The Crystalline Structure of Metals. 117 



Joly has shown that when crystals of a salt are immersed in their 

 mother-liquor, growth of one at the expense of others will take place. 



It should be added that solution of one crystal into the intervening 

 film of eutectic, along with deposit on the neighbouring crystal from 

 the eutectic, may occur as a consequence of differences of orientation, 

 producing differences of " solution pressure " apart from actual 

 electrolysis, but the fact that growth has not been observed to 

 occur except in strained crystals favours the view that the action is 

 electrolytic. 



Some further results which have been deduced from the above 

 hypothesis have been verified by experiment. It follows from the 

 hypothesis that an inter-crystalline boundary containing no eutectic 

 would be an impassable barrier to crystalline growth, but if the eutectic 

 could in any way be supplied, growth across the boundary might take 

 place. In an absolutely pure specimen of lead, there would be no 

 eutectic at the inter-crystalline junctions, but as extremely minute 

 traces of impurity would suffice to set up the action, it is almost hope- 

 less to verify the hypothesis in this way. Some experiments on the 

 cold welding of lead have, however, borne out our conclusions. Two 

 clean, freshly scraped lead surfaces will unite under great pressure in 

 the cold state, and if a piece so welded be annealed, we find that the 

 crystalline growth due to the annealing, with very rare exceptions, 

 never crosses the inter-crystalline boundary formed by the welding 

 surface. To test whether the presence of some eutectic would allow 

 growth to take place, we have scattered small quantities of a more 

 fusible metal over the freshly scraped surfaces of lead before squeezing 

 them together. Then, after- a cold weld had been made by pressure, 

 .on annealing by exposure to 200 C. it was found that crystal growths 

 frequently crossed the line of the weld, as the above theory led us to 

 expect. This experiment has been repeated many times with the 

 uniform result that whenever a small quantity of eutectic, or of an 

 impurity capable of forming a eutectic with the lead, was scattered 

 over the clean surfaces before welding, a distinct growth of crystals 

 across the boundary took place as a resxilt of annealing. On the other 

 hand, a large number of welds were made without introducing any/ 

 impurity, and with very rare exceptions they showed no growth across 

 the boundary, even after the annealing process was continued for some 

 weeks. In rare exceptions a minute amount of growth across the 

 boundary was observed, but these may fairly be accounted for by the 

 almost unavoidable presence of traces of impurity. The result as a 

 whole goes far to confirm this solution theory of crystalline growth in 

 annealing. 



VOL. LXVII. 



