380 
PROFESSOR J. A. EWING ON THE EFFECTS OF STRESS AND 
strain, distortion is accompanied by certain complex changes of density, which depend 
on the arrangement of the particles. Thus, if a bag of shot or sand he shaken into the 
stable arrangement which corresponds to maximum density, and then be subjected to 
stress by (say) compression between two planes, the density will at first diminish, and 
then, as the stress is increased, will pass a minimum and begin to increase. So the 
thermoelectric quality of a piece of iron which has been shaken into a condition of 
molecular stability changes, first, towards (say) positive and, later, towards negative 
(fig. 31), when stress is applied and increased. Here, however, we are dealing with a 
quality, which, unlike density, is directional ; and the changes have, in fact, the 
reverse signs from those just stated if the direction of the applied stress is reversed. 
Thus, let a (fig. 34) be a point expressing the relation of thermoelectric E.M.F. to 
Fig. 34. 
load, which is reached by tapping the metal or by exposing it to the process of 
demagnetising by reversals. If from this point we go on loading we have the curve 
a b. If from the same point we had begun to unload we should have had the 
curve a c. 
The property of dilatancy, which Professor Reynolds has shown that granular 
media possess, does not in itself afford any clue to the changes which a directed 
quality like that now under examination suffers when the condit ion of stress is varied. 
But the same considerations (which explain why, in the changes of volume accom¬ 
panying strain in a granular medium after vibration, there is a point of inflection 
similar to that which is the most striking feature of these thermoelectric curves) 
suggest that a granular medium may be the kind of molecular mechanism required to 
account for the characteristics which have been described in this paper, and that the 
remarkable feature in the hysteresis of thermoelectric quality with regard to stress 
commented on in §§13 and 41 is to be attributed to a periodic arrangement of 
granules. The difficulty of any dynamical explanation is not a little increased by the 
fact that the variations of magnetism which accompany variations of stress, while 
otherwise not very dissimilar to the variations of thermoelectric quality, exhibit none 
of this inflection, either when a change takes place from loading to unloading (or from 
