ground with diamond powder on a cast-iron mill for three hours
without its being at all worn, but that, on changing its
direction with respect to the grinding surface, the same edge was
ground away.

9. Employment of materials of little value. The skins used by
the goldbeater are produced from the offal of animals. The hoofs
of horses and cattle, and other horny refuse, are employed in the
production of the prussiate of potash, that beautiful, yellow,
crystallized salt, which is exhibited in the shops of some of our
chemists. The worn-out saucepans and tinware of our kitchens,
when beyond the reach of the tinker's art, are not utterly
worthless. We sometimes meet carts loaded with old tin kettles
and worn-out iron coal-skuttles traversing our streets. These
have not yet completed their useful course; the less corroded
parts are cut into strips, punched with small holes, and
varnished with a coarse black varnish for the use of the
trunk-maker, who protects the edges and angles of his boxes with
them; the remainder are conveyed to the manufacturing chemists in
the outskirts of the town, who employ them in combination with
pyroligneous acid, in making a black die for the use of calico
printers.

10. Of tools. The difference between a tool and a machine is
not capable of very precise distinction; nor is it necessary, in
a popular explanation of those terms, to limit very strictly
their acceptation. A tool is usually more simple than a machine;
it is generally used with the hand, whilst a machine is
frequently moved by animal or steam power. The simpler machines
are often merely one or more tools placed in a frame, and acted