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would then be arranged in precisely such a manner as is 
seen to be the case in such a rock having cleavage, the 
agreement being most perfect in every particular, both in 
kind and amount, as seen in sections cut in each direction. 
Though such calculations may be fully relied on, yet, to 
satisfy myself that they were correct, I have tested them by 
actual experiment. Having mixed some scales of oxide of 
iron with soft pipe-clay, in such a manner that they would 
be inclined evenly in all directions, like the flakes of mica 
in Water-of-Ayr stone, I changed its dimensions artificially 
to a similar extent to what has occurred in slate rocks. 
Having then dried and baked it, I rubbed it to a perfectly 
flat surface, in a direction perpendicular to pressure, and in 
the line of elongation which would correspond to that of dip 
of cleavage, and also, as it were, in its strike, and in the plane 
of cleavage. The particles were then seen to have become 
arranged in precisely the same manner as theory indicates 
that they would, and as is the case in natural slate ; so much 
so, that, so far as their arrangement is concerned, a drawing 
of one could not be distinguished from that of the other. 
Moreover, it then admitted of easy fracture into thin flat 
pieces in the plane corresponding to the cleavage of slate, 
whereas it could not in that perpendicular to it. Even in 
clay, which has but few very unequiaxed particles, a most 
distinct lamination is produced by changing its dimensions 
as described above, but it would not cleave perfectly, no 
more than will natural slate of similar mineral composition; 
and, moreover, one cannot obtain their firm, uniform structure. 
It is a fact well worthy of remark, that in well cleaved 
slates, on each side of the larger rounded grains of mica, 
in the line of cleavage, the particles are arranged evenly at 
all angles, over small triangular spaces, having their bases 
towards the grain. This is just the part which would be 
protected from change of dimensions by its presence; and 
