622 president's address- — section h. 



general principle explains why, as experiment has shown, hard copper 

 suffers a much greater proportionate loss of strength than soft 

 annealed copper from the effects of moderate heating. 



In the light thrown upon the character of these phenomena by 

 obsei'vations with the microscope, several hypotheses have been put 

 forward in attempts to explain their real nature. Mr. G. T. Beilby, 

 in the Phil. Mag. for 1904, wrote a very suggestive paper, in which 

 he interprets the microscopic observations as showing that metals 

 may ordinarily occur in two distinct solid states, one which he 

 speaks of as the amorphous state, in which the molecules ai^e 

 arranged just anyhow, and the other in which the polarity of the 

 molecules has caused them to become arranged in the orderly 

 manner that produces the crystalline form. In tlie one case we have 

 the molecules with no systematic arrangement whatever, like the 

 units in an ordinary crowd of people: in the other we have them 

 arranged like soldiers in the orderly formation of a battalion. Or, we 

 may liken the amorphous solid to an instantaneous photograph of 

 the molecules when in a liquid condition. All metals seem to be 

 capable of existing in these two states, which differ not only in their 

 me<;hanical properties, but their optical, electrical, and thermo- 

 chemical properties are so distinctive as to make it appear that the 

 metal must be in two different forms. When in the amorphous state 

 the formation of surfaces of slip under stress is impossible, and con- 

 sequently there is no plasticity; so that, contrary probably to our 

 ordinary notions, it is the amorphous state that is the hard form 

 and the crystalline state that is the soft or malleable form of the 

 metal. A considerable mass of metal never appjears to exist wholly 

 in the amoi-phous state. It is always crystalline for the most part 

 even after being subjected to the severest hammering. But this 

 treatment may partly break the crystals up, and there is evidence 

 of the existence of this amorphous structure on the surface, and 

 between fragments of the broken crystals. Mr. Beilby holds the 

 view that a metal cannot pass from the hard phase to the crystalline 

 or vice versa without going through an intermediate mobile phase 

 in which the molecules, if not actually in the liquid state, have at 

 any rate something of the freedom of movement belonging to that 

 condition. The microscopic examination of polished metals, for 

 instance, always shows on the surface a layer of what looks like 

 hardened viscous fluid which has to be removed by etching before 

 the civstalline structure beneath is exposed. Mr. Beilby supposes 

 that tJie same sort of thing occurs at all surfaces where slip has 

 taken place. Thus, when a metal is strained beyond its yield point, 

 movements take place in the various grains along the slip surfaces 

 we have previously discussed, and between these sui-faces molecules 

 are set free that form a thin mobile layer which immediately passes 

 over into the hardened or amorphous state. Thus exceedingly thin 

 layers of metal in the hardened amorj)hous condition are formed 

 throughout the whole mass of the strained metal. '"' Slipping is easy 

 so long as fresh mo-\ang surfaces are forthcoming for the mobile 

 phase; but when all available crystalline phase has become encased 

 in the unyielding amorphous phase, plasticity under these particular 

 stresses comes to an end." In this way he accounts for the remark- 

 able phenomenon of the yield point, at which the metal passes through 



