ON THE CRYSTALLINE STRUCTURE OF METALS. 365 



explanation, then, is that in this grain the crystalline elements are oriented in such a 

 w;kv that the intersection of two sets of cleavage or gliding planes is jHirallel to the 

 Mil-face, and consequently their traces on the surface are parallel to one another. 

 This seems to be an accidental occurrence, possibly favoured by the condition under 

 which the sjxjcimen crystallised, namely in contact with cold glass. In many other 

 instances a more or less close approximation to such jmrallelism has been observed, 

 resulting in systems of slip-bands intersecting at a very small angle ; in that case the 

 two positions where slip-bands appear under oblique light are nearly, but not exactly, 

 180 apart. Seen under vertical light, with low magnification, such intersecting 

 systems produce the appearance which is exemplified in the central grain in fig. 18, 

 Plate 1'J. 



The relation of slip-lxinds to the geometrical air-pits in cadmium is illustrated by 

 the photographs, figs. 26, 27, and 28. In these examples the metal was cast against 

 glass in such a way as to produce air-pits, and was then strained sufficiently to 

 develop slip-bands. It appears that the slip-bands are always parallel to one side of 

 the geometrical pits the two phenomena thus confirming the views which have been 

 advanced above as to the crystalline structure of the metal. Figs. 29 and 30 show 

 the relation of slip-bands to geometrical etched pits in iron. It will be observed that 

 the slips are not generally jwirallel to a side of the etched figures, but in specimens 

 that have been more severely strained than those here illustrated, one set of slip- 

 bands in each grain is generally found to be parallel to one side of the etched pits, 

 while the other systems intersect these sides diagonally. The observations point to 

 the conclusion that in iron slipping occurs most readily along the octahedral planes, 

 although slips parallel to the sides of the cubical crystals are also found. 



The development of slip-bands in strained metal throws what appears to us to be a 

 IH-W light on the character of plastic strain. Plasticity is due to the occurrence of 

 these slips. When metals are strained beyond their limit of elasticity the deforma- 

 ti<>n occurs through sliding over one another of the elementary portions of which each 

 crystalline grain is built up. 



The sliding which gives rise to slip-bands is of finite amount, and occurs at a limited 

 number of places. " Flow " or plastic strain in metals is not a homogeneous shear 

 such as occurs in the flow of a viscous fluid, but is the result of a limited number of 

 separate slips. 



The conception that metals adapt themselves to the new shapes imposed upon them 

 when they undergo plastic deformation by means of slips along cleavage or gliding 

 planes within each crystalline grain, leads naturally to the supposition that the 

 crystalline elements themselves undergo no deformation in this process. The portions 

 >t the metal between one surface of slip and the next may remain undeformed, except 

 elastically, under all stresses. The effect of a stress sufficient to produce plastic strain 

 is a n. i logons to that of a tractive force overcoming the static friction between two 

 Mil-faces. 



