362 REPORTS ON THE STATE OF SCIENCE.—1912. 
carbon is not used in the free state, but in the form of a steel rich 
in carbon. If the process be one of true diffusion, carbon should tend 
to pass from places of high to those of low concentration. This is 
found to be the case. When two pieces of steel, containing respec- 
tively 0°90 per cent. and 0°15 per cent. of carbon, are placed in imme- 
diate contact and heated, air being excluded, carbon passes from the 
high into the low carbon steel, the gain of weight of the one being 
almost exactly equal to the loss of weight of the other.** Experiments 
on this point have been made with great care, a cylinder of nearly 
pure iron being bored through longitudinally, and fitted over a core of 
steel containing a much larger proportion of carbon.°® After heating 
for ten hours in vacuo at 950° to 1,050° the outer jacket was turned off, 
and the layer immediately adjoining the core was analysed. The diffu- 
sion of carbon was clearly proved in this manner. 
The fact that the materials used in the practical cementation of 
iron are commonly not pure carbon, but contain combined nitrogen, 
has led on several occasions to the supposition that the carbon diffuses 
in the form of a compound of nitrogen, such as cyanogen, or that a 
nitride of iron takes part in the process, as well as the carbide. The 
cyanogen theory is due to W. Stein,’ but need not-be further con- 
sidered here, as there is no reason to suppose that nitrogen is respon- 
sible for the. transport of carbon within the metal, whatever may be 
the case in regard to the formation of the external layer of highly 
carburised iron. It is certain that the presence of cyanides and of 
gaseous carbon compounds greatly accelerates diffusion, but it does not 
follow that the process of diffusion into the interior differs in any 
respect from that which occurs when elementary carbon is the cement- 
ing agent.°? Very rapid cementation is brought about by the action of 
mixtures of the oxides of carbon on iron,** especially when the mixture 
in equilibrium with solid carbon at the required temperature is used. 
It is not necessary to assume that the gases penetrate to the interior 
of the metal and so convey the carbon; on the contrary, the experi- 
ments support the view that the function of the gas is merely that of 
reacting with the iron to form a superficial layer rich in carbon, from 
which diffusion inwards can then proceed. The investigations of 
Arnold and McWilliam and of others have clearly established the fact 
that the carbon does not diffuse in the free state, but as a carbide, 
Fe,C, although the further assumption by the above-named authors, 
that an intermediate carbide, Fe.C, is formed, has not met with 
general acceptance. 
As the carbide is not appreciably soluble in a-iron it is to be 
expected that cementation should not take place below 690°, the lowest 
temperature at which austenite, the solid solution of carbide in y-iron, 
can exist’ in a stable state, and this appears to be generally true, 
% G, P. Royston, Jowr. Iron. and Steel Inst., 1897, i. 166. 
9 J. O. Arnold and A. McWilliam, ibid., 1899, i. 85 
" Polyt. Centr., 1851, 897. 
® A, Ledebur, Stahl und Risen, 1906, 26, 72. 
% JF, Giolitti, Jour. Iron and Steel Inst., 1911, ii. 307; summarising a series of 
papers contained in Gazzetta, 1910-1911. 
