236 



Mr. A. Mallock. The Physical 



[June 6, 



further extension increases very rapidly, so rapidly indeed as to 

 suggest that the structure of the material brings some sort of 

 mechanical stop into action. See Diagram III. 



Diageam III. 



V, I I I I I I I I -I- 1 I I I I I -\ 1-1 1 I I- I I l I I l-l 1 I I I 1 1 11 1 -I -I I L_J L_J 



100 200 300 400 500 600 700 800 900 



. ■ J*ound.5 . ; . . • 



Several mechanical mixtures such as putty (chalk and oil), damp 

 clay, and sand and water exhibit similar properties. If a lump of 

 putty be well rolled or beaten it will be found to be slightly elastic, 

 but beyond the elastic limit to be easily stretched for a certain distance 

 and then to become almost hard, at the same time the appearance of 

 the surface changes from a smooth, oily character to a dull granular 

 one. 



The explanation in this case is that the hard particles of the mix- 

 ture are, in its undisturbed state, separated each from its neighbours 

 by a wall of fluid of finite thickness. When the material is distorted 



