262 SUMMARY OF CURRENT RESEARCHES RELATING TO 



he found that the effect of the carbon was to limit the amount of the 

 copper which could be alloyed with the iron. 



Crystalline Structure of Metals. * — The above was the subject of 

 the Bakerian Lecture by Prof. Ewing and Mr. Kosenhain before the 

 Eoyal Society. The writers dealt with a hranch of the subject hitherto 

 somewhat overlooked, viz. the effects of strain. They believe that they 

 have established the fact that the structure of metals is crystalline, even 

 under conditions which might be supposed to destroy crystalline struc- 

 ture. They found that ihe plastic yielding of metals when severely 

 strained occurs in such a manner that the crystalline structure is pre- 

 served. The distinction which is often drawn between crystalline and 

 non-crystalline states in metals appears to be unfounded. The difficulty 

 of obtaining a good surface on the more fusible metals (e.g. lead, zinc, 

 and tin) by polishing was avoided by pouring the molten metal upon 

 glass or polished steel, in contact with which it was allowed to solidify. 

 In the case of lead, another method of obtaining a good surface was also 

 used. A face of the specimen was freshly cut to remove the tarnish, and 

 was then pressed against a smooth surface of plate glass. Whenever a 

 sufficient pressure could be reached without breaking the glass, a very 

 beautiful surface was obtained. In some specimens a quantity of air- 

 bubbles appeared arising from the imprisonmont of air between the 

 metal and the glass surface, or arising from air occluded or dissolved in 

 the metal itself. These bubbles always took a geometrical form, and 

 they were of great assistance in elucidating the phenomena. It was 

 found that, although the " grains " (or crystal aggregates) suffered de- 

 formation, the individual crystals preserved the same orientation. The 

 effect of strain was in reality to cause a slipping of one grain in whole 

 or in part over another. The effects of such movements were to cause 

 a series, or several series, of lines in the microscopic field, and their 

 true nature was recognised by oblique illumination. 



In their second paper f on this subject the authors state that their 

 object was to study the phenomena of annealing. It is well known that 

 prolonged annealing tends to produce large crystals in iron and steel. 

 But even short exposure at a suitable temperature produces complete 

 recrystallisation, and it has been suggested that these changes occur at 

 critical points corresponding to the arrest-points in the cooling of the 

 metal. These arrest-points indicate evolutions of heat, and it is natural 

 to suppose that they are evidences of rearrangement of the structure of 

 the metal. It was hoped that this change could be observed under the 

 Microscope ; but, although the experimental difficulties of keeping a 

 specimen under microscopic observation while it was being heated were 

 successfully overcome, the attempt to watch the recrystallisation of iron 

 failed. It was found that it could not be expected to see the process of 

 recrystallisation in any metal where etching, staining, or relief polishing 

 is needed to differentiate the constituents. Attention was therefore 

 turned to more fusible metals, especially lead. With this metal great 

 success was obtained, and it was found that in lead which has been 

 severely strained recrystallisation goes on at all temperatures, from that 



* Phil. Trans., exciii. (1899) pp. 353-75 (14 pie. of 51 microphotos). 

 t Op. cit., exev. (1900) pp. 279-301 (13 pis. of 38 microphotos). 



