420 Professor J. A. Ewing [May 18, 



uniform brightness of the grain under oblique light, the sudden 

 variations of brightness which occur as the direction of the incident 

 light is changed, and the evidence which high power microscopic 

 examination gives of geometrical forms in the texture of the grain. 

 In many cases^the etching produces pits here and there on the grain, 

 and in each grain these pits have one and the same orientation. 

 Numerous lantern slides of micro-photographs were exhibited in 

 illustration of these points, and the experiment was shown of directly 

 projecting on the screen the light reflected from grains of a piece 

 of iron, prepared by Mr. Stead and lent to the lecturer by Sir W. 

 Roberts-Austen, in which the granular structure was exceptionally 

 large. 



The lecturer proceeded to give an account of recent researches 

 made by him in conjunction with Mr. W. Eosenhain, in which the 

 effects of straining and the subsequent influence of temperature in 

 causing partial annealing were particularly examined. 



By watching the polished surface of the metal during straining, 

 it was seen that as soon as the elastic limit was passed and the metal 

 began to take " permanent set," a lar»e number of lines appeared on 

 the surface of each grain. Seen under vertical illuminations, these 

 were black, and looked like crevasses, but oblique illumination showed 

 them to be really steps or abrupt changes of level. It was shown 

 that these were produced by sudden slips occurring on cleavage or 

 " gliding " planes in the crystal. Plastic yielding on the part of the 

 metal took place by means of these slips, which in cases of severe 

 strain were seen to occur on two, three, four, or perhaps more sets of 

 independent planes. Micro-photographs were exhibited, showing the 

 slips in iron, lead, gold, copper and other metals. In many cases, 

 severe straining was also found to develop twin crystals. 



Twin crystals were also a usual characteristic of metal, which, 

 after being severely strained in the process of manufacture, had been 

 more or less annealed. This was readily seen by examining rolled 

 copper after it had been softened by heat. But perhaps the most 

 striking instance was to be found in sheet lead. Ordinary plumbers' 

 lead usually showed a very large crystalline structure, with many 

 brilliant examples of twin crystals. This led the lecturer and his 

 colleague to suspect that prolonged exposure to atmospheric temper- 

 ature was sufficient to cause crystalline growth to occur in lead, and 

 they succeeded in verifying this supposition. The growth which 

 goes on after severe straining is a function of the time as well as of 

 the temperature to which the metal is exposed. At comparatively 

 high temperature it occurs fast, and the metal quickly reaches an appa- 

 rently stable state. At lower temperature, it goes on more slowly, 

 and its progress may be traced for weeks or months. 



Photographs were exhibited illustrating the gradual process of re- 

 crystallisation in pieces of lead which were severely strained by com- 

 pression, and were then kept under observation at various constant 

 temperatures, one of which was the ordinary temperature of a room. 



