392 PROFESSOR W. C. ROBERTS-AUSTEN ON THE DIFFUSION OF METALS. 
Experimental Results. 
The following tables, A, B, C, D, embody the results of four diffusion experiments, 
made simultaneously, two with gold and two with platinum, in straight vertical 
tubes. They serve to indicate the method of calculating the diffusivity of the 
respective metals from the data afforded by the experiments. In the case given in 
Table A, for example, the alloy of gold and lead was allowed to diffuse upwards into 
pure lead in the way already described. This comparatively rich alloy of gold and 
lead containing 30 per cent, of gold, occupied 2T08 centims. of the length of the tube. 
The tube (the whole length of which was about 16 centims.) was subsequently cut 
into sections, each of which was 1'054 centims. long, or half the length occupied by 
the rich alloy of gold. The reason for the adoption of this length was as follows :— 
In Graham’s experiments on the diffusion of salts, which form the basis of Stefan’s 
tables, the concentrated solution from which diffusion started, occupied two of the 
sections into which the contents of the diffusion cylinders were divided for analysis. 
The length of one of these sections/or half of the length of the portion of the tube 
occupied by the alloy from which diffusion takes place, is denoted in the calculation 
by the letter h. In Table A there are fourteen such sections, and these are numbered 
consecutively in column 1, while column 2 gives the weight in grammes of the lead- 
gold alloy obtained from each of the respective sections and of the pure gold extracted 
from it. Column 3 gives the percentage of gold present in each section calculated 
from the numbers given in column 2. In column 4 these percentages of gold have 
been corrected so as to give the amounts of gold in equal volumes of the “ solution. 
It must be remembered that the density of the gold-lead alloys increases with the 
percentage of gold, so that the concentration of the gold, that is, the weight present 
in equal volumes of the alloy, is not truly represented by the percentage given in 
column 3. A correction of sufficient accuracy may, however, be introduced by 
assuming that the fluid densities of the alloys are proportional to their calculated 
densities, and the numbers so corrected are given in column 4. 
