412 PROFESSOR W. C. ROBERTS-AUSTEN ON THE DIFFUSION OF METALS. 
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 
0-64 
0-00025 
0-039 
2 
2-33 
0-00069 
0-030 
0-005 
3 
2-02 
0-00030 
0-015 
II. 1 
0-80 
0-00031 
0-039 
2 
2-06 
000060 
0-029 
0-004 
3 
1-40 
0-00027 
0-019 
Gold in Lead at 100°. 
In the following, and concluding experiment of the series, the lead cylinders, 
0'45 centim., were maintained in a water oven at a temperature of 100° for forty-one 
days. Each cylinder had a plate of very pure gold pressed against its surfaced end. 
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 
0-22 
0-00013 
0-059 
2 
119 
0-00006 
0-005* 
0-00002 
3 
1-98 
II. 1 
0-22 
0-00034 
0T55 
2 
1-52 
o-oooio 
0-007 
0-00002 
3 
1-43 
It is remarkable that gold placed at the bottom of a cylinder of solid lead, 
7 centirns. long, should, at 250°, appear in notable quantity at the top of it in less 
than a month. The diffusivity of gold in solid lead is, however, slow when compared 
with diffusion in the fluid metal. The diffusivity of gold in lead at 500° was shown 
in the first part of this paper to be 3‘0 In solid lead, on the other hand, the 
diffusivity at 250° is 0'03, or too^Ii of the diffusivity at 500°. It is also clear that gold 
will diffuse into solid lead at the very moderate temperature of 100°, which is 225° 
below its melting point, a fact which must be considered to be remarkable, and one 
the existence of which has hitherto been unsuspected. The diffusivity is, however, 
only r oo^o oo -th of that which occurs in fluid lead. 
any complication which might arise if the cylinder were heated by fusing gold on to the lead. At the 
conclusion of the experiment the gold alloy was simply detached by breaking it off the lead cylinder. 
* It may be thought that 0‘005 per cent, of gold is rather a small quantity to measure w r ith accuracy, 
but it represents no less than If oz. of gold per ton of lead, an amount of precious metal far in excess of 
that which assayers are accustomed to.deal -with in the valuation of am-iferous commercial lead. 
