ON DIFFUSION IN SOLIDS i 359 
A slight increase of conductivity was noticed in the first hour, owing 
to the conversion of electrolytic copper into the better-conducting 
annealed form, but after this a continuous decrease of conductivity 
was observed, the minimum being practically reached after 140 hours, 
when the value attained was exactly that of a homogeneous alloy of the 
same composition. Exactly similar results were obtained when gold 
wires were coated with silver or copper. In a further experiment the 
area of contact of the two metals was greatly increased by coating a 
copper wire, 0°075 mm. diameter, with nickel and copper alternately, 
until thirty layers of each metal had been deposited. The thickness of 
the layers gradually diminished towards the outside, ultimately reach- 
ing a value of about 2°5 yw. Such a wire became homogeneous, as 
shown by microscopical examination as well as by the determination 
of the conductivity, after less than two hours at 1,000°, whilst diffusion 
proceeded at an appreciable rate at 500°. These authors are now 
engaged on experiments with still finer layers of the two metals, with 
the object of studying diffusion at still lower temperatures. 
The attempt has been made to draw a distinction between diffusion 
in metals and that in fluids on the ground that the penetration of 
oxygen into iron or steel in the ordinary process of tempering does 
not proceed indefinitely, but ceases at a certain finite depth, dependent 
only on the temperature.*! This is, however, inconsistent with the 
facts, as it is now known that the tempering colours, by which the 
thickness of the oxidised filrn is judged, are dependent on time as well 
as on temperature, and that although the diffusion of oxygen in such 
cases is slow, the process continues to greater and greater depths if 
given sufficient time. 
Cementation and Decarburisation of Iron. 
The process of converting iron into steel by heating in contact with 
solid carbon has undoubtedly been practised from ancient times, 
although it is not described by ancient authors.’ The examination of 
early implements sometimes shows that the iron has been superficially 
hardened by this means. For example, a Sinhalese chisel of the fifth 
century showed carburisation at the edge, the mass of the chisel being 
soft iron made by the direct process.** It is suggested that the process 
is of Indian origin. An accurate description of cementation, as well 
as of the converse process of decarburising iron by heating in hema- 
tite, is given by R. A. F. de Réaumur.™* As regards a theoretical 
explanation, the fact of reaction between solid substances was felt to 
be a difficulty, and an hypothesis was proposed, according to which 
the active cementing substance was not carbon, but gaseous hydro- 
earbons derived from. impurities in the charcoal, or oxides of carbon.7* 
1 C, Barus and V. Strouhal, Bull. U.S. Geol. Surv., 1886, 27, 51; C. Barus, Nature, 
1889, 41, 369. 
® J. Percy, ‘ Metallurgy, Iron and Steel ’ (London, 1864), p, 110. 
® Sir R. Hadfield, Proc. Roy. Soc., 1912, 864, 94 ; Jour. Iron and Steel Inst., 1912, i. 
u “TL Art de convertir le fer forgé en acier, et l'art d’adoucir le fer fondu’ (Paris, 
1722), 
7% F. Le Play, Ann. Chim. Phys., 1836 [ii.],.62, 291; F. Le Play and A. Laurent, 
ibid., 1837 [ii.], 65, 408. 
