146 
SORBY: CHARACTERISTIC CONSTITUENT OF STEEL. 
laminae are often of extreme thinness, those of the soft iron being 
often about iosoo^^^ i^^^^^ thickness, and those of the hard 
substance only ^oJooth, so that we have alternating ridges and 
grooves about 00*^1 i^^ch apart. The only satisfactory explana- 
tion for this remarkable structure appears to be that at a high 
temperature a stable compound of iron and carbon exists, which is 
not stable at a lower temperature, but breaks up with the two 
substances named above, which certainly are stable at both a high 
and low temperature. 
What we see in examining an ingot of cast steel of medium 
temper is that comparatively large crystals are formed on solidifi- 
cation. As the temperature becomes somewhat lower these break 
up into smaller crystals; and finally these resolve themselves into 
the extremely thin alternating plates, the direction of which is 
probably determined by the previous crystalline structure. So far 
as I am aware no strictly parallel case is known to occur in any 
other substance, but at the same time it must be admitted that it 
has not been specially looked for with very high power, and might 
well enough occur under such circumstances that it could not be 
recognized even if present. 
Independent of the interest that may be attached to such a 
remarkable structure, it seems to me that the facts throw mucli light 
on the hardening and tempering of steel. It is possible and indeed 
probable that when red hot steel is suddenly cooled by plunging it in- 
to cold water, the compound of iron and carbon which is stable at a 
high temperature is suddenly fixed, before it has, so to speak, time to 
break up, and retains properties intermediate between 'those of the 
soft iron and the intensely hard but l^rittle compound, that is to say, 
combines great hardness with strength. On again raising the temper- 
ature somewhat, so as to temper the hardened steel, we can easily 
understand that the two constituents separate out to a gi-eater or less 
extent, so as to give rise to a structure like that met with when the 
steel is slowly cooled. As far as I can learn, this view of the subject 
agrees perfectly well with all the facts seen by studying different 
kinds of iron and steel with high magnifying powers. 
