Physical and Chemical Behavior of Solids. 207 



In the discussion of the effects of pressure on solids,* it is 

 of the utmost importance to notice that the effects of compres- 

 sion depend upon its mode of action ; they are different, 

 namely, according as we are dealing with pressure uniform in 

 all directions (true hydrostatic pressure or cubic static pres- 

 sure) or with a mode of compression which does not satisfy 

 this condition ;f in other words, the effects vary — as indeed 

 is almost obvious — according as the solid retains its form 

 unchanged or undergoes deformation. 



It is also essential to note that thermodynamic reasoning is 

 applicable only to those processes which are, ideally at least, 

 reversible, but not to irreversible processes or changes of state.:]; 

 Moreover, we must recognize that even with reversible pro- 

 cesses we can draw useful conclusions only when the conditions 

 are such that the rate at which equilibrium can be established 

 is appreciable, or in other words, only when we are dealing 

 with cases of true, as distinguished from apparent or false, 

 equilibrium. 



The importance of these distinctions has not been properly 

 appreciated by many writers. In fact, this neglect is respon- 

 sible for most of the existing confusion concerning the effect 

 of high pressures and the significance of this effect in geolog- 

 ical phenomena. In view of this confusion it has seemed to 

 the writers necessary to discuss at some length the differences 

 between the effects of uniform and of non-uniform pressures 

 and to go somewhat fully into the experimental work on which 

 some of the assertions in the literature have been based, if we 

 are to have a clear notion of the nature and range of the con- 

 clusions which may properly be drawn from this work. 



The General Effects of Uniform Pressure. 



The effects produced by pressure which is uniform in all 

 directions are in general completely and easily reversible ; the 

 original state of the system being regained when the pressure 

 is removed. Consequently the magnitude and direction of 



* The term solid we use throughout • this paper to denote a crystalline 

 aggregate. 



f A non-uniform compression can always be resolved into a uniform pres- 

 sure and a shearing stress, the latter being the effective factor in producing 

 deformation or distortion. 



X A change of state, or the process by which it is effected, is called 

 reversible when the system can be restored to its original condition without 

 giving rise to any residual effect in the surroundings. A change in state, or 

 a process, is irreversible when this is not the case (e. g. the production of 

 heat by mechanical friction or by electrical resistance). After an irreversi- 

 ble change has taken place, it is in general possible to restore the system to 

 its original condition, but only by doing upon it a larger quantity of work 

 than was obtained from it, so that the original condition of the surroundings 

 is not reproduced. 



