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SLATES. 3 95

polyhedra become converted into laminae, separated from
each other by surfaces of weak cohesion, and the infallible
result will be a tendency to cleave at right angles to the
line of pressure.

Further, a mass of dried mud is full of cavities and fis-
sures. If you break dried pipe-clay you see them in great
numbers, and there are multitudes of them so small that you
cannot see them. A flattening of these cavities must take
place in squeezed mud, and this must to some extent facilis
tate the cleavage of the mass in the direction indicated.

Although the time at my disposal has not permitted me
duly to develop these thoughts, yet for the last twelve
months the subject has presented itself to me almost daily
under one aspect or another. I have never eaten a biscuit
during this period without remarking the cleavage devel-
oped by the rolling-pin. You have only to break a biscuit
across, and to look at the fracture, to see the laminated
structure. We have here the means of pushing the anal-
ogy further. I invite you to compare the structure of this
slate, which was subjected to a high temperature during
the conflagration of Mr. Scott Russell’s premises, with that
of a biscuit. Air or vapor within the slate has caused it
to swell, and the mechanical structure it reveals is precisely
that of a biscuit. During these inquiries I have received
much instruction in the manufacture of puff-paste. Here
is some such paste baked under my own superintendence.
The cleavage of our hills is accidental cleavage, but this is
cleavage with intention. The volition of the pastry-cook
has entered into its formation. It has been his aim to pre-
serve a series of surfaces of structural weakness, along
which the dough divides into layers. Puff-paste in prepa-

to accomplish this he first compressed it. The mould was conical, and
permitted the lead to spread out a little laterally. The lamination was
as perfect as that “of slate, and it quite defeated him in his effort to ob-
tain a granular powder.

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