CAPS, PERCUSSION. It is explained in Arms the earlier kind of fire. arms Were discharged. The percussion-cap is a modern contrivance for this purpose. It depends on the property , possessed by several chemical substances of exploding by a blow et; percus sion. Fulminating mercury was the first sub stance employed in this way; but as soon as it became known that a slight blow would ex plod.e, certain powders, and that this explosion would ignite gunpowder, numerous improve ments were introduced by degrees. Carious salts and other chemical compounds, such as some of the chlorates, nitrates, and fulminates, will explode in this way, thy most familiar example of which is afforded by the 'hicifers' or congreves ' now sold so cheaply in the streets, and which ignite so readily by friction (in effect a series of minute percusSions): When the method was first introduced, there was some difficulty in causing the explosion of the mixture to ignite the gunpowder. But this is now effected by putting the detonating mixture into a little copper box or cell called a cap, which is adjusted over the touch-hole, and so arranged that a smart blow bursts the cap and explodes, its contents : the little cell itself is destroyed, so that a new one is re quired for each firing. The size of the cell,
or percussion-cap,' the nature of the mixture, the quantity,employed with each clifirge, and the mechanism for firing it, have been the subjects of many improvements within the last few Years, some of which are patented. The caps are now made in large numbers at Birmingham, fir, much, the same maiiiier as metal buttons, blanks being cut out of sheet copper or Mixed metal, and 'stamped or pressed into the proper shape. One of the recent inventions has been to make the cap double, or one _cap within another, With the mixture between the two, and a small hole in the inner one to communicate With the gun powder.
Many contrivances have also been suggested caps under a modified form to larger guns i and the detonating compounds themselves, have been brought into use, for exploding the,,conteiitA of bbiribs and Shells.