PROFESSOR W. C. ROBERTS-AUSTEN ON THE DIFFUSION OF METALS. 411 
Gold in Lead at 200°. 
The next experiments were made at 200° and only lasted ten days. The results 
of two of these are given below. In No. I the cylinder of lead was 2‘5 centims. 
long, and had a plate of gold fused on to its end. In No. II the cylinder was 
I'O centim. long, and the plate of gold was merely held against the carefully surfaced 
end by means of a binding screw. 
Number of section 
(counting from 
base). 
Weight of alloy 
in grammes. 
Weight of gold 
in grammes. 
Gold per cent. 
Diffusivity in sq. 
centims. per diem. 
I. 1 
LoO 
0-2147 
14-3 
2 
1-05 
lost 
3 
2-00 
0-00055 
0-0275 
4 
2T4 
0-000185 
0-0086 
0-007 
5 
2T8 
0-000022 
o-ooio 
6 
3-95 
trace 
7 
4-60 
none 
II. 1 
0-39 
0-00082 
0-210 
2 
0-49 
0-000140 
0-029 
0-008 
3 
0-82 
0-000187 
0-023 
4 
212 
0-000220 
o-oio 
5 
2-50 
0-000040 
0-002 
There are many other results of similar experiments at 200°, which closely agree 
with those given. There is, however, one point that must not be lost sight of. The 
lead cylinder was solid, but the experiments just described were conducted at a 
higher temperature than the melting-point of the eutectic alloy of gold and lead, 
which melts at nearly 200°. It may be that directly gold diffuses into lead it forms 
an alloy which is fluid at the temperature of the experiment and, therefore, though 
the lead itself is solid, it may, nevertheless, contain the gold in the form of a fluid 
alloy which diffused as such. Great care was consequently taken to ascertain 
whether gold would still diffuse in solid lead at a temperature well below the melting- 
point of the eutectic alloy (200°), and for this purpose the temperature chosen 
was 165°. 
Gold in Lead at 165°. 
The length of the cylinder in Experiment 1 was 0‘64 centim., and it was maintained 
at this temperature for thirty days. The length of the cylinder in Experiment 2 was 
0’60 centim., and it was heated for twenty-nine days. In each case diffusion started 
from a 5 per cent, alloy of gold and lead, pressed against the end of the lead cylinder,* 
and there was no need, therefore, to reject the first section of the cylinder. 
* This precaution, -was necessary, as the diffusion must be very slow, and it was important to avoid 
3 G 2 
