RESIDENTIAL ADDRESS. 



661 



liquid state. As a rule, the actual process of crystal-building goes on delidritieally ; 

 bi'anches shoot out, aud from them other branclies proceed at right angles, leaving 

 interstices to be tilled in later. We Lave, therefore, to conceive of the molecules 

 as piling themselves preferably in rows rather than in blocks, though ultimately 

 the block form is arrived at. In this position of maximum stability each molecule 

 has its six poles touching poles of contrary name. 



Now comes a point of particular importance. Imagine two neighbouring 

 molecules in the same block to be turned round, each through one right angle, in 

 opposite senses. They will now each have five poles touching five poles of con- 

 trary name, but the sixth pole Avill touch a pole of the same name as itself. They 

 are still stably situated, but much less stably than in the original configuration, 

 and they will revert to that configuration if set swinging through an angle 

 sufficient to exceed the limited range within which they are stable in the new 

 position. 



Similarly we may imagine a group of three, four, or more molecules, each to 

 be turned through a right angle, thereby constituting a small group witli more or 

 less stability, but always with less than would be found if the normal configura- 

 tion had been preserved. The little group in question may be made up of molecules 

 in a row, or it may be a quartette or block, or take such a form as a T or L. 



Fig. 3. 



A sufficient disturbance tends to resolve it into agreement with the norme.! tactics 

 of the molecules which build up the rest of the grain. 



It is conjecturally possible that small groups of this kind, possessing little 

 stability, may be formed during the process of crystallisation, so that here and 

 there in the grain we may have a tiny patch of dissenters keeping one another in 

 countenance, but out of complete harmony with their environment. 



If this happens at all during crystallisation, it would seem less likely to happen 

 in free crystallisation from a liquid state than in the more constrained process 

 that occurs when a metal already in the solid state recrystallises at a temperature 

 far below its melting-point. Though rare or absent in the first case, it might 

 occur frequently in the second. There are differences in the appearance of crystal 

 grains under the microscope in metal as cast and in metal as recrystallised in the 

 solid state, of which this may be the explanation. It may also explain a 

 difference pointed out by Rosenhain,' that the slip lines in cast metal are straight 

 and regular, whereas in wrought iron and other metals which have recrystallised 

 in the solid they rarely take a straight course across the crystal, but proceed in 

 jagged, irregular steps. These may be due to the presence here and there of 

 small planes of weakness, resulting from the existence of what I have called 

 dissenting groups. Again, these groups, possessing, as they do, less stability than 



' Rosenhain, 'The Plastic Yielding of Iron and Steel,' Journal of the Iron and, 

 Steel Institute, No. 1 for 1904, p. 335. 



