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On the Diffusion of Metals. 



[Feb. 20 r 



nates over the other," is shown to have afforded ground for the 

 anticipation that metals would diffuse into each other at temperatures 

 far below their melting points. Reference is then made to the im- 

 portant work by Spring in 1886 on the lead-tin alloys, which retained 

 a certain amount of molecular activity after they had become solid, 

 and special importance is attached to the proof afforded by Spring, that 

 alloys may be formed either by the strong compression of the finely 

 divided constituent metals at the ordinary temperature (1882) or 

 (1894) by the union of solid masses of metal compressed together at 

 temperatures which varied from 180° in the case of lead and tin, to- 

 400° in the case of copper and zinc ; tin melting at 227° and zinc 

 at 415°. 



The evidence as to the volatilisation of solid metals is then traced, 

 and allusion is made to the expression of Robert Boyle's belief,, 

 that even such solid bodies as glass and gold might respectively 

 "have their little atmospheres, and might in time lose their weight."" 



Merget's experiment on the evaporation of frozen mercury is 

 quoted in relation to Gay-Lussac's well-known discovery that the 

 vapours emitted by ice and water both at 0° C, are of exactly equal 

 tension. 



Demarcay's experiments on the volatilisation of metals in vacuo at 

 comparatively low temperatures is connected with the evidence 

 afforded by Spring (1894), that the interpenetration of two metals at 

 a temperature below the melbing point of the more fusible of the 

 two is preceded by volatilisation. 



The author then points out that, interesting as the results of the- 

 earlier experiments are, as affording evidence of molecular inter- 

 penetration, they do not, for the purpose of measuring diffusivity. 

 come within the prevailing conditions in the ordinary diffusion of 

 liquids, in which the diffusing substance is usually in the presence of 

 ji large excess of the solvent, a condition which has been fully main- 

 tained in the experiments on the diffusion" of liquid metals described 

 in the first part of the paper. Yan't Hoff has made it highly prob- 

 able that the osmotic pressure of substances existing in a solid solu- 

 iion is analogous to that in liquid solutions, and obeys the same laws : 

 nnd it is probable that the behaviour of a solid mixture, like that of 

 :i liquid mixture, Avould be greatly simplified if the solid solution 

 were very dilute. 



The author proceeds to describe his own experiments on the diffu- 

 sion of solid metals. They are of the same nature as in the case or: 

 fluid metals, except that the gold, which is the metal chosen for 

 examination, was placed at the bottom of a solid cylinder of lead 

 instead of a fluid one. 



In the first series of experiments, cylinders of lead, 70 mm. long, 

 with either gold, or a rich alloy of gold and lead at their base, were 



