﻿48 Dr. W. Peddie on 



stable equilibrium to another, that rotation causing, in its 

 turn, the permanent relative displacement of the molecules 

 which occurs in the u set :" and this, of course, is quite a 

 plausible hypothesis. In the homogeneous permanent torsional 

 strain the final configuration of a given group of molecules 

 is essentially similar to the initial configuration ; yet we may, 

 since a position of unstable equilibrium has been passed, say 

 that the old configuration has broken down and that a new 

 one has been formed ; so that the loss of energy may be 

 ascribed to the work done in breaking down molecular con- 

 figurations. 



There seems to me to be no need to ascribe such a special 

 type to the rupture of the configurations. On the contrary, 

 I think it more likely that the permanent molecular rotation 

 is a consequence of the permanent set which ensues on com- 

 plete rupture of configurations — i. e. rupture in which mole- 

 cules in given configurations part company, new groups 

 being formed. It appears to be most probable that the loss 

 of energy in this total rupture greatly exceeds, group by 

 group, the loss due to the cause which Wiedemann contem- 

 plates. 



Such total rupture takes place in all liquids and in plastic 

 solids ; and, by analogy, we conclude that it takes place in 

 all solids to a greater or less extent. Indeed we do not 

 need to rest upon analogy. The evaporation of ice below 

 the freezing-point is due to such rupture ; and so, probably, 

 as has been remarked, is the characteristic smell of metals, 

 such as freshly cut copper or iron. At a given temperature, 

 the molecules in a given configuration are in rapid motion 

 with a certain average speed, the actual speed of a molecule 

 being sometimes greatly in defect, sometimes greatly in 

 excess, of this average. When the excess is sufficiently great, 

 the molecule will pass beyond the range of the molecular 

 action of the remaining constituents of the group, rupture of 

 the configuration will occur, and the potential energy of 

 deformation will be transformed into heat energy. In a 

 sufficiently long interval of time such rupture must occur, 

 according to the kinetic theory. In a given state of strain 

 it occurs more readily the higher the temperature ; at a given 

 temperature it occurs more readily the greater the strain. 



It is easy to see that, if a molecule is under directional 

 control by molecular forces, rotation will in general ensue 

 on rupture of a given group. A dynamical illustration will 

 make this plain. 



Let PQ represent a pendulum, with a massive bob Q, 

 supported at the point P. Let the pendulum be situated 



