658 TRANSACTIONS OF SECTION G. 



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, placing its first brickbat at random, and then piling the others side by side 

 with the first in geometrical regularity of orientation until the pile, or the branches 

 it shoots out, meets the advancing pile of a neighbour; and so the structure goes 

 on, until the whole space is entirely filled by a solid mass containing as many grains 

 as there have been nuclei from which the growth began. 



We now know that this process of crystal growth occurs not only in the 

 solidification of a metal from the liquid state, but in many cases during cooling 

 through a ' critical ' temperature when the metal is already solid. We know also that 

 the process may in certain conditions go on slowly at very moderate temperatures. 

 We know also that the process of annealing is essentially the raising of the metal 

 to a temperature at which recrystallisation may take place, though the metal 

 remains solid while this internal rearrangement of its particles goes on. Whether 

 crystallisation occurs in solidifying from the liquid or during the cooling of an 

 already solid piece it results in the formation of an aggregate of grains, each one 

 of which is a true crystal. Their size maybe large or small — in general, quick 

 cooling means that crystallisation starts from many nuclei, and the resulting grains 

 are consequently small ; with very slow cooling you get a gross structure made up 

 of grains of a much larger size. 



For simplicity of statement I shall ask you in what follows to confine your 

 attention to simple metals, omitting any reference to alloys. Alloys present many 

 complexities, into which we need not at present enter. AVith simple metals every 

 crystalline grain is made of the same substance : the elementary Ijrickbats are all 

 exactly alike, though there may be the widest variation from grain to grain as 

 retTards the form of the grain, and also as regards the direction in which the ele- 

 mentary brickbats are piled. In any one grain they are piled with perfect regu- 

 larity, all facing one way, like a regiment of perfectly similar soldiers formed up 

 in rows, where each man is equidistant from his neighbours, before and behind, as 

 well as to right and to left. Or perhaps I might compare them to the well-drilled 

 flowers of an early Victorian wall-paper. 



It was shown by Mr. Rosenhain and myself that when a piece ol metal is 

 strained beyond its limit of elasticity, so that permanent set is produced, the 

 yielding takes place by means of slips between one and another portion of each 

 crystal grain. A part of each crystal slides over another part of the same crystal, 

 as you might slide the cards in a pack. It is as if all the soldiers to one side of 

 a given line were to take a step forward, those on the other side remaining as 

 they were, or as if all the men in the front rows took a step to the left, while 

 those in the rows behind kept their places. In other words, the plasticity which 

 a metal possesses is due to the possibility of shear on certain planes in the crystal 

 that are called ' cleavage ' or ' gliding ' planes. Plastic yielding is due to the 

 occurrence of this shear ; it ma}^ take place in three or more directions in a 

 single grain, corresponding to the various possible planes of cleavage, and in each 

 direction it may happen on few or many parallel planes, according to the extent of 

 the strain to which the piece is subjected. Examine under the microscope the 

 polished surface of a piece of metal which has been somewhat severely strained 

 after polishing, and you find that the occurrence of this shear or slip is manifested 

 on the polished surface by the app?arance of little steps, which show themselves as 

 lines or narrow bands when looked at from above. To these we gave the name of 

 slip-bands. Just as the piece of metal is an aggregate of crystal grains, the change 

 of shape which is imposed upon it in straining is an aggregate effect of the multi- 

 tude of little slips which occur in the grains of which it is made up. Each grain, 

 of course, alters its form in the process. 



Speaking broadly, this distortion of the form of any one grain by means oi: 



' Ewing and Rosenhain, ' The Crystalline Structure of Metals,' Bakerian Lecture 

 Ph'tl. Trans. Boy. Soc, vol. cxciii. A, 1899. 



