408 PROFESSOR W. C. ROBERTS-AUSTEN ON THE DIFFUSION OF METALS. 
this precaution will be obvious as the union of the two compressed cylinders might 
easily be effected by the fusion of an eutectic alloy with a relatively low melting point. 
I observed in 1887 that an electro-deposit of iron on a clean copper plate will adhere 
so firmly to it that when they are severed by force a copper film is actually stripped 
from the copper plate and remains on the iron, thus affording clear evidence of the 
interpenetration of metals at the ordinary temperature. I found that this interpene¬ 
tration of copper and iron will take place through films of electro-deposited nickel.* * * § 
Mylius and Fromm have shown that metals interpenetrate and form alloys when they 
are precipitated by electrolysis from their aqueous solutions. - ! 
The diffusion of metals in each other must be closely connected with the evaporation 
of solid metals or alloys at temperatures far below their melting points, and it w^ill be 
well before describing the new experiments on diffusion in solid metals to briefly 
recall the facts which are already known. It is not necessary to go further back for 
definite views on the subject than to the time of Boyle, | who thought that “even such 
(bodies) as are solid may respectively have their little atmospheres,” . . . “for” 
he adds, “ no man, I think, has yet tried whether glass, and even gold, may not in 
length of time lose their weight.” 
Boyle’s opinion was correct, for mercury which has been frozen by extreme cold 
does, as Merget§ showed two centuries later, evaporate into the atmosphere surround¬ 
ing it ; a fact which is of much interest in connection with Gay Lussac’s well-known 
observation that the vapours emitted by ice and by water both at 0°C. are of equal 
tension. Demar<2Ay|| has proved that in vacuo metals evaporate sensibly at lower 
temperatures than they do at the ordinary atmospheric pressure, and he suggests 
that even metals of the platinum group will be found to be volatile at comparatively 
low temperatures. 
Thus he finds that cadmium volatilizes at 1G0°, zinc at 184°, and lead and tin at 
360°, and subsequently Spring^ (1894) demonstrated that zinc is volatile at atmos¬ 
pheric pressure at about 300°, cadmium at about 400°, while even copper is slightly 
volatile at the latter temperature. 
Moissan** has stated quite recently that the vapour tension of solid silicon enables 
it to unite with iron and chromium by true “cementation ” at a temperature of 1200°, 
which is much below the fusing point of these metals. 
It must be borne in mind that the interesting facts recorded by the various experi¬ 
menters whose names I have cited hardly come within the prevailing conditions in 
* ‘ Journ. Iron and Steel lust.,’ Part I., 1887, p. 73. 
f ‘ Ber. der D. Chem. Gesell.,’ vol. 27, 1894, p. 630. 
1 ‘ Collected Works.’ Shaw’s edition, 1738, vol. 1, p. 400. 
§ ‘ Ann. de Chirm et de Pliys.,’ vol. 25, 1872, p. 121. 
|| ‘ Comptes Rendus,’ vol. 95, 1882, p. 183. 
Loc. cit., 1894, p. 42. 
** ‘ Comptes Rendus,’ vol. 121, 1895, p. 621. 
