282 
PROFESSOR J. A. EWING AND MR. W. ROSENHAIN 
end ; the tube could be exhausted by means of a Fleuss pump. The specimen was 
placed with its polished and etched surface as near to the top of the tube as possible, 
and a 22 millims. objective was used. When the specimen became really hot, the 
radiation would have endangered the objective, and this difficulty was met by 
keeping a blast of air directed partly upon the lower end of the objective and 
partly upon the surface of the test-tube. The apparatus was thus kept quite cool 
without in any way disturbing the distinctness of the image. 
We found that with the best vacuum we could obtain, even with the addition 
of phosphoric anhydride drying tubes, our specimens always tarnished if kept at a 
red heat for any length of time. We are inclined to ascribe this to the action of 
gases occluded in the metal and given out on heating. The tarnishing, however, 
was not very rapid, and a specimen could safely be heated up to bright red once before 
it became noticeable. This enabled us to make observations which very clearly showed 
that, even with the greatest experimental refinement, we could not hope to observe 
the process of re-crystallisation microscopically. We have found that if a strained, 
polished and etched specimen, showing distinctly elongated crystals, be gradually 
heated in vacuo no change whatever is visible in the outlines of the crystals : on 
removing the specimen from the apparatus it is sometimes found coated with a thin 
blue tarnish, and on examination with the microscope it is seen that this tarnish has 
not obliterated the original crystals, but in fact differentiates them by various 
colours on different crystals. If this tarnish be now polished off, as may easily be 
done by the use of rouge alone, and the specimen be re-etched, an entirely new set 
of crystals is revealed. This was readily established by drawing the outlines of the 
original crystals at a marked spot on the specimen before and after the light 
re-polishing. Simple re-polishing and etching a similar specimen, without annealing, 
produces only very small changes in the pattern. We may therefore conclude that 
in the above experiment the iron did re-crystallise when at a red heat, but that this 
re-crystallisation did not immediately affect the pattern on the surface. The 
explanation is obvious when we realise that the pattern seen in the microscope is 
due to the differences of level and texture in the surface of the specimen which have 
been produced by the action of the acid used in etching. This pattern, although 
in its origin dependent upon the actual crystalline structure of the metal at the 
time when the surface was etched, remains as a mere mask, beneath which re-orienta¬ 
tion of crystalline elements may go on without affecting its appearance. The etched 
pattern is in this respect in the same position as a scratch or mechanical marking of 
the surface, and the persistence of the etched pattern after annealing is simply an 
extension into more minute detail of the well-known fact that the external shape 
of a piece of metal is not in general affected by annealing, although the crystalline 
structure is entirely changed. 
These considerations show that we cannot expect to see the process of re-crystallisa¬ 
tion in any metal where etching, staining, or relief polishing is needed to differentiate 
