3. Page 155

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THE THERMAL TREATMENT OF METALS.

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steel,* has shown that this is the case ; the point of main importance being the existence of a substance which Sorby called the “pearly constituent” in soft steel. This pearly constituent is closely related to the carbide of iron, Fe₃C, of Abel, and is probably a mixture of Fe₃C and pure iron. Its presence is dia

IRON and CARBON.
(0-2 to 1-5 per cent of CARBON.)
FIG. 36.

grammatically indicated in Fig. 36. The diagram will serve for
the purpose of illustration to indicate the appearance when soft, hardened, and tempered steel are respectively heated with a solvent which acts gently on the mass.
By the kindness of my friend, M. Osmond, I am able to pass
from mere diagrammatic illustrations to actual photographs  of

* Journ. Iron and Steel Inst, No. 1 (1887), p. 255.