212 Johnston and Adams — High Pressures on the 



establishing a parallelism between the brittleness of a metal 

 and any other of its properties, just as it appears to be value- 

 less when applied to the case of cleavage or of gliding planes 

 in crystals. The discussion of these phenomena formed no 

 part of the plan of the present paper ; we wish, however, to 

 state explicitly our conviction that the occurrence of cleavage 

 and of gliding planes, which renders possible some degree of 

 permanent distortion of a crystal, is a phenomenon which may 

 well be totally different in character from the permanent 

 deformation of a crystalline aggregate ; further, that the views 

 which we advocate with reference to the latter case and the 

 conceptions which petrologists make use of to account for 

 the former case,* are by no means mutually exclusive, and 

 may indeed supplement one another. 



Calculation of the Effect of Unequal Pressure Upon the 

 Melting Point of Crystalline Substances. 



In order to determine the magnitude of the effect of unequal 

 pressure upon the melting point of crystalline substances we 

 must make use of thermodynamical reasoning, which is, as we 

 have noted above, applicable only to those processes which 

 may be classed as reversible. f JS[ow it might at first sight 

 appear that this particular process does not satisfy the condi- 

 tions for reversibility ; for, according to our hypothesis, it 

 results in a practically irreversible permanent deformation of 

 the body as a whoie. But this total deformation we regard as 

 the aggregate effect of a large number of what might be termed 

 individual meltings, and there is no more reason to doubt the 

 thermodynamic reversibility of these individual meltings than 

 there is to question the validity of applying thermodynamics 

 to the case of melting under uniform pressure. TammannJ 

 indeed has stated that he doubts the thermodynamic admissi- 

 bility of the derivation of the formula for the lowering of 

 equilibrium temperature by unequal pressure, but this view of 

 the matter is not shared by all of those who have considered 

 the question ; for instance, it is not shared by Roozeboom,§ 

 OstwaldJ LeChatelier,T or by JSernst,** who writes "Die 

 thermodynamishe Behandlung auch dieses Falles ist exakt 

 durchzniuhren." 



*It may be noted that these conceptions are of little help in accounting 

 for, or correlating, the experimental observations discussed in the body of 

 the present paper. 



fSee footnote p. 207. 



| Ann. Phys. (4), vii, 198, 1902; Krystallisieren und Schmelzen (Leipzig, 

 1903), pp. 173-81. 



§Heterogene Gleichgewichte, i, 213. 



|| Lehrbuch der Allgemeinen Chemie, 2 Aufl., ii, II, 374-379. 



IT Zs. phys. Chem., ix, 338, 1902. 



**Theoretische Chemie, 5 Aufl., p. 663. 



