Manchester Memoirs, Vol. Iv. (191 1), No. V'lt. 3 



which is not yet fully understood. If the body changes 

 shape, a certain disturbance amongst the molecules is 

 inevitable. The molecules probably become re-arranged, 

 and if the tensile force is sufficiently great, certain groups 

 of molecules may be broken up, whilst other groups will 

 only be distorted, coming back to their original position 

 when the stress is removed. Viewed from a rather 

 different standpoint, these molecular changes may be 

 divided into two classes, each producing its own charac- 

 teristic phenomena, viz. : — 



(a) A displacement of the position of the mole- 

 cular groups relatively to their positions of equilibrium. 

 {b) A change in the dimensions of the inter- 

 molecular spaces. 



Both of these effects take place simultaneously, and 

 it is necessary to differentiate between them. The first 

 of these, namely — the movement of the molecular groups 

 past one another, is a cause of heating if we regard the 

 effect as being of the nature of internal friction. In the 

 second case, in order to overcome the attraction between 

 the molecules, work has to be expended on them to 

 increase their distance apart ; this absorbs energy and is 

 consequently a source of cooling. 



When work is performed on a body due to a change of 

 stress, this work must either go towards increasing the 

 potential energy, or the kinetic energy of the body. The 

 kinetic energy of the molecules re-appears as heat, and 

 thus an alteration in the state of strain of a body results 

 in a change in temperature. There is, however, a con- 

 dition under which no thermal change takes place ; this 

 occurs when the whole of the work done goes towards 

 increasing the potential energy of the body. If, during a 

 given change of strain, the increase of potential energy is 

 greater than the work expended on the specimen, then 



