PROFESSOR W. C. ROBERTS-AUSTER OR THE DIFFUSION OF METALS. 413 
Welding of Gold anal Lead, and Diffusion of Gold in Lead at the Ordinary 
Temperature. 
The fact that two clean surfaces of lead will weld together at the ordinary tem¬ 
perature is well known. It may be well, however, to state in connection with the 
experiments of DEMATigAY 7 (see p. 408) on the volatilization of metals in vacuo at 
comparatively low temperatures that if the ends of small bars of gold or silver be 
surfaced, pressed against lead, and maintained for four days in vacuo at a temperature 
of only 40°, the interpenetration of the two metals will be so complete that their 
separation can only be effected by the application of a load of 70 lbs. per square inch 
of the sectional area of the bars, or no less than of the breaking strain of lead. 
It remains to be seen whether diffusion can be measured in solid lead at the 
ordinary temperature, and, with this object in view, cylinders have been prepared 
and set aside for future examination. 
In searching for evidence of diffusion in solid metals at the ordinary temperature, 
it will be well to examine certain alloys used in art metal-work by the Japanese, who 
often employ an alloy of copper containing a small proportion of gold (called Shalcudo), 
which is soldered or welded in alternate layers w T ith pure copper. The gold in the 
copper enables it to assume a beautiful purple patina when it is treated with suitable 
pickling solutions, which leave the pure copper of a red colour. In this way very 
singular banded effects are produced. Many of the specimens are centuries old, and 
I have attempted, by grinding away the existing patina and re-pickling the surface, 
to ascertain whether the widening of the coloured bands would show that, in the 
course of time, gold had diffused from the Sliakudo layers and had passed into the 
copper. I believe that there is evidence that it does do so, but the enquiry is full of 
difficulty, and needs training in micrography, of which my friend, M. Osmond, is a 
master. We propose to study this part of the subject together, and I only allude to 
it here because, if diffusion occurs in copper, silver, and gold at the ordinary tempera¬ 
ture, its results should be revealed in the products of this ancient oriental art. 
Diffusion of Gold in Solid Silver. 
A short cylinder of silver, containing 20 per cent, of gold, was pressed against the 
carefully-surfaced end of a cylinder of pure silver, 1 centim. in diameter and 2'5 
centims. long. These cylinders were kept in an annealing furnace for ten days, at a 
temperature which never exceeded 800°, and was, therefore, 160° below the melting- 
point of silver (960°). The lowest melting point of the gold-silver series of alloys is 
850°. The cylinder was then cut into sections in the usual way, and the amount of 
diffused gold determined by analysis. As the temperature was intermittent the true 
diffusivity cannot be taken, but I am satisfied that the diffusivity of solid gold in 
solid silver at 800° is of the same order as that of gold in lead at 200°. It would 
O 
