a sudden and violent expansion of an aerial or other elastic fluid, by which it instantly throws off any obstacle that happens to be in the way, sometimes with great force, and in such a manner as to produce astonishing effects upon adjacent objects.

Explosion differs from expansion, which is a gradual and continuous power, acting uniformly for some time, whereas the former is always sudden, and only of momentary duration. The expansions of solid substances do not terminate in violent explosions, on account of their slowness, and the small space through which the metal, or other expanding substance, moves; though their strength may be equally great with that of the most active aerial fluids. Thus we find, that though wedges of wood, when moistened, will cleave solid blocks of stone, they never throw them to any distance, as is the case with gunpowder. On the other hand, it is seldom that the expansion of any elastic fluid bursts a solid substance without projecting the fragments of it to a considerable distance.

The reasons of this are, first, the immense velocity with which the aerial fluids expand, when affected by a considerable degree of heat; and, secondly, their celerity in acquiring heat and being affected by it, which is much superior to that of solid substances. Thus, air, heated as much as iron when brought to a white heat, is expanded to four times its bulk; but the metal itself will not be expanded the five hundredth part of its bulk. In the case of gunpowder, which is a violent and well-known explosive substance, the velocity with which the flame moves is calculated by Mr Robins, in his Treatise upon Gunnery, to be no less than seven thousand feet in a second, or little less than seventy-nine miles per minute. Hence the impulse of the fluid is inconceivably great, and the obstacles on which it strikes are hurried off with vast velocity, though much less than that just mentioned; for a cannon bullet, with the greatest charge of powder that can be conveniently given, does not move at a greater rate than two thousand four hundred feet per second, or little more than twenty-seven miles per minute. The velocity of the bullet, again, is promoted by the sudden propagation of the heat through the whole body of air, as soon as it is extricated from the materials of which the gunpowder is made; so that it is enabled to strike all at once, and thus greatly to augment the momentum of the ball. That this contributes much to the force of the explosion, is evident from what happens when powder is wetted or mixed with any substance, which prevents its taking fire all at once. In this case the force of the explosion, even when the same quantity of powder is made use of, cannot be compared to that of dry powder.

Upon these principles we may conclude that the force of an 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 celerity with which the degree of heat affects the whole of the expansive fluid. These three take place in the greatest perfection where the electric fluid is concerned, as in cases of lightning, earthquakes, and volcanoes. In violent lightning, where the electric fluid collects itself into balls, the strength of the explosion is proportional to the quantity. Every one has heard of the prodigious effects of lightning when it happens to strike buildings, trees, or even the most solid rocks; and in some cases, where the quantity of electricity is still greater than in any flash of lightning, we hear of still more tremendous consequences ensuing. Dr Priestley gives an instance of a large fireball, or quantity of electric matter, rolling on the surface of the sea, which, after rising up to the topmast of a ship of war, burst with such violence, that the explosion resembled the discharge of hundreds of cannon fired at once. Great damage was done by it; but there is not the least doubt that most of its force was spent on the air, or carried down to the sea by the mast and iron work of the ship. Considering, indeed, that in all cases a great part of the force of electrical explosions is dissipated in this manner, it may reasonably be doubted whether they can be measured by any method applicable to the mensuration of other forces.

In those cases where the electrical matter acts like common fire, the force of the explosions, though exceedingly great, is capable of mensuration by comparing the distances to which the bodies are thrown with the weight of such bodies. This is most evident in volcanoes, where the projections of the burning rocks and lava manifest the greatness of the power, at the same time that they afford a method of measuring it. By means of the electric fire which kindles the volcanoes, the aerial fluids are suddenly restored to their elastic state; and not only so, but their natural elasticity is greatly augmented, so that the explosions take place with great violence. The case is the same with gunpowder.

Next in strength to the aerial vapours are those of aqueous and other liquids. The most remarkable effects of these are observed in steam-engines; but there is one particular case from which it has been inferred that aqueous steam is vastly stronger than the flame of gunpowder. This is when water is thrown upon molten copper; for here the explosion is so strong as almost to exceed imagination; and the most terrible accidents have been known to happen from such a slight cause as one of the workmen spitting in the furnace where copper was being melted.

In explaining the phenomenon in question, Dr Black supposes that the mere heat of the metal applied to the Explosion. aqueous steam produces the explosion; and in proof of this he alleges, that copper imbibes a greater quantity of heat during fusion than any other metal. Aqueous steam, however, seems to be too slow for producing such sudden and violent effects. Explosions, it is true, will be occasioned by it, but then it must be confined for a considerable time; whereas the effects of water thrown upon molten copper are instantaneous.

It may now be asked, why such explosions do not take place when water is thrown upon the surface of any other metal, for instance iron, when in a state of fusion? In answer to this it may be observed, that though water is decomposed by being applied to red-hot iron in the form of steam, yet there is a possibility, that when the same element is applied in substance to the fluid metal, no decomposition may ensue. Something like this indeed happens with copper itself; for, notwithstanding the violent effects which take place on the contact of water in substance with the molten metal, no explosion happens from aqueous steam being blown upon its surface. On the contrary, the upper part of the metal is thus cooled, and forms itself into cakes, which are afterwards taken off, and new ones formed in the same manner. Neither does aqueous steam affect red-hot copper in the manner that it does iron in the same state. A decisive proof that the explosion is not occasioned by the mere heat of the aqueous steam may be deduced from the example of molten glass, which produces no explosion though we pour water upon it in that state; and yet the heat of glass in a state of fusion is undoubtedly equal at least to that of molten copper. It may be observed, however, that in all cases where a very hot body is thrown upon a small quantity of water in substance, an explosion will follow; but here the water is confined, and suddenly rarefied into steam, which cannot escape without throwing off the body which confines it. Examples of this kind frequently occur where masons or other mechanics are employed in fastening cramps of iron into stones; if there happen to be a little water in the hole into which the liquid lead is poured, the latter will fly out in such a manner as sometimes to occasion serious accidents. Occurrences of this kind have sometimes happened in founderies, when large quantities of molten metal have been poured into wet moulds. In these cases, the sudden expansion of the aqueous steam throws out the metal with great violence; and if any decomposition take place at the same time, so as to convert the aqueous into a serial vapour, the explosion will be still greater.

To this last kind of explosion we must refer that which takes place on pouring cold water into boiling or burning oil or tallow. Here the case is much the same whether we pour the oil on the water, or the water on the oil. In the former case, the water, which lies at the bottom, is rarefied into steam, and explodes; in the latter, it sinks down through the oil by its superior specific gravity, and explodes as it passes along. In either case, however, the quantity of aqueous fluid must be but small in proportion to that of the oil; a very great quantity would put out the flame, or destroy the heat, in whatever way it might be applied.

Another kind of explosion is that which takes place in solid substances, where we can scarcely suppose either aqueous or aerial vapours to be concerned. The most remarkable of these are the "volcanic bombs" mentioned by Sir William Hamilton in his account of the great eruption of Vesuvius in the year 1779. They were large pieces of lava, and burst like bombs as they fell to the ground; but he does not inform us whether their bursting was attended with any great violence or not. Other examples Exponent are the bursting of electrical globes when put in motion; of other glass vessels exploding spontaneously, and seemingly without any cause; and the bursting of large cast metal vessels in the act of cooling. These are all so similar to one another, that it is probable they depend on one general cause; and all of them agree in this, that the extreme parts are considerably cooled, whilst the internal remain very hot.

The effects of explosions, when violent, are felt at a considerable distance, by reason of the concussions they give to the atmosphere; for all of them act upon the atmospherical fluid with the same force which they exert upon terrestrial substances subjected to their action. Sir William Hamilton relates, that in consequence of the explosions of Vesuvius in 1767, the doors and windows of the houses at Naples flew open if unbolted, and one door was burst open though it had been locked; and the same thing happened several years ago, when the Stobbs powder-mills, near Dalkeith, containing several tons of powder, blew up.