464 Effects of Strain on the Crystalline Str ucture of Lead. [May 28, 



also shown illustrating the similar, though much slower, progressive 

 growth which goes on in a strained crystal at ordinary atmospheric 

 temperature. 



The final experiments were carried out in order to decide the ques- 

 tion, whether the recrystallisation which is apparent immediately after 

 re-etching a severely strained crystal is a direct and instantaneous 

 effect of the strain, or is a growth which occurs during the interval of 

 time that has elapsed between the straining and the examination. 

 The author has satisfied himself that the structural re-arrangement 

 does not occur during the act of straining, like the re-arrangement (by 

 twinning) which one can produce by straining a crystal of calcite. It 

 occurs after the strain has taken place, during the interval of time that 

 elapses before the specimen is etched for examination, and though it 

 requires only a short interval of time for its development, it is to be 

 classed with the progressive growth demonstrated by Ewing and 

 Rosenhain in their observations on the crystals of ordinary lead after 

 straining. 



The manner in which this was demonstrated was as follows : — If the 

 re-arrangement took place during the act of straining by successive 

 twinning, the slip lines should, as the strain proceeded, form in new 

 directions over the parts which had been already altered by the strain, 

 and thus as more and more strain was given, various new systems of 

 slip lines should appear, so grouped as to exhibit the twin character of 

 the crystals produced. It was, however, found that the slip lines gave 

 no indication of any such change, but ran in directions which were 

 parallel all over the strained crystal. But when such a strained 

 specimen was re-etched, or when a second strain was given after a 

 short interval of time (about 5 minutes), the patches of new orienta- 

 tion then became visible — in the first case by the difference in orienta- 

 tion of the etched pits, and in the second by the appearance of 

 numerous new systems of slip lines, parallel over certain small areas, 

 but bearing no relation to the lines already produced by the first 

 strain. Even when the strain was continued to the limit of fracture 

 no re-arrangement could be observed during its application. 



It is to be concluded that there is no broad distinction between the 

 change which is visible (on etching) almost immediately after strain- 

 ing and that which is observed to go on after any considerable lapse of 

 time, or when the specimen is slightly warmed. 



