368 PROFESSOR J. A. KNVINi: AM) Ml!. W. ROSKNHAIN 



definite angle. The twinning which we have found in many st mined metals corre- 

 sponds to the twinning observed in calcite by BAUMHATTER and REITSCH, and subse- 

 quently produced in isolated crystals of antimony and bismuth by MUGGE.* 



BAUMHAUER found that by forcing a knife blade into a crystal of calcite at the 

 proper angle a portion of the crystal could be made to swing over into the twinned 

 position. This implies a corresponding change of orientation in the crystalline 

 elements. The experiment may be said to afford an example of plasticity in a 

 crystal, but it is not entirely analogous to the plasticity by pure sliding which is 

 found in iron and in most other metallic crystals. In the process of twinning by 

 strain there is both slip and rotation of the elements. 



The existence of twin crystals in certain metals became apparent when systems of 

 slip-bands were found like those shown in figs. 34, 35, and 36. These are photographs 

 from specimens of copper. They show that certain of the crystalline grains are crossed 

 by twin lamellae, the twin planes being defined by a sudden change in the direction 

 of the slip-bands. Where several such twin lamellae occur in one crystalline grain, 

 we have a periodic structure with alternate systems of parallel slip-bands. The 

 change of orientation in passing from one lamella to another is constant. It is clear 

 that in these examples we have true cases of twin crystallisation. An example of 

 twinning in gold, as seen under vertical light, is given in fig. 37, and fig. 38 is a 

 photograph of twins in gold, seen under oblique light. 



The question arose whether these twin crystals were a feature in the primitive 

 crystallisation of the metal, or whether they were subsequently produced as a conse- 

 quence of strain. They were first seen in wrought copper (namely in a piece of rolled 

 plate), which had been raised to a bright red heat before polishing. We next 

 examined specimens of copper, gold, silver, and lead, each in the cast state, and in 

 none of these found any appearance of twinning. For the purposes of this examina- 

 tion only a slight strain was applied. The same pieces were then wrought, that is to 

 say they were severely strained (namely, by cold hammering) and they were again 

 examined both before and after annealing at a red heat. The result showed that the 

 violent strain produced by working the metal had developed twins in specimens where 

 none could be seen before, and that the twins were still found in the wrought specimens 

 after annealing. Fig. 39 is a photograph of a "twin" in cold-hammered copper (not 

 heated after hammering) ; incidentally it illustrates the persistence of crystalline 

 structure after violent deformation. Still more striking in this respect is the appear- 

 ance shown in fig. 40. The specimen in this instance was an ordinary piece of 

 plumber's sheet lead ; the surface was scraped bright with a knife, and was then 

 squeezed against a piece of plate-glass in a vice, thus producing a beautiful surface. 

 The specimen was then very slightly bent in the fingers to develop slip-bands, 

 and on examination under a high power it showed the appearance reproduced in the 

 photograph. 



* See P. GROTH'S ' Physikalische Krystallographie.' Voss, Leipzig. 



