372 PROFESsol; J. A. EWINC AM> Ml. 1 . \V. IJosKNIIAIN 



from one point, giving an appearance which resembles roughly the ribs of ;in umbrella. 

 Kiif. 47 incidentally shows the boundary of two such areas. 



In the course of experiments on the lead-l>isnuith eutectic, specimens were obtained 

 showing comparatively large isolated crystallites. When the piece was strained these 

 crystallites were found to exhibit slip-hands. Kxamples are ^iven in figs. 50 and 51. 

 These are interesting as showing the development of dip-bands in bodies which are 

 evidently fully developed crystals, even as to external form. 



A study of the micro-structure of alloys suggests a possible explanation of the 

 peculiarities they present in regard to variation of electrical conductivity with 

 temperature. The two constituents may behave individually as pure metals in this 

 respect, but if their coefficients of expansion are different the closeness of the joints 

 between them will depend on the temperature. Thus, if the more expansible metal 

 exists as plates or separate pieces of any form within the other, the effect of heating 

 will be to make the joints between the two conduct more readily, with the result of 

 reducing the increase of resistance to which heating would otherwise give rise, and in 

 extreme cases with the effect even of producing a negative temperature coefficient. 



Reviewing the general results of the experiments, we consider that they establish 

 the view that the structure of metals in general is crystalline, and remains crystalline 

 when the form of the metal is altered by strain, plastic yielding being due to 

 slips on cleavage or gliding planes within each individual crystalline grain, and partly 

 (in some metals) to the production of twin crystals. In a pure metal, when straining 

 is carried far enough to produce fracture, the crystalline grains suffer cleavage, and 

 the cleavage surfaces thus developed give to the fracture its characteristically crystal- 

 line appearance. In impure metals fracture may occur through the parting of grains 

 from one another at their boundaries. In both cases, however, the plastic yielding 

 which precedes fracture takes place by slips in the manner we have described. 



In conclusion we should like to express our indebtedness to Sir W. HOBERTS- 

 AUSTEN, Mr. T. ANDREWS, and Professor ARNOLD, for giving us at the outset of our 

 work the benefit of their large experience in preparing specimens of metals for 

 microscopic examination. Messrs. HEYCOCK and NEVILLE and Mr. A. HUTCHINSON 

 have assisted us materially by various suggestions, and by supplying specimens for 

 examination. We have also to thank Mr. ANDREWS, Mr. STEAD, Mr. HADFIELD, 

 Professor HICKS, and Messrs. EDGAR ALLEN and Co. for special specimens of iron. 



The work described in this paper was carried out in the Engineering Laboratory 

 at Cambridge. 



To facilitate reference to the illustrations an index is added in which brief par- 

 ticulars are given of the subject of each photograph. 



