August 23, 1906] 



NA TURE 



419 



In ;inv one grain they are piled witli perfect regularily, 

 all facing one way, like a regiment of perfectly similar 

 soldiers lormed 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 llowers of an early Victorian wall-paper. 



It was shown by .Mr. Rosenhain and myself that when 

 .1 piece of 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 

 cryst.il 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 

 l<ept 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 may take place in three or more direc- 

 tions 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 

 appearance 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 multitude 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 of slips leaves it still a crystal. If part 

 of the group of brickbats moves forward, keeping parallel 

 to themselves and to the others, the formation remains 

 regular, except that a step is formed on the outermost 

 rows ; the orientation of the elements continues the same 

 throughout. Considerations which I shall mention presently 

 lead to some qualification of this statement. I now see 

 reason to believe that in the process of slip there is a 

 disturbance of the elementary portions or brickbats adjoin- 

 ing the plane of slip, which may alter their setting, and 

 thereby introduce to a small extent some local departure 

 from the perfectly homogeneous orientation which is the 

 characteristic of the true crystal. In very severe straining 

 there may even be a wide departure from true crystalline 

 character. We shall recur to this later ; but meanwhile it 

 will suffice to say that substantially the slip which is 

 involved in a plastic strain of moderate amount is a bodily 

 translation, parallel to themselves, of part of the group 

 of elementary brickbats or molecules which build up the 

 grain. If a crystal the form of which has been altered, 

 even largely, by such straining is cut and polished and 

 etched it appears, under the microscope, to be to all intents 

 and purposes as regular in the tactical grouping of its 

 elements as any other crystal. 



Further, in the process of straining we have, first, an 

 elastic stage, extending through very small movements, in 

 which there is no dissipation of energy and no permanent 

 set. When this is exceeded, the slip occurs suddenly : the 

 work done in straining is dissipated ; if the straining force 

 is removed a strain persists, forming a permanent " set "; 

 if it continues to act it goes on (within certain limits) pro- 

 ducing augmented strain. In general a large amount of 

 strain may take place without the cohesion between the 

 gliding surfaces being destroyed. Immediately after the 

 strain has occurred there is marked fatigue, showing itself 

 in a loss of perfect elasticity ; but this will disappear with 

 the lapse of time, and the piece will then be harder than 

 at first. If, on the other hand, a process of alternate 

 straining back and forth be many times repeated, the piece 

 breaks. 



These are now familiar facts. Can wc attempt to explain 

 them on the basis of a molecular theory which will at the 

 same time offer a clue to the process of crystal-building 

 as we find it in metals? I venture to make this Address 

 the occasion of inviting attention to some more or less 

 speculative considerations which may be held to go some 

 little way towards furnishing the material for such an 

 explanation. 



At the Leeds Meeting of this Association, in 1890, it was 

 my privilege to bring forward certain contributions to the 

 ni()lecular theory of magnetism, and to show a model which 

 demonstrated that the rather complex phenomena of 

 ntagnctisation were explainable on the very simple assump- 

 tion that the magnetic molecules are constrained by no 

 other forces than those which they mutually exert on one 

 another in consequence of their polarities.' From this 

 were found to result all the chief phenomena of permeability 

 and magnetic hysteresis. Let us attempt to-day to apply 

 considerations of a similar character to another group of 

 physical facts, namely, those that are associated with the 

 crystalline structure of metals and with the manner of 

 their yielding under strain. Just as in dealing with mag- 

 netic phenomena, I take as starting-point the idea that the 

 stability of the structure is due to mutual forces exerted 

 on one another by its elementary parts or molecules, and 

 that the clue to the phenomena is to be sought in the 

 play of these mutual forces when displacement of the 

 molecules occurs. 



Iron and most of the useful metals crystallise in the 

 cubic system ; for simplicity we may limit what has to be 

 said to them. Imagine a molecule possessing polarity 

 equally in three directions, defined by rectangular axes. 

 VVe need not for the present purpose inquire to what the 

 polarity along the axes is due ; it will suffice to assume 

 that the molecule has six poles, three positive and three 

 negative, and that these repel the like and attract the un- 

 like poles of other molecules. We may make a model by 

 using three magnetised rods fixed at right angles to one 

 another at their middle points. I imagine, further, that 

 the molecule has an envelope in the shape of a sphere, 

 which touches the spherical envelopes of its neighbours, 

 and assume that these spheres may turn on one another 

 without friction." 



Think now of the process of crystal-building with a 

 supply of such spherical molecules for brickbats. Starting 

 with one molecule, let a second be brought up to it and 

 allowed to take up its place under the action of the polar 

 forces. It will have a position of stability when a positive 

 pole in molecule A touches (or lies in juxtaposition to) a 

 negative pole in molecule B, with the corresponding axes 

 in line, and when the further condition is satisfied that 

 the axes in molecule B the poles of which are not touched 

 by A are stably situated with respect to the field of force 

 exerted by the poles of A. 



In other words, wc have this formation : — 



in? and Rosenhain, " The Crystallit 

 .n Lecture, Phil. Trans. Roy, Soc. vol. 



NO. 1 92 I, VOL. 74] 



■ A, 1899. 



For convenience of represenlalion in the diagram the poles 

 are distinguished by the letters N. and S., but it must 

 not be assumed that the polarities with which we are here 

 concerned have anything to do with magnetism. 



Suppose, now, that the crystal is built up by the arrival 

 of other molecules, each of which in its turn assumes the 

 position of maximum stability consistent with formation in 



' " Contributions to the Molecular Theory of Induced Masnetism," 

 Pnc. Roy.Soc.,\o\. xlviii., June 19, 1890, or I'hil. -Vag , September, 1890. 



- Or. let the envelope be a shell of any form, inside of which the axes 

 of polarity are free to turn as a rigid system. 



