But this is not all. A _dry_ wheel, although it were formed of the finest steel, could barely scratch the surface of an agate: or, if great force were exerted at the winch, would splinter it. The lapidary has need of diamond-powder, emery, and rotten-stone. He makes use also of a peculiar kind of oil, and has a jar of water within reach for ordinary purposes. The oil is called “brick-oil.” It comes from coal-tar, and does not heat by friction, as common oil would: consequently, it neither burns the operator’s fingers, nor injures his specimens by causing the wheel to glow too fiercely. Of the above materials, the most expensive is the diamond. Although obtained by crushing “bort,” of little intrinsic value, it never costs the lapidary less than twenty-five shillings per _carat_; and he can do nothing without it. Emery, on the other hand, which is a coarse variety of corundum, is cheap enough. The first wheel put on the spindle is an extremely thin one, it can scarcely be too thin, and is made of tin or of the _softest_ steel. Its disk should lie exactly level in the plane of the horizon, and it must not have the shadow of a notch or bend upon its delicate edge. This wheel the lapidary wets, along its entire rim, with oil or water, by means of a feather. A small portion of the diamond, ready mixed with oil, is then applied to the edge of the wheel, the latter being made to revolve gently, until it has taken up the mixture from his finger. The wheel thus primed is now set in motion, at first slowly, but by degrees more rapidly, and the pebble being steadily pressed, not _pushed_, against it, the diamond eats into the metal, and the metallic edge armed with an adamantine tooth eats into the solid stone, and at length saws it asunder. In this way an agate of a couple of inches diameter, and of average hardness, will be neatly divided in less than a quarter of an hour. The two surfaces thus obtained are then inspected, and if the stone be judged worthy, the lapidary proceeds to _polish_ either or both of them. There is always some difference between the two, a slice having been taken out of the pebble answering to the thickness of the wheel; hence the importance of using a delicate disk, especially for valuable agates. To accomplish the polishing, the thin wheel of steel is now exchanged for a much thicker one of lead, which takes its place upon the spindle. It is no longer the edge, but the ribbed upper surface which is now to do the work; and to charge it for this, it is smeared over with rotten-stone dipped in water. This heavy wheel is then set spinning at a rapid rate, and the pebble is pressed flat upon it with great force; the heavier the hand the better. As it slidders about, the action of the rotten-stone, which is very fine in the grain, gives it by degrees a high polish. And this polish is durable, and will preserve the agate from the corroding effect of our atmosphere.