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X. B aker r ax Lecture.— On the Diffusion of Metals. 
By W. C. Robe RTS-Austen, C.B., F.R.S., Professor of Metallurgy , Royal College 
of Science ; Chemist of the Mint. 
Received and Read February 20, 1896. 
[Plates 7, 8.] 
PART I.—DIFFUSION OF MOLTEN METALS. 
Gold, Platinum, and Rhodium, diffusing in Molten Lead and in 
Molten Bismuth. 
The diffusion of molten and solid metals lias long demanded investigation, their 
molecular mobility being of great interest in relation to the constitution of matter, 
and its results of much industrial importance. 
The analogy of alloys to ordinary saline solutions has often been pointed out, and 
many experiments have recently been devoted to comparing the action of osmotic 
pressure in saline solutions and in alloys, as measured by the lowering of the freezing 
point which is caused by the addition to the solvent of a small quantity of another 
body.* The general effect is the same whether the solvent is a liquid like water or a 
molten metal. Very little attention has, however, been given to the consideration of 
the molecular movements which enable two or more molten metals to mix spon¬ 
taneously and form a truly homogeneous fluid mass, although it is by such an 
investigation that the analogy of an alloy to a saline solution may reasonably be 
expected to be more clearly revealed than by any other method of research. A single 
example of the spontaneous mixing of two metals may be useful. In preparing the 
alloy of gold and copper used for coinage, some 33 kilos, of gold and 3 kilos, of copper 
are melted together in a single crucible, and the results of assays on the first and the 
last portions of metal poured from the crucible, seldom differ by more than one 
ten-thousandth part. Such a fluid mass of standard gold owes this remarkable 
uniformity in composition not only to the mechanical stirring by which the blending of 
* Heycock and Neville, ‘Trans. Chem. Soc.,’ vol. 55, 1889, p. 666; vol. 57, 1890, pp. 376 and 656; 
vol. 59, 1891, p. 936; vol. 61, 1892, pp. 914 and 888 ; vol. 65, 1894, pp. 31 and 65 ; vol. 67, 1895, p. 1024; 
and Robebts-Austen, ‘ Proc. Roy. Soc.,’ vol. 49, 1891, p. 347. 
MDCCCXCVI.—A. 
23.6.96 
