llti Prof. J. A. Ewing. 



large crystals are visible. In this state the metal is strong and tough, 

 bending quite noiselessly. Aftrr exposure to 200 C. for half an hour, 

 it shows on etching a large brilliant structure, and the metal is then 

 wrak and brittle, and when bent, breaks along well marked cleavage 

 planes and' emits a " cry " like that of tin. 



In cadmium the recrystallisation is comparatively slow at 200 C., 

 and a time effect has been observed ; the action is rather different from 

 that observed in lead. In cadmium the size to which the crystals 

 grow appears to be much more uniform ; no arms or branches are 

 thrown out and no twin-lamellae are found. 



The final section of the paper deals with an hypothesis, which is 

 advanced as an attempt to explain the mechanism of the growth of 

 crystals in apparently solid metal.* According to this hypothesis, the 

 metallic impurities which are present in a metal, play an important 

 part in the action. When a metal solidifies from the fluid state, the 

 metallic impurities ultimately crystallise as a film of eutectic alloy in 

 the inter-crystalline junctions ; when fairly large quantities of such 

 eutectics are present, the microscope reveals their presence as an inter- 

 crystalline cement, such as that formed by " pearlite " in slowly cooled 

 mild steel ; very minute quantities of eutectic, however, will be 

 invisible and yet capable of forming a thin film of fusible cement. 

 We conceive that the changes of crystalline structure which go on 

 while the piece is in the solid state are accomplished by the agency of 

 eutectic films between the crystals, in dissolving metal from the sur- 

 faces of some crystals and depositing it on others. When a metal is 

 severely strained, these films of eutectic will be also strained and in 

 many places broken, thus allowing the actual crystals to come into 

 contact with one another. The difference in the rate of etching of 

 adjacent crystals and the phenomena of the electrolytic transfer, in an 

 acid solution, of lead from one crystal to another in the same mass of 

 metal, support the supposition that there is a difference of electric 

 potential between the crystal faces which are brought into contact 

 by severe strain. If it be assumed that a film of eutectic alloy when 

 fluid, or even when in the pasty condition that precedes fusion, can act 

 as an electrolyte, we may regard any two crystals thus in contact, with 

 a film of eutectic interposed in places, as a very low-resistance circuit, 

 and the growth of the positive crystal at the expense of the negative 

 would result. Moreover, such growth would be more rapid at 

 higher temperatures, and its rate at a given temperature would vary 

 in different specimens according to the nature and quantity of the 

 impurities present. That an alloy can act as an electrolyte has not 

 been established experimentally, but the assumption is supported by 

 the close general analogy between alloys and salt solutions. This 

 analogy extends to the very question of the growth of crystals, as 



* It is proper to say that this hypothesis is due to Mr. Rosenhain. J. A. E. 



