1896.] 



On the Diffusion of Metals, 



283 





1c. 







3*] 9 



at 500°. 



„ bismuth 



4-52 







4-65 







4-14 



>> 





3-18 



»> 



Rhodium in lead . . . . 



3-04 



)> 



Platinum in lead. . . . 



1-69 



at 490°. 





3 ; 03 





Gold in mercury 



072 at 11°. 



In order to afford a term of comparison, it may be stated that the 

 •diffusivity of chloride of sodium in water at 18° is 1*04. 



The author at present refrains from drawing any conclusion as to 

 the evidence which the results afford respecting the molecular con- 

 stitution of metals. It is, however, evident that they will be of 

 value in this connection, because, with the exception of the gases, 

 they present the simplest possible case of diffusion which can occur — 

 the diffusion of one element into another. 



Thns the relatively slow rate of diffusion of platinum as compared 

 with gold, points to its having a more complex molecule than the 

 latter. 



Part II. — Diffusion of Solid Metals. 



The second part of the paper is devoted to the consideration of the 

 diffusion of solid metals. ]\Iuch of the evidence is historical, for 

 there has long been a prevalent belief that diffusion can take place in 

 -solids, and the practice in conducting important industrial operations 

 supports this view. In this connection the author cites two truly 

 venerable " cementation" processes. The object in the first of these 

 is the removal of silver from a solid gold-silver alloy, while the second 

 is employed in steel-making by the carburisation of solid iron. In 

 both of these processes, however, a gas may intervene, though the 

 carburisation of iron by the diamond, which had been effected in vacuo 

 by the author, suggests that if a gas does intervene in the latter case, 

 its quantity must be very minute. In connexion with the mobility of 

 various elements in iron the work of Colson, of Osmond, and of 

 Moissan is specially referred to. 



The author points out that in 1820 Faraday and Stodart showed 

 that platinum will alloy with steel at a temperature at which even 

 the steel is not melted, and they express their interest in the forma- 

 tion of alloys by cementation, that is by the union of solid metals. 



The remarkable view expressed by Graham, in 18G3, that the 

 M three conditions of matter (liquid, solid, and gaseous) probably 

 always exist in every liquid or solid substance, but that one predomi- 



u 2 



