On the Diffusion of Gold in Solid Lead. 103 



photographs of the gold extracted from the successive layers of a 

 cylinder are, moreover, appended ; the magnification being in all cases 

 the same (56 diameters). 



It may be thought that the amounts are but small, but from the 

 point of view of the assayer, who is accustomed to determine minute 

 quantities of precious metal in large masses of material, the results 

 assume very substantial proportions. Thus the amount of gold found 

 in the richest layer of lead represents no less than 1 oz. 6 dwts. of 

 gold per ton, which could be profitably extracted, while the amount 

 in even the poorest layer is 1^ dwt. per ton. 



The significance of these results may perhaps be made clearer if it 

 is stated that the amount of gold which would diffuse in solid lead 

 at the ordinary temperature in 1000 years is almost the same as 

 that which would diffuse in molten lead in a single day, provided no 

 more gold is supplied in either case than can be held in solution. This 

 will serve to show how important temperature is in relation to diffusion. 

 As an example of the relative effects of temperature on this purely 

 physical change and on a chemical change, it may be interesting to 

 refer to the case of the dissociation of auric chloride. At the 

 ordinary temperature, the tri-chloride of gold is very stable though 

 it decomposed rapidly at 180, and my colleague, Dr. Rose,* has shown 

 that though the decomposition of auric chloride may be perceptible at 

 a temperature of 70, it would nevertheless require, at that tempera- 

 ture, about twenty-five years for its nearly complete change into mono- 

 chloride. 



I believe, with Eobert Boyle, that though solid gold may have its 

 " little atmosphere," " no man has yet tried whether gold may not in 

 time lose its weight," but the rate at which gold can possibly evaporate 

 into the air at the ordinary temperature must be far less than that at 

 which it diffuses into lead. This shows that the action of a solvent 

 for the gold is necessary, and this solvent is provided by bringing gold 

 into contact with solid metallic lead. 



I would express my warm acknowledgment to Dr. A. Stansfield, who 

 aids me in conducting the Metallurgical Laboratory at the Royal 

 College of Science, for the care he has devoted to the tedious manipu- 

 lation involved in these experiments. His help has given me great 

 confidence in the accuracy of the results. It may be well to add that 

 I propose to prepare suitable cylinders of lead and gold on the lines 

 indicated in this paper, and to offer them to the National Physical 

 Laboratory with a view to their being examined after such a lapse of 

 time as may be deemed fully adequate. 



[Note, May 28. In the Graham Lecture, delivered at Glasgow on the 

 18th of April last, after speaking of the diffusion of gold in solid lead, 



* ' Journ. Chem. Soc.,' rol. 67 (1895), p. 904. 



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