CONSTITUTION* OF BODIES. 



in a homogeneous solid, are not in similar circumstances in this 



Thus we may suppose that in a certain number of groups the ordinary 

 agitation of the molecules is liable to accumulate so much that every now and 

 then the configuration of one of the groups breaks up, and this whether it is 

 in a state of strain or not. We may in this case assume that in every second 

 a certain projwrtion of these groups break up, and assume configurations cor- 

 responding to a strain uniform in all directions. 



If all the groups were of this kind, the medium would be a viscous fluid. 



But we may suppose that there are other groups, the configuration of which 

 is so stable that they will not break up under the ordinary agitation of the 

 molecules unless the average strain exceeds a certain limit, and this limit may 

 be different for different systems of these groups. 



Now if such groups of greater stability are disseminated through the sub- 

 stance in such abundance as to build up a solid framework, the substance will 

 be a solid, which will not be permanently deformed except by a stress greater 

 than a certain given stress. 



But if the solid also contains groups of smaller stability and also groups 

 of the first kind which break up of themselves, then when a strain is applied 

 the resistance to it will gradually diminish as the groups of the first kind 

 break up, and this will go on till the stress is reduced to that due to the 

 more permanent groups. If the body is now left to itself, it will not at once 

 return to its original form, but will only do so when the groups of the first 

 kind have broken up so often as to get back to their original state of strain. 



This view of the constitution of a solid, as consisting of groups of mole- 

 cules some of which are in different circumstances from others, also helps to 

 explain the state of the solid after a permanent deformation has been given 

 t-i it. In this case some of the less stable groups have broken up and assumed 

 new configurations, but it is quite possible that others, more stable, may still 

 retain then* original configurations, so that the form of the body is determined 

 by the equilibrium between these two sets of groups ; but if, on account of 

 rise of temperature, increase of moisture, violent vibration, or any other cause, 

 the breaking up of the less stable groups is facilitated, the more stable groups 

 may again assert their sway, and tend to restore the body to the shape it 

 had before its deformation. 



