=Definition of an Explosion=: An explosion is a chemical reaction which is completed in an exceedingly short period of time with the evolution of a large quantity of gas at a very high temperature. If this reaction occurs in a body which is closely confined, the expansive effect of the highly heated gases produces disruptive effects. If the suddenness of the reaction is very great, disrupted action upon solid objects in contact with the body may be obtained even when it is unconfined, because the cohesion of these objects can be overcome more readily in an instant of time than the inertia of the surrounding air. This has given rise to the popular error that nitro-glycerine and other high explosives act downwards; as a matter of fact they act with equal force in all directions. It is evident, therefore, that the violence of an explosion depends upon three things, namely, the time occupied completing the reaction of the explosive body; the temperature produced by the reaction, upon which directly depends the expansive forces of the resultant gases; and the quantity of gas evolved by the reaction. A fourth consideration--whether the products of the reaction are the result of one set of chemical actions occurring simultaneously; or whether the set of new compounds react upon each other, producing a second set of compounds. LECTURE III. STUDY OF A FEW OF THE PRINCIPAL TYPES OF RIFLE AND HAND GRENADES. =General.= The undermentioned types of grenades, empty, are issued for instruction:-- (a) Grenade, 303-inch, short rifle, No. 3, Mark 1. (J. pattern.) (b) Grenade, hand, No. 1, Mark 1. (c) Grenade, hand, No. 5, Mark 1. (d) Grenades, hand, Nos. 6 and 7, Mark 1. These grenades were also known as (a) Hales Rifle Grenade; (b) R. L. impact or percussion grenade; (c) Mill’s pattern grenade; (d) R. L. grenades, friction, time. Of these (a) is for firing from the point .303-inch short rifle. Its range is about 200 yards. The remainder are intended to be thrown by hand. Hand grenades can be of two classes, Heavy and Light. Heavy grenades weigh about 2 lbs. and light about 1 lb. It is considered that a man can throw a 2-lb. weight about 30 yards and a pound-weight about 50 yards. Heavy grenades project fragments of some weight a fair distance, while light grenades rely for effect principally on the blast of detonation. The effective area of a light grenade is mainly local, 6 yards diameter, but when possible it should not be thrown less than 20 yards in the open, as stones, etc., thrown up by the explosion would be dangerous to the thrower. The danger area of the heavy grenade is about 30 yards in diameter, and, therefore, it should not be thrown less than 25 yards in the open. With both grenades the thrower should cover the eyes at the moment of explosion and protect himself, as small fragments of metal may carry further than the distance mentioned.