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ration must not be handled too much; it ought, moreover,
to be rolled on a cold slab, to prevent the butter from melt-
ing, and diffusing itself, thus rendering the paste more ho-
mogeneous and less liable to split. Puff-paste is, then,
simply an exaggerated case of slaty cleavage.

The principle which I have enunciated is so simple as
to be almost trivial; nevertheless, it embraces not only the
cases mentioned, but, if time permitted, it might be shown
you that the principle has a much wider range of applica-
tion. When iron is taken from the puddling-furnace it is
more or less spongy, an aggregate in fact of small nod-
ules: it is at a welding heat, and at this temperature is
submitted to the process of rolling. Bright, smooth bars
are the result. But, notwithstanding the high heat, the
nodules do not perfectly blend together. The process of
rolling draws them into fibres. ‘Here is a mass acted upon.
by dilute sulphuric acid, which exhibits in a striking man-
ner this fibrous structure. The experiment was made by
my friend Dr. Percy, without any reference to the question
of cleavage.

Break a piece of ordinary iron, and you have a granular
fracture; beat the iron, you elongate these granules, and
finally render the mass fibrous. Here are pieces of rails
along which the wheels of locomotives have slidden; the
granules have yielded and become plates. They exfoliate
or come off in leaves; all these effects belong, I believe,
to the great class of phenomena of which slaty cleavage
forms the most prominent example.1

[I would now lay more stress on the lateral yielding,
referred to in the note at the bottom of page 394, accompa-
nied as it is by tangential sliding, than I was prepared to
do when this lecture was given. This sliding is, I think,
the principal cause of the planes of weakness both in
pressed wax and slate-rock. J. T. 1871.]

1 For some further observations on this subject by Mr. Sorby and
myself, see Philosophical fifagazine for August, 1856.

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