﻿Molecular Structure of Metals. 257 



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 a gliding " planes. Plastic yielding is 

 due to the occurence of this shear ; it may 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 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 move 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 r see reason to believe 

 that in the process of slip there is a disturbance of the 

 elementary portions or brickbats adjoining the plane of slip, 

 which may alter their setting, and thereby introduce to a 

 small extent some local departure from the perfectly homo- 

 geneous 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 sub- 

 stantially 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 whose form has been 

 altered, even largely, by such straining is cut and polished 



Phil. Mag. S. 6. Vol. 12. No. 69. Sept. 1906. S 



