410 PROFESSOR W. C. ROBERTS-AUSTEN ON THE DIFFUSION OF METALS. 
cut up into sections, and the amount of precious metal in the respective sections was 
determined by analysis. It should be observed that gold and lead are singularly well 
adapted for such an investigation, as the amount of precious metal diffusing can, by 
the aid of a delicate balance, be determined with wonderful precision. The balance 
used was a short-beam one, with a pointer at each end of the beam, moving over a 
graduated ivory scale, the divisions of which were read by means of fixed lenses. 
This balance was specially designed for the verification of the weights used in gold 
assaying at the Mint. Its maximum load is 0'5 gramme, and it will readily indicate 
the five hundred thousandth part of a gramme. 
Gold in Lead at 251°. 
The results of two experiments are shown in the following tables. 
The total length of the cylinder was 6 '5 centims., in the case of Experiment I, and 
7*0 centims. in No. II ; fragments of gold were fused on their lower ends. Time, 
thirty-one days. 
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 
26-9 
0-0824 
0-3064 
2 
22-2 
0-0053 
0-0239 
3 
19-6 
0-0015 
0-0076 
0023 
4 
22-3 
0-00013 
0-0006 
5 
24-6 
0-00002 
o-oooi 
II. 1 
2-83 
0-1803 
0-3700 
2 
2-55 
0-0020 
0-0784 
3 
312 
0-0013 
0-0417 
4 
2-84 
0-0006 
0-0211 
0-03 
5 
2-99 
0-00026 
0-0087 
6 
5-92 
0-00013 
0-0022 
7 
6-03 
0-00001 
0-0002 
8 
4-40 
In calculating the diffusivity in this and in the following experiments, the initial 
concentration of the solid lead-gold alloy from which diffusion starts was deduced 
from the general trend of the concentration curve as plotted from the above figures. 
The first section in each case, which of course contained pieces of partially alloyed 
gold, was neglected. 
