Intelligence and Miscellaneous Articles. 525 



and platinum in molten lead. He points out that although the action 

 of osmotic pressure in lowering the freezing-point of metals has 

 been carefully examined, very little attention has been devoted to 

 the measurement, or even to the consideration, of the molecular 

 movements which enable two or more metals to form a truly 

 homogeneous fluid mass. The absence of direct experiments on 

 the diffusion of molten metals is probably explained by the want of 

 a sufficiently accurate method. Ostwald had stated, moreover, 

 with reference to the diffusion of salts, that " to make accurate 

 experiments in diffusion is one of the most difficult problems in 

 practical physics," and 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 

 temperatures, which led to the abandonment of the earlier work, 

 w r as overcome when the author arranged his recording pyrometer, 

 and the use of thermo-junctions in connexion 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 care- 

 fully 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 ascertained 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 Pick's law, by the differential equation 



dv __ 7 d 2 v 

 dt doc 2 ' 



In this equation x represents distance in the direction in which 

 diffusion takes place, v is the degree of concentration of the diffu- 

 sing metal, and t is the time ; Jc is the diffusion constant, that is, 

 the number which expresses the quantity of the metal in grams 

 diffusing through unit area (1 square centim.) in unit time (one 

 day) when unit difference of concentration (in grams per cubic 

 centim.) is maintained between the two sides of a layer 1 centim. 

 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. 



