OB THE CRYSTALLINE STRUCTURE OF MET M s .".f.'.i 



T \\iiniiiig, as revealed by slip-bands, has l>een observed in nickel ; in tliis case the 

 specimen was a virgin casting, but the twin (shown in tin- photograph, fig. 41) \\.-i- 

 observed only after very severe straining, and was. in .-ill probability, produced by 

 that straining. 



In /inc, tin, and cadmium twins cither occur verv freely in the cast metal, or else 

 arc very readily produced by the slight strain which is applied t<> develop slip-l>ands. 

 Surtiices of these metals, produced by casting against glass, show twin hands even 

 under fairly low [towers when the sj>ecimen is slightly U-nt ; tin- twin kinds then 

 appear as .shad. (! Lands riinnin- MTOM tin- '-rvsj .dime gNUO* Verv (V.-. pi. !it 1 v the 

 twin hands run on continuously across two or more grains, with more or less change 

 in direction when they cross a boundary. 



A specimen of cadmium showing this feature is photographed in tig. 4'J. This 

 particular specimen presents another peculiarity ; it was prepared by casting against 



U 'lass. and in this instance the -lavs fuAtt "as j < ; t , -i , t ';. ,| ,., II V -i\eli | c-c .n-i, 1,-| a 1,1. 



slo]ie, with the result that the metal solidified in a long strip while it was running 

 down the glass. On examining the under face of this strip two modes of 

 crystallisation were observed. Part of the surface there showed a very small 

 structure with no direction of greatest length. On another part there were large, 

 grains very considerably longer in the direction of the length of the strip than in a 

 transverse direction. The photograph, fig. 42, is taken from an area showing these 

 long grains. When the piece was strained by bending, twin lamellae appeared in a 

 more or less transverse direction, passing across from one elongated grain to the next 

 with only a slight change in direction. The twin hand in one grain is associated with 

 a twin kind in a neighbouring grain, the bands being continuous except for a change 

 of direction as they pass from grain to grain. 



We have observed twinning in gold, silver, copper, lead, nickel, zinc, tin, and 

 cadmium. It does not appear to occur in iron. 



The facility with which most metals undergo twinning as a consequence of strain 

 sho\\s that there are in general two modes by which plastic yielding takes place in 

 an air^re^ate of i rvstals. One is by simple slips, where the movements of the 

 crystalline elements are purely translators', and their orientation is preserved 

 unchanged. The other is lv twinning, when rotation occurs through an angle which 

 is the same f..r each molecule in the twinned group. Both modes are often found in 

 a single specimen M|' metal, and even in a single crystalline grain. Thus, in gold or 

 copper, it is verv usual to find, on examining a strained sj>ecimen that one portion t 

 a ^rain is covered with simple slip lines, while another jMirtion of the same grain 

 shows inn- ,i|- more lamell.-e which are twinned with respect t<> the rest of the grain. 



On surfaces prepared by casting against glass, particularly with cadmium, but also 



with /.inc and tin, a curious feature often occurs which is closely associated with the 



facility these metals show in developing twins. The appearance in question is that of 



an apparent duplication of the inter-granular Ixumdaries, as seen in the cadmium 



\ OI* \'iii. A. :' r. 



