The Crystalline Structure of Metals. 113 



rearrangement of the crystalline structure of iron occurs when the 

 metal is heated to redness, and it is .believed that such changes are 

 associated with the evolutions of heat which are indicated by " arrest 

 points" during the cooling of iron. We hoped that by keeping a 

 polished and etched surface of the strained metal under microscopic 

 observation while the specimen was gradually heated, we should see a 

 more or less sudden change in the crystalline pattern at a temperature 

 corresponding to one of the " arrest points." This attempt, however, 

 to watch the process of recrystallisation failed, although the experi- 

 mental difficulties of keeping a specimen under microscopic observa- 

 tion while it was being heated were successfully overcome. The 

 specimen was electrically heated in a vessel with a thin glass or mica 

 window, and the microscope-objective was kept cool by directing a 

 strong blast of cold air on it and on the surface of the window. In 

 one set of experiments the specimen was kept in an atmosphere of pure 

 hydrogen during the heating, but it was found to become so much 

 tarnished as to obliterate the crystalline pattern. At a red heat, how- 

 ever, the uniform luminous surface of the specimen was seen to develop 

 a number of dark patches which, on slightly raising the temperature, 

 spread over the entire field. No corresponding change was visible 

 during cooling, but the phenomenon would recur every time the speci- 

 men was heated, provided it had been cooled below redness after the 

 previous heating. This phenomenon was absent when the specimen 

 was heated in a vacuum, and we believe that it indicates a chemical 

 action between hydrogen and iron, possibly corresponding to the 

 hydrogen arrest point discovered by Sir W. Eoberts- Austen.* 



In the next series of experiments the specimen was heated in a 

 vacuum. On prolonged heating the specimen still became tarnished, 

 but at first the crystalline pattern remained visible up to a bright red 

 heat. No change in the pattern was observed, but subsequent polishing 

 and etching of the same surface showed that a real change of crystal- 

 line structure had occurred. The original etched pattern on the sur- 

 face had persisted after heating, simply because the differences of level 

 and surface texture on which it depended had in no way been disturbed 

 by the recrystallisation. Any crystalline pattern seen under the 

 microscope, whether it be produced by etching or relief polishing, 

 consists either of coloured surface deposits, or of steps, or pits, or other 

 differences of level in the surface, and these differences of superficial 

 texture, like mechanical scratches, are not affected by rearrangement 

 of the crystalline elements. Coloured surface deposits would also 

 remain unaffected. All attempts to observe the actual process of 

 recrystallisation must therefore be unsuccessful. 



The next section of the paper deals with the changes of crystalline 

 structure which go on in lead and other metals at comparatively low 

 * Alloys Research Committee Report, ' Proc. Inst. Mech. Eng.,' 1899. 



