amination of the cuts under the microscope, or even a
6 diameter lens, certainly also tends to confirm.

What happens appears to my non-scientific eyes to be this.

Glass is one of the most fissile or "splittable" of all materials; but
it is so just in the same way that ice is, and just in the opposite way
to that in which slate or talc is.

Slate or talc splits easily into thin layers or laminae, _because it
already lies in such layers_, and these will come apart when the force
is applied between them: but _it will only split into the laminae of
which it already is composed, and along the line of the fissures which
already exist between them_.

Glass, on the contrary (and the same is true of ice, or for that matter
of currant-jelly and such like things), appears to be a substance which
is the same in all directions, or nearly so, and therefore as liable to
split in one direction as in another, and is so loosely held together
that, once a splitting force is applied, the crack spreads very rapidly
and easily, and therefore smoothly and in straight lines and in even
planes.

The diamond, or the wheel-cutter, is such a force. Being pressed on to
the surface, it forces down the particles, and these start a series of
small vertical splits, sometimes nearly through the whole thickness of
the glass, though invisibly so until the glass is separated. And mark,
that it is the _starting_ of the splits that is the important thing;
there is no object in making them _deep_, it is only wasted force; they
will continue to split of themselves if encouraged in the proper way
(see Plates IX. and X.). Try this as follows.

Take a bit of glass, say 3 inches by 2, and make the very smallest dint
you can in it, in the middle of the narrowest dimension. You cannot make
one so small that the glass will hold together if you try to break it
across. It will break across in a straight line, springing from each end
of the tiny cut. The cut may be only 1/8 of an inch long; less--it may
be only 1/16, 1/32--as small as you will, the glass will break across
just the same.

Why?

Because the cut has _started_ it splitting at each end; and the material
being the same all through, the split will go straight on in the
direction in which it has started; there is nothing to turn it aside.

So also the pressure of the wheel starts a continuous split, or series
of splits, _downwards_, into the thickness of the glass. No matter how
small a distance th