196 SCIENCE PROGRESS 



entirely indistinguishable. The detection of impurities by 

 means of the microscope is therefore far more difficult in 

 worked than in cast or annealed specimens. The effect on a 

 homogeneous alloy is well seen on etched surfaces of rolled 

 sheet brass. Still more severe distortion is seen in hard-drawn 

 wires, in spun sheet metal and in cold-pressed objects such as 

 cartridge cases. 



The facts which have to be explained in the mechanical 

 deformation of metals are the plastic yielding of the crystal 

 grains, which distinguishes a metal from a material such as 

 sandstone — the grains of which are usually separated by 

 pressure before any great deformation of the stone as a whole 

 is produced — and the remarkable increase of hardness which is 

 the consequence of the cold-working of most metals. The two 

 properties, plastic yielding and increase of hardness, are 

 intimately connected but it is only in quite recent years that 

 either of them has been satisfactorily explained and several 

 points still remain obscure. Both properties depend on the 

 minute internal structure of the crystals. 



Viewed in the gross, there is considerable analogy between 

 the behaviour of crystalline and of amorphous materials under 

 a mechanical stress sufficient to produce deformation. Thus, 

 if a rectangular block with polished surfaces be compressed 

 either uniformly over one face or locally by means of a knife- 

 edge, systems of lines appear on the remaining faces and these 

 lines have the same general form and direction whether the 

 material examined be wax, hard gelatin or metal. The arrange- 

 ment of lines can be calculated mathematically and is inde- 

 pendent of the nature of the material. The differences between 

 amorphous and crystalline materials become obvious whenever 

 the deformation is studied more minutely. Whilst a fracture 

 in an amorphous substance may occur in any direction indif- 

 ferently, a crystalline substance has definite planes of weakness 

 along which rupture takes place by preference. Moreover, an 

 amorphous substance may undergo considerable permanent 

 change of shape without the development of any fracture, 

 however minute, provided only that sufficient time be allowed 

 for the deforming force to exert its effect. Examples of this 

 are seen in the slow sagging of glass tubes supported only 

 at the ends and in the remarkable experiments with brittle 

 cobbler's wax which have been made familiar by Lord Kelvin. 



