and a photograph merely shows a dark,
structureless area.

[Illustration: FIG. 49.--Coarse-grained martensite, polished and
etched with nitric acid and magnified 50 times. Made by Prof. Chas.
Y. Clayton.]

Sorbite is believed to be an early stage in the formation of pearlite,
when the iron and iron carbide originally constituting the solid
solution (austenite) have had an opportunity to separate from each
other, and the iron has entirely passed into the alpha modification,
but the particles are yet too small to be distinguishable under
the microscope. It also, possibly, contains some incompletely
transformed matter. Sorbite is softer and tougher than troostite,
and is habitually associated with pearlite. Its components are
tending to coagulate into pearlite, and will do so in a fairly
short time at temperatures near the lower critical, which heat will
furnish the necessary molecular freedom. The normal appearance,
however, is the cloudy mass shown in Fig. 52.

Pearlite is a definite conglomerate of ferrite and cementite containing
about six parts of the former to one of the latter. When pure, it
has a carbon content of about 0.95 per cent. It represents the
complete transformation of the eutectoid austenite accomplished by
slow-cooling of an iron-carbon alloy through the transformation
range. (See Fig. 46.)

[Illustration: FIG. 50.--Quenched high-carbon steel, polished,
etched and viewed at 100 magnifications. This structure is called
martensite and is desired when maximum hardness is essential. Photograph
by H. S. Rawdon.]

[Illustration: FIG. 51.--Martensite (light needles) passing into
troosite (dark patches). 130 X. From a piece of eutectoid steel
electrically welded.]

[Illustration: FIG. 52.--Sorbite (dark patches) passing into pearlite
(wavy striations). Light Areas are Patches of Ferrite. 220 X. From
a piece of hypo-eutectoid steel electrically welded.]

These observations are competent to explain annealing and toughening
practice. A quickly quenched carbon steel is mostly martensitic
which, as noted, is a solid solution of beta iron and cementite,
hard and brittle. Moderate reheating or annealing changes this
structure largely into troostite, which is a partly transformed
martensite, possessing much of the hardness of martensite, but with
a largely increased toughness and shock resistance. This toughness is
the chief characteristic of the next material in the transformation
series, sorbite, which is