﻿Molecular Structure of Metals. 255 



alteration in form. Again, the engineer recognizes the merit, 

 for structural purposes, of plasticity as well as strength, and 

 in many of his operations he makes direct use of that pro- 

 perty, as in the drawing of wires and tubes or the flanging 

 of plates. He is concerned, too, with the hardening effect 

 that occurs in such processes when work is expended in per- 

 manently deforming a metal in the cold state, and also with 

 the restoration to the normal condition of comparative softness 

 which can he brought about by annealing. Nor can he afford 

 to be indifferent to the phenomena of " fatigue " in metals, 

 which manifest themselves when a piece is subjected to 

 repeated alterations or variations of stress — fatigue of strength 

 and fatigue of elasticity, which, like physiological fatigue, 

 admits under some conditions of rest-cure, inasmuch as it 

 tends to disappear with the lapse of time. No apology need 

 be made in selecting for a Presidential Address to Section Gr 

 a subject that touches so many points of direct practical 

 interest to engineers. It is a subject which has for me the 

 additional attraction of lying in the borderland between 

 engineering and physics — a borderland in which I have often 

 strayed, and still love to stray, and I enter it to-day even at 

 the risk of wandering into regions which, to engineers, may 

 seem a little remote from home, regions where the landscape 

 has, perhaps, a suspicious likeness to that of the country over 

 which the learned men of section A hold rule. 



To engineers, quite as much as to physicists and chemists, 

 we owe in recent years an immense extension of knowledge 

 regarding the structure of metals. This has come about 

 mainly by the intelligent use of the microscope. Take any 

 piece of metal, in the state in which an engineer makes use 

 of it, polish and lightly etch its surface, and examine it under 

 the microscope, and you find that it is a congeries of a mul- 

 titude of grains, every one of which may be proved to be a 

 crystal. It is true that the boundaries of each grain have 

 none of the characteristics of geometrical re<mlaritv which 

 one is apt to look for in a crystal, but the grain is a true 

 crystal for ail that. Its boundaries have been determined by 

 the accident of its growth in relation to the simultaneous 

 growth of neighbouring grains — the grains have grown, 

 crystal fashion, until they have met, and the surface of 

 meeting, whatever shape it may happen to take, constitutes 

 the boundary. But within each grain there is the true 

 crystalline characteristic — a regular tactical formation of the 

 little elements of which the crystal is built up. It is as if 

 little fairy children had built the metal by piling brickbats in 

 a nursery. Each child starts wherever it happens to be, 



