360 REPORTS ON THE STATE OF SCIENCE.—1912. 
As, however, contact with a block of graphite was found to suffice 
when the iron was heated in a porcelain tube, the further assumption 
was made** that carbon vapour was formed under such conditions. 
This was the beginning of a controversy that has been renewed at 
intervals down to the present day. ‘The views of Le Play and Laurent 
were opposed by Gay-Lussac, who, whilst admitting that much in 
the process of cementation is obscure, closes his vigorously written 
memoir 77 with the words :— 
‘Tl n’est plus permis aujourd’hui d’avoir une foi aveugle au 
principe si banalement répété des anciens chimistes, corpora non agunt 
nisi soluta. I] est certain, au contraire, que tous les corps, solides, 
liquides ou aériformes, agissent les uns sur les autres, mais que, des 
trois états des corps, l'état solide est le moins favorable 4 l’exercice 
de l’affinité.’ 
The adoption of van’t Hoff’s view of the nature of solid solutions 
would reconcile Gay-Lussac’s conclusion with the principle of the 
older chemists. 
The view that cementation is a process of true diffusion in the 
solid state was distinctly advanced by experiments which showed that 
the curve of distribution of carbon with depth at any given tempera- 
ture had the form of a diffusion curve.** The memoir of Mannesmann 
is particularly valuable for its mass of experimental evidence. The 
form of the curve has since been redetermined with greater exactness,’° 
confirming previous conclusions. Mannesmann’s experiments showed 
that under suitable conditions cementation only took place where the 
iron was actually in contact with solid carbon, whereas, if the active 
agent had been traces of gaseous compounds, the action should have 
been general, and not localised. Charcoal always contains absorbed 
gases, which are removed only with difficulty, and special interest 
therefore attaches to experiments in which graphite or diamond is used 
in place of the amorphous modification. As far back as 1865 it had 
been shown ®° that iron was carburised by direct contact with either 
graphite or diamond in an atmosphere of hydrogen. In such early 
investigations, however, no account was taken of oxides of carbon 
occluded in the iron, and the experiments have been repeated on many 
occasions by other observers, with contradictory results. Thus it was 
found §' that diamond would carburise iron at 1,160° in an atmosphere 
of nitrogen, and that the cementation was actually more rapid than 
when other modifications of carbon were used. Similarly, the direct 
contact of diamond with iron in a vacuum, the degree of which is not 
stated, was found to bring about cementation.*? On the other hand, 
it has been stated that the presence of gaseous compounds is necessary 
7 A. Laurent, Ann. Chim. Phys. 1837 [ii.], 65, 417. 
71 Ann. Chim. Phys., 1846 [iii.], 17, 221. 
78 R, Mannesmann, Verh. Ver. Befird. Gewerbefl., 1879, 58, 31; A. Colson, Compt. 
rend., 1881, 98, 1074. 
7 W. C. Roberts-Austen, Jour. Iron and Steel Inst., 1896, i. 139. 
80 F, Margueritte, Ann. Chim. Phys., 1865 [iv.], 6, 55. 
8 W. Hempel, Ber., 1885, 18, 998. 
® W. C. Roberts-Austen, Jour. Iron and Stcel Inst., 1890, i. 91. 
