Physical and Chemical Behavior of Solids. 231 



defined) even at ordinary pressure. There is, however, no 

 basis of any kind for the statement that high pressure causes a 

 decrease in the viscosity of solids. It is true that stressed 

 solids flow whenever the magnitude of the stress is sufficient 

 to overcome the resistance to flow offered by the body ; but all 

 the evidence tends to indicate that this resistance to flow 

 increases when the body is exposed to the action of high uni- 

 form pressure. 



In view of the lack of definiteness of our present notions of 

 solid viscosity and of the uncertainty as to the significance of 

 the numerical data on viscosity of solids, it behooves us to 

 exercise great caution in the use of the idea of viscosity in any 

 argument dealing with solid systems. But our present inabil- 

 ity to make advantageous use of the idea matters the less, — 

 especially in applications to geological problems — because 

 viscosity affects only the rate of change of the system, but not 

 its final state. To quote the concise statement of Ibbetson : 



" The amount of viscous resistance of a given solid or fluid, 

 at a given uniform temperature, depends only upon, and 

 increases continually with the rate at which shear takes 

 place, and invariably vanishes with this rate: — or in other 

 words, infinitely small resistance is offered by viscosity to 

 infinitely slow moving. 



" The existence of viscosity in a material does not therefore 

 affect the conditions of equilibrium under stress, but only 

 resists and modifies the process (other than simple [uniform] 

 dilatation and compression) by which a body passes from one 

 state of strain to another ; a relation being introduced between 

 the magnitude of the stress producing the change of state and 

 the time occupied by the change. 



"Although a fluid may, in virtue of its viscosity, offer 

 immense resistance to sudden or rapid distortions, yet any 

 shearing stress, however small, will suffice to produce any 

 required amount of flow, however great, provided that it be 

 applied continuously for a sufficient length of time P* 



Residual Effect of Compression on the Density of Solids. 



This question, which is closely related to the general ques- 

 tion treated in this section, has been fully treated in another 

 communication. f In this paper it is established that a residual 

 effect — a decrease of density — is observed when, and only 

 when, the material (supposed homogeneous, i. e. free from 

 ordinary vacuoles or inclusions^) has been subjected to deforma- 



* Ibbetson, op. cit., p. 175. The italics are Ibbetson's. 



f J. Am. Chem. Sue, xxxiv, 563-84, 1912 ; Zs. anorg. Chein., lxxvi, 274-302, 

 1912. 



X It is obvious that any result might be obtained with a substance which 

 does not satisfy these conditions. 



