bo4 REPORTS ON THE STATE OF SCIENCE.—1912. 
In both of these processes, nowever, gases intervene in the reaction, 
and a porous mass remains, so that it would be rash to attribute the 
extraction of the metal to solid diffusion without further evidence. 
The diffusion of one metal into another has an important influence 
on the technical process of electroplating. For instance, it was found # 
that when a thick layer of copper was deposited electrolytically on a 
sheet of platinum, and the compound sheet was heated to dull redness, 
the copper oxidised and peeled off, leaving a surface which could be 
cleaned by nitric acid and then presented the appearance of the original 
platinum. On again heating to redness, however, a fresh layer of 
copper oxide was formed, and this process could be repeated more than 
once. The diffusion of an electrolytically deposited layer may even 
take place at the ordinary temperature. ‘Thus a thin layer of copper or 
brass deposited on zinc slowly disappears, the zinc resuming its greyish- 
white appearance, and this occurs even when the outer surface is pro- 
tected from atmospheric action by a layer of lacquer.*' 
A platinum wire coated electrolytically with silver and drawn to a 
smaller diameter through a draw-plate becomes completely soluble in 
nitric acid, showing that alloying has taken place.*° 
A layer of silver, deposited on copper, may be absorbed in course of 
time if sufficiently thin. As copper is capable of dissolving about 5 per 
cent. of silver in the solid state, absorption will cease when this con- 
centration has been reached. It has been suggested by W. Guertler *° 
that the absorption might be hindered by the interposition of a thin 
layer of electrolytic iron, in which silver is not soluble to a measurable 
extent. On the other hand, the interposition of a layer of gold would 
facilitate diffusion, as this metal forms solid solutions in all proportions 
with silver and with copper, and also increases the mutual solubility 
of those metals. Copper will diffuse into iron-through an intervening 
layer of nickel.*? 
A few minor instances of diffusion into metals at the ordinary 
temperature may be noticed. The penetration of sulphur into silver, 
blackening it to a considerable depth, has been recordeéd,** as well as 
the gradual diffusion of iron into silver in the course of several years.*® 
The earliest systematic investigation of diffusion in the solid state 
is due to W. Spring, whose experiments have suggested those of most 
subsequent workers in the subject. - It has been observed by 
Spring °° that various powdered solids could be formed into a compact 
mass by the application of high pressure, and further experiments 
showed that the compression of mixtures of metals with arsenic *? or 
with sulphur *? brought about the formation of arsenides or sulphides 
43 G. Gore, ‘The Art of Electro-Metallurgy ’ (London, 1877), p. 47. 
44 G, Langbein and W. T. Brannt, ‘ The Electro-Deposition of Metals’ (Phila- 
delphia, 1891), p. 133. 
4° Observation of Rouma, quoted by W. Spring, Ber., 1882, 15, 595. 
46 * Metallographie ’ (Berlin, 1910), i., 183. 
47 W. C. Roberts-Austen, Jowr. Lron and Steel Inst., 1887, i., 73. 
48 Homberg, Mém. Acad. Roy. Sci., 1713, 306. 
4 J. H. Howell, Nature, 1906, 78, 464. 
© Bull. Acad. Roy. Belg., 1878 [ii.], 45, 746. 
1 Tbid., 1883 [iii.], 5, 229. 2 Ibid., 492. 
