406 WRIGHT AND HOSTETTERI CRYSTAL GROWTH 



tions represented by the thermodynamic equations obtain. In 

 case both the solid and the liquid are under the same pressure 

 (hydrostatic compression) there is no difficulty in conceiving of 

 a thermodynamic cycle which is reversible and from which the 

 ordinary Clapeyron equations for uniform pressure can be de- 

 rived. But in the case of a crystal under load the crystal phase 

 is under a state of pressure different from that of the liquid; the 

 pressure throughout the system is no longer uniform; in the pas- 

 sage of the material from one state to the other a pressure factor 

 enters and tends to render uncertain the reversibility of the proc- 

 ess. Nevertheless, its reversibility has been assumed either 

 directly as self-evident, or indirectly by postulating conditions 

 (semipermeable membranes, etc.) which are experimentally not 

 attainable ; but no direct evidence of the reversibility of the proc- 

 ess has heretofore been offered, so far as the writers can ascer- 

 tain, in support of this fundamental assumption. 



The relations between a strained solid and its liquid have been 

 discussed at various times by different authors under the head- 

 ings of " non-uniform," " one-sided," "differential," or " unequal" 

 pressure. Each of these titles is suggestive of the fact that in 

 these discussions pressure was assumed to be unequal on the 

 various parts of the system being investigated The equations 

 between the various factors involved in the treatment of such a 

 system have all been developed from the theoretical side, and 

 none of them has ever been subjected to a thorough experimental 

 demonstration. This lack of experimental evidence on the sub- 

 ject probably accounts for the diversity of effects postulated for 

 a given pressure acting non-uniformly; — the effects so postu- 

 lated vary a thousand-fold and should have been submitted to 

 experimental test long ago. 1 



Perhaps the first investigator to study the effects of non- 

 uniform pressure was James Thomson 2 who in 1862 made a careful 

 distinction between stresses applied only to the solid phase of 



1 It may be mentioned here that preliminary results on the influence of non- 

 uniform pressure on solubility indicate that the effect so produced is very small. 

 Hostetter, J. C. J. Wash. Acad. Sci., 7: 79. 1917. 



2 Thomson, J. Phil. Mag., (4) 24: 395. 1862; Proc. Roy. Soc, 11: 473. 



