These are little hollow cups of copper having a fulminating substance
at the bottom, so that when put on to the “nipple” of the gun and
struck by the “hammer,” the fulminating powder explodes, and the
spark passing down the hole in the nipple discharges the gun. To
prepare the fulminating powder for these caps, let 100 grains of
mercury be dissolved in a measured ounce-and-a-half of nitric acid,
and when cold let two ounces of spirits of wine be added, and the
whole put into a Florence-oil flask made perfectly clean, and let it
be placed in the open air; copious fumes will pass off and a violent
action take place, during which a white crystalline powder will be
deposited; as soon as all action has ceased and the liquid cooled,
pour the whole on a filter of blotting paper, and let the fluid pass
through, wash the powder which remains on the filter with a little
water, and let it dry, without heat. This is fulminating mercury,
which is a highly dangerous compound, and should be kept in a bottle
with a cork, and not a stopper, as the friction of this against
the neck of the bottle might cause an explosion. At the Arsenal at
Woolwich is a machine (shown at the head of this article) which
makes the caps complete. It is fed by a band of thin copper about
two inches wide, out of which pieces are punched in the form of a
thick cross, leaving the perforated copper as shown in fig. 1; these
pieces are punched or “struck up,” and expelled as perfect caps of
the form of fig. 2, at the rate of about 1000 per minute. The caps
have next to receive their charge of fulminating powder, which is
done by dropping them into a perforated plate (fig. 3), capable of
receiving many hundreds; this is covered over by two other plates,
each perforated to correspond, but the upper one made to shift, so
that in one position the holes correspond, and in the other they do
not, but remain as small shallow dints. A portion of the fulminating
powder is put on this plate and scraped all over it by means of a
piece of paste-board (fig. 4), so as to fill all the little dints;
the plate is then shifted and the holes made to correspond, when the
powder falls through into the percussion caps, each one of which thus
receives a definite charge. The next process is to press it down
into the cap, so as to prevent it falling out. For this purpose the
plate full of caps is removed to a machine (fig. 5), having a row of
little stoppers, which are moved rapidly up and down, the pressure
being exactly regulated by flat leaden weights, suspended so as to
give only so much pressure as will consolidate the powder, without
exploding it. They are next removed to a third machine called the
“varnishing machine” (fig. 6). This has a trough of varnish, made
by dissolving shellac in spirits of wine, into which a row of wires
dip, and by a turn of the hand convey the minute portion of varnish
on their points into the caps, row after row. This varnish dries in
a few minutes, and causes the fulminating powder to adhere; the caps
are now complete. The old form of caps was a simple short cylinder,
but it is found better to allow the four little flaps to remain on,
that the right end may be distinguished and instantly placed on the
nipple the proper side downwards, which cannot otherwise so readily
be done in the dark, and when the hands are benumbed with cold.