406 PROFESSOR W. C. ROBE RTS-AUSTEN ON THE DIFFUSION OF METALS. 
tions of iron, and of iron and carbon, atfords, moreover, a striking proof of the 
molecular mobility of solid iron at a temperature which is at least 600° below its 
melting point. 
The history of the formation of alloys by cementation will be traced subsequently. 
The Penetration of Solid Metals by Gases. —This subject was, as is well known, 
investigated by Graham, “ the leading atomist of his generation,” but before his 
attention was specially directed to it, a mass of experimental evidence led him in 
1863 to express views of singular interest in the beautiful paper'" which embodied 
his “ speculative ideas respecting the constitution of matter.” In this paper he 
pointed out that in solids, some of the molecules may still be in the liquid or even 
the gaseous condition, and his words are very definite. He says, “the three con¬ 
ditions [solid, liquid, and gaseous] probably always co-exist in every liquid or solid 
substance, but one predominates over the others. . . . Liquefaction or solidi¬ 
fication may not, therefore, involve the suppression of either the atomic or the 
molecular movement, but only the restriction of its range.” He subsequently, in 
1866,t gave singular point to these speculations by his discovery of the penetration 
of solid metals by gases. 
After Graham’s death, evidence as to the molecular mobility of metals came 
slowly. E. WiedemannJ showed, in 1878, that solid metals were not necessarily 
inert, as changes which involve atomic movement take place in bismuth-lead alloys, 
and the clear evidence thus afforded of allotropic change in the solid, recently formed 
the subject of some experiments of my own.§ 
Confirmation of the accuracy of Graham’s views as to the co-existence of liquid 
and gaseous molecules in a solid, was afforded twenty years later .by Professor 
W. Spring. || In 1886 his admirable investigation on the solidification of alloys 
of lead and tin, afforded him experimental evidence that in these alloys active mole¬ 
cular movement is continued after the alloys have become solid. He says, and it 
is well to quote his interesting words, “ on serait porte k penser qu’entre deux mole¬ 
cules de deux corps solides il y a un va-et-vient perpetuel d’atomes,” and he adds, “if 
the two molecules are of the same kind, chemical equilibrium will not be disturbed, 
but if they are different this movement will be revealed by the formation of new 
substances.” 
Formation of Alloys by Cementation. —The fact that alloys can be formed by the 
union of two metals at a temperature below the melting point of the more fusible of 
the two has long been known to metallurgists, and perhaps the most striking fact 
in the more modern history of the subject was recorded in 1820 by Faraday and 
* ‘Phil. Mag.,’ February, 1864; ‘ Collected Papers,’ p. 299. 
f ‘ Phil. Trans. Roy. Soo.,’ 1866, pp. 399-439. 
t ‘ Wied., Ann.,’ 3, 1878, p. 237. 
§ Second Report Alloys Research Committee, ‘ Proc. Inst. Mech. Engineers,’ 1893, p. 127. 
|| ‘ Bull, de TAcad. Royale de Belgique,’ vol. 11, 1886, p. 355. 
