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 these go in, the glass will come asunder directly pressure is applied.
Now, if you press too hard in cutting, another thing takes place.
Imagine a quantity of roofing-slates piled flat one on top of another, all the piles being of equal height and arranged in two rows, side by side, so close that the edges of the slates in one row touch the edges of those in the other row, along a central line.
Wheel a wheelbarrow along that line over the edges of both.