The Crystalline Structure of Metals. 115 



from the liquid state, the space between the branches filling in as 

 solidification proceeds. 



A marked feature observed in several specimens was the large and 

 rapid growth of one or two individual crystals ; in several instances 

 such individuals grew until they were some hundreds of times larger 

 than their neighbours. We have not been able to discover the deter- 

 mining cause of such growth nor, in general, why one crystal should 

 grow at the expense of its neighbour. Generally the most aggressive 

 crystals were found near the edges of the specimen. It is noticeable 

 that at times a crystal which has already grown considerably is 

 swallowed up by a more powerful neighbour. 



Some light is thrown on the nature of these actions by the fact that 

 this growth only occurs in crystals that have been subjected to severe 

 plastic strain. By casting the metal in a chill mould, specimens of 

 lead can be obtained having a crystalline structure quite as minute 

 as that found in a severely strained specimen, but this structure 

 remains unchanged at temperatures which produce rapid change in a 

 strained specimen. 



The investigation of the effects of such comparatively moderate 

 temperatures was extended to other metals, viz., tin, zinc, and 

 cadmium. In tin, the various phenomena of crystallisation from the 

 fluid state are strikingly illustrated on a large scale by the thin layer 

 of that metal which constitutes the surface of commercial tin-plate. 

 The effects of rapid and slow solidification in producing small or large 

 crystals respectively are well marked, and an examination of the 

 etched surface of tin-plate under the microscope reveals beautiful 

 geometrical markings or pits, whose oriented facets produce the well- 

 known selective effect of oblique illumination. The study of the 

 crystalline structure affords an explanation of the nature and method 

 of production of patterns in " moiree metallique," a process which 

 has long been in use for the decoration of articles manufactured of tin- 

 plate. 



In tin, also, we find that the smallest structure obtainable by 

 quenching the melted metal in water remains unchanged at all tem- 

 peratures up to the melting point ; on the other hand, specimens 

 whose crystalline structure has been modified by great plastic strain 

 exhibit phenomena of recrystallisation at lower temperatures similar 

 to those observed in lead. In a piece of strained tin, an hour's expo- 

 sure to 150 C. produced complete recrystallisation. Exposure to 

 lower temperatures for this short time produced no visible change, but 

 we have not investigated gradual time effects in this metal. The 

 behaviour of strained zinc and cadmium is analogous to that of tin 

 and lead. Exposure to 200 C. is sufficient to produce rapid recrystal- 

 lisation in both zinc and cadmium. This is particularly marked in the 

 case of ordinary sheet zinc. On etching commercial sheet zinc,' no 



