows: 



Aa = 0.000025 ] 



A^ = 0.00027 [ 



ACp = 6.9 [kgm. cm./gm.] J 



A/? was found by direct experiment and the others were computed. 

 The value found by Block ^^ for A/3 is considerably less than that 

 found here, 0.00017 against 0.00027. 



At the triple point it was established by direct experiment that 

 I is more compressible than II. The difference of compressibility 

 increases somewhat at the lower temperatures. At the triple point 

 the difference is of the order of 0.000005. The difference of compres- 

 sibility between liquid and I calculated from this is about O.O4I5. 

 This value may probably be counted on with some certainty because it 

 is not much affected by changes in the value of Aan. The differences 

 in expansion between the two phases are, however, very susceptible 

 to errors in the compressibility. One cannot conclude with certainty 

 which of I or II is more expansible. The liquid is certainly more 

 expansible than I and the difference is of the order of 0.0002. More 

 than this cannot be stated. The differences of specific heats between 

 the liquid and the two solids are also very susceptible to slight errors ; 

 small changes in the constants may change the sign. The difference 

 of specific heat between the two solids is not so sensitive, however, 

 and it seems fairly certain that the specific heat of I is lower than that 

 of II, perhaps a surprising result. The difference is small, and of the 

 order of 0.3 kgm. cm. per gm. 



