EXPLOSION, in natural philosophy, a sudden and violent expansion of an. ae rial or other elastic fluid, by which it in stantly throws off any obstacle that hap pens to be in the way, sometimes with in credible force, and in such a manner as to produce the most astonishing effects. It diflers from expansion in this, that the latter is a gradual and continued power, acting uniformly for some time ; whereas the former is always sudden, and only of momentary duration. The expansions of solid bodies do not terminate in violent explosions, on account of their slowness, and the small space through which the metal, or other expanding substance, moves. Thus wedges of dry wood dri ven into stone, and wetted, will cleave the most solid blocks, but they never throw the parts to any distance, as is the case with gunpowder ; but the expansion of elastic fluids will burst solid substan ces, and throw the fragments a great way off : for this two reasons have been as signed : 1. The immense velocity with which aerial fluids expand, when sud denly affected with high degrees of heat : and 2. The great celerity with which they acquire heat, and are affected by it. As
an example, air, when heated as much as iron, when brought to a white heat, is ex panded to four times its bulk ; but the metal itself will not be expanded the 500th part of the space. In the case of gunpowder, which is well known as an explosive substance, the velocity with which the flame moves is estimated at 7000 feet in a second. Hence the im pulse of the fluid is inconceivably great, and the obstacles on which it strikes are hurried off with vast velocity, viz. at the rate of 27 miles per minute. The velo city of the bullet is also promoted by the sudden propagation of the heat through ,the whole body of air, as soon as it is ex tricated from the materials of which the gunpowder is made, so that it strikes at once. Hence it has been inferred, that explosion depends first on the quantity of elastic fluid to be expanded : secondly, on the velocity it acquires by a certain_ degree of heat : and, thirdly, on the ce lerity with which the degree of heat af fects the whole expansive fluid.