The Crystalline Structure of Metals. 



175 



metal which is polished and etched after severe straining are referred 

 to as illustrating the same point. The persistence of crystalline struc- 

 ture is demonstrated by micro-photographs of the section of a bar of 

 Swedish iron which had been rolled cold from a diameter of f inch to 

 a diameter of J inch without subsequent heating. The outline of the 

 grains is much distorted, but the orientation of the crystalline ele- 

 ments remains constant within each individual grain. 



The slips in metals which exhibit a cubical crystalline structure on 

 etching are in some instances parallel to the faces of the cubes, and are 

 very frequently inclined to the faces, apparently along the octahedral 

 planes. Stepped lines are frequently seen, and also lines which appear 

 curved probably in consequence of numerous steps which are unre- 

 solved even under the highest powers. In exceedingly plastic metals 

 such as lead, copper, and gold, the lines are particularly straight. A 

 piece of lead cast against glass to produce a smooth surface gives* 

 when slightly strained, a splendid display of slip-bands, and the 

 boundaries of the grains are sharply defined by the meeting of the 

 lines on one grain with those on its neighbours. Another way to get 

 a clear lead surface for the purpose of showing slip-bands is to press a 

 freshly cut piece of the metal with considerable force against a smooth 

 object. Photographs of slip-bands in iron, gold, silver, lead, copper, 

 and other metals are given in the paper. 



"When a metal is fractured the grains do not as a rule part company 

 at their boundaries, but split along cleavage surfaces. It is to this that 

 the crystalline appearance, obvious in many fractures, is due. 



In several metals the authors find that " twinning " takes place in 

 the crystalline structure as an effect of strain. Samples of copper, 

 which in the original cast state gave no evidence of the existence of 

 twin crystals, were hammered or otherwise wrought, and were then 

 found to be full of twins. The twinning produced in this way sur- 

 vived after the wrought copper had been raised to a red heat and 

 allowed to cool. Similar results were obtained in gold and in silver ; 

 the metal in the cast state did not show twins, but they were found 

 after the metal had been wrought and subsequently softened by 

 annealing. An example of twinning was observed in nickel after the 

 application of a somewhat severe strain. Twins were readily developed 

 in cadmium by strain, apparently as a result of the slight strain which 

 was applied for the purpose of developing slip bands. They were also 

 found in lead, zinc, and tin, either as a primitive feature in the crystal- 

 lisation or produced by straining. The twinning frequently takes the 

 form of a large number of parallel bands within a single grain, and a- 

 twin band due to strain in one grain is sometimes associated with a 

 twin band in neighbouring grains, the bands being continuous except 

 for a change in orientation in passing from grain to grain. 



Photographs of twin bands in copper, gold, lead, and other metals are 



