404 PROFESSOR W. C. ROBERTS-AUSTEN ON THE DIFFUSION OF METALS. 
PART II.—DIFFUSION OF SOLID METALS. 
Gold, Diffusing into Solid Lead. 
The experiments described in the first part of this paper naturally suggested the 
enquiry whether gold would still permeate lead if the temperature were maintained 
at a point far below the melting point of lead. Would diffusion take place through 
solid lead at the ordinary temperature, or must a certain amount of viscosity be 
given to it by the application of a moderate heat ? These were questions which 
demanded attention. 
Historical. 
The history of the diffusion of solids is full of interest, and it may be convenient, 
as far as possible, to group the facts which are known, rather than to deal with them 
in strict chronological order. 
“ Kernel Roasting .”—There has long been a prevalent belief that diffusion can take 
place in solids, and the practice in conducting certain important industrial processes 
supports this view. One of these processes, which is of comparatively ancient date, 
has certainly been employed since 1692 at Agorclo, and its results are as follows : 
When lumps of cupriferous iron pyrites are subjected to very gradual roasting with 
access of air, the copper becomes concentrated as a “ kernel ” of nearly pure sulphide 
of copper in the centre of a mass of ferric oxide, while, at the same time, the silver 
originally present in the ore travels outwards and forms a glistening shell on the 
exterior. These complicated changes must be effected in the solid by a-movement 
allied to diffusion. 
Cementation Processes .—Of all the processes which depend on the diffusion of 
solids probably the most interesting is the truly venerable one by which silver may 
be recovered from either plates or globules of solid gold by “ cementation,” the name 
being derived from the “ cement ” or compound in which the plates were heated. Its 
nature was indicated by Pliny, and the manipulation it involved was minutely described 
by Geber in the 8th century, as well as by many of the early metallurgists ; Savot, # 
for instance, pointed out in the early part of the 17th century that “cementation” 
will deprive gold of the silver it contains, “ however small ” the amount of the latter 
metal may be, so that it will be evident that the elimination of the silver from the 
centre of a mass of solid gold must also be effected by an inter-molecular movement 
allied to diffusion. The evidence, however, is not conclusive, because gaseous chlorine 
intervenes, and may even play an important part in the penetration of the solid 
metal. 
In another ancient “cementation” process, the conversion of strongly heated but 
* ‘ Discours sur les Medailles antiques,’ 1627, p. 76. 
