210 Johnston and Adams — High Pressures on the 



small. In general, therefore, the influence of uniform pressure 

 upon reactions between solids is likely to be very slight. 



The General Effects of Non-uniform Compression. 



The effects produced by exposure to non-uniform compres- 

 sion are in general permanent ; that is, the original state of 

 the system is not re-established when the compression ceases. 

 The effect of differential compression may be resolved into two 

 parts : that due to a (smaller) uniform pressure and that due to 

 a shearing stress,* the latter being the preponderating effect. 

 The mere fact that " flow," or deformation, of the com- 

 pressed material occurred is sufficient evidence that unequal 

 stresses were set up ; in other terms, that the compression was 

 not uniform. 



In most of the work recorded in the literature the solid sub- 

 stances were subjected to non-uniform compression, as is 

 evident from the author's description of his apparatus and 

 method of working. The discussion of this work (postea, 

 p. 223), therefore, brings out in a more detailed way the 

 effects produced on subjecting solid substances to compression 

 of this type. Before proceeding to this, however, we shall 

 consider whether we can adopt any plausible theoretical 

 attitude which will harmonize the various observations of the 

 effect of non-uniform pressure. 



Such a standpoint can be attained if we assume that every 

 permanent deformation of a crystalline aggregate is conditioned 

 by, and consequent upon, a real melting ;f the whole of the 

 solid aggregate does not of course melt, but only those por- 

 tions (particles) of it which at any instant bear the brunt of 

 the load. This assumption, which in an indefinite form has 

 frequently and more or less unconsciously been made use of,J 

 is not so violent as at first sight it might seem to be; as we 

 hope to show in what follows. 



Granting for the moment that the above assumption is cor- 

 rect, it follows that the forces which produce deformation are 

 competent to cause the melting point to be lowered to about 

 the ordinary temperature ; otherwise we must suppose that the 

 heat developed during the friction accompanying deformation 

 is sufficient to raise the temperature of some of the particles to 

 their ordinary melting-point — a supposition which is less prob- 

 able, for Spring found that he could in his apparatus subject 



* In precisely the same way a tensile stress can be resolved into a dilata- 

 tion and a shearing stress. 



f Evidence in support of the plausibility of this assumption is adduced 

 later (p. 219). 



% For instance, it is common knowledge that there is a general parallelism 

 between the mechanical properties of the metals and their melting points, 

 those with the higher melting points being the stronger, and vice versa. 



