MancJiester Memoirs, Vol. li. (1907), No. 15. 13 



by way of explaining the process of crystal building, and 

 the characteristics which the crystal exhibits under 

 strain.* Just think of the various things that these brick- 

 bats have to explain. In the first place, they have to 

 explain the regular tactical formation in the crystals ; 

 next they have to explain cohesion ; then they have to 

 explain how the metal is elastic, that is to say, how it can 

 undergo a certain small amount of strain without any 

 permanent set, with complete recovery of its original 

 form when the stress is removed ; next they have to 

 explain how, when it is further strained, it passes an 

 elastic limit, and how it is capable of plastic strain, under- 

 going permanent set and giving rise to dissipation of 

 energy; finally, they have to explain those various 

 phenomena of fatigue I have just briefly touched upon. 

 The molecule or individual brickbat that I imagine the 

 crystal to be built up of is a piece which has three axes of 

 polarity. Think of it as having six poles, with a pair 

 of opposite poles at the ends of each of the three axes, and 

 the axes at right angles to one another. 



The polarity is not magnetic, although we may con- 

 veniently represent it in a model by means of magnetism. 

 Whatever the polarity is due to, you must suppose that a 



* Report Brit. Assoc, York, 1906. Address of President, Section G. 



