Physical and Chemical Behavior of Solids. 253 



only), and its density. The viscosity is of minor importance, 

 since it affects only the rate with which the final condition is 

 attained, but not the position of equilibrium. The viewpoint 

 adopted in this paper serves to harmonize the outstanding dis- 

 crepancies in the literature pertaining to the effect of high 

 pressure on solids. 



Under geological conditions, high uniform pressure is always 

 accompanied by fairly high temperature, which is a very much 

 more efficient agency in effecting changes than high uniform 

 pressure is. The effect of unequal pressure is, as we have 

 seen, many times as great as that of uniform pressure ; never- 

 theless even with unequal pressure, the effect of change of 

 temperature is so very important that the two factors must be 

 considered simultaneously. Moreover, the extent to which a 

 reaction proceeds (for example MSiO t + C0 2 K > MC0 3 + Si0 2 

 in presence of water*) depends upon the relative solubilities 

 (or rather relative insolubilities), under the particular condi- 

 tions, of the various substances concerned in the reaction. 

 The effect of uniform pressure on solubility is usually insignifi- 

 cant; that of temperature, on the other hand, is commonly 

 large and very different for different substances. Hence, tem- 

 perature alone may easily cause the reversal of a reaction (as 

 in the case above) and in this regard far outweighs in general 

 the effect of unequal pressure ; with uniform pressure alone 

 such a reversal is unlikely. 



Finally it is to be noted that we can determine the effects 

 of compression on a solid system only if we can define the 

 character of the compression (with reference to its approach to 

 uniformity or otherwise) as well as its magnitude ; and even 

 then, only when the requisite thermal and other data, charac- 

 teristic of the system, are available. But these data, when 

 once they are available, should — at least if the viewpoint of 

 the present paper proves to be satisfactory — provide a basis for 

 attacking the very fundamental question of the limiting stress- 

 difference at which flow sets in for any solid aggregate, and 

 the variation of this limit with the temperature of the material, 

 and hence, in the case of a rock mass, with its depth below the 

 surface of the earth. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C, January, 1913. 



* If no solvent is present, the position of equilibrium depends upon the 

 relative volatility of the various substances under the specified conditions. 

 This statement is exactly analogous to that made in the text, if we remember 

 that volatility may be regarded as solubility in a vacuum. 



Am. Jour. Sci.— Fourth Series, Vol. XXXV, No. 207.— March, 1913. 

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