282 



Prof. W. C. Roberts-Austen. 



[Feb. 20, 



the difficulties are obviously increased when molten metals diffusing 

 into each other take the place of salts diffusing into water. 



The continuation of the research was mainly due to the interest 

 Lord Kelvin had always taken in these experiments. The want of 

 a ready method for the measurement of comparatively high tempera- 

 tures, which led to the abandonment of the earlier work, was over- 

 come when the author arranged his recording pyrometer, and the use 

 of thermo-j unctions in connection with this instrument rendered it 

 possible to measure and record the temperature at which diffusion 

 occurred. Thermo-junctions were placed in three or more positions 

 in either a bath of fluid metal or an oven carefully kept hotter at the 

 top than at the bottom. In the bath or oven, tubes filled with lead 

 were placed, and in this lead, gold, or a rich alloy of gold, or of the 

 metal under examination, was allowed to diffuse upwards against 

 gravity. The amount of metal diffusing in a given time was ascer- 

 tained by allowing the lead in the tubes to solidify ; the solid metal 

 was then cut into sections, and the amount of metal in the respective 

 sections determined by analysis. 



The movement in linear diffusion is expressed, in accordance with 

 Fick's law, by the differential equation 



dv _ ^d~v 

 dt d 2 x 



In this equation x represents distance in the direction in which 

 diffusion takes place, v is the degree of concentration of the diffusing 

 metal, and t is the time ; h is the diffusion constant, that is, the 

 number which expresses the quantity of the metal in grams diffusing 

 through unit area (1 sq. cm.) in unit time (one day) when unit differ- 

 ence of concentration (in grams per c.c.) is maintained between the 

 two sides of a layer 1 cm. thick. The author's experiments have shown 

 that metals diffuse in one another just as salts do in water, and the 

 results were ultimately calculated by the aid of tables prepared by 

 Stefan for the calculation of Graham's experiments on the diffusion 

 of salts. 



The necessary precautions to be observed and the corrections to 

 be made are described at length, and the values of the diffusivity of 

 various metals in lead are then given. 



The values for Tc, the diffusivity, given in sq. cm. per day, are as 

 follows : — 



