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 destructive effects upon adjacent objects.
With regard to solids, explosion differs from expansion, which is a continuous power, acting uniformly for some time; whereas the former is 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, the velocity with which the flame moves is calculated by Robins (Treatise upon Gunnery) to be no less than 7000 feet in a second, or little less than 79 miles in a 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 Explosion, that just mentioned; for a cannon ball, with the greatest charge of powder that can be conveniently given, does not move at a greater rate than 2400 feet in a second, or little more than 27 miles in a minute. The velocity of the ball, 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. In violent lightning, where the electric fluid collects itself into balls, the strength of the explosion is proportional to the quantity. The violent effects of lightning when it strikes buildings, trees, or even the most solid rocks, is well known; and in some cases, where the quantity of electricity is still greater than in any flash of lightning, still more tremendous consequences ensue. Dr Priestley gives an instance of a large fire-ball, 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 is impossible to measure it by any method applicable to the mensuration of other forces.
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 incomparably 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 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.
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 water is decomposed by being applied to red-hot iron in the form of steam, and one of its elements enters into combination with the iron. It may be observed, that in all cases where a very hot body is thrown upon a small quantity of water in the liquid state, an explosion will follow. 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 molten metal has been poured into wet moulds. In these cases, the sudden expansion of the aqueous steam throws Exponent out the metal with great violence; and if any decomposition take place at the same time, so as to convert the aqueous into an aerial vapour, the explosion will be still greater.
A similar explosion takes place on pouring cold water into boiling or burning oil or tallow. The same effect follows 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.
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. The same effect has been observed on the explosion of powder-magazines.
**EXPO**NENT, or INDEX, in *Algebra*. See *Algebra*, vol. ii., p. 491.
Exponent of the ratio, in arithmetic, is the quotient arising when the antecedent is divided by the consequent of the ratio. Thus 4 is the exponent of the ratio of 20 to 5; for \( \frac{20}{5} = 4 \).
**Exponential Calculus**, the method of differencing, or finding the fluxions of exponential quantities, and of summing up those differences, or finding their fluents.
**Exponential Curve** is that the nature of which is defined or expressed by an exponential equation; as the curve denoted by \( x^a = y \), or by \( x^a = y \).
**Exponential Equation** is one in which is contained an exponential quantity; as the equation \( a^x = b \), or \( x^a = ab \), &c.
**Exponential Quantity** is that the power of which is a variable quantity; as the expression \( a^x \), or \( x^a \). Exponential quantities are of several degrees and orders, according to the number of exponents or powers.
**Exposing of Children**. This was a common practice among the Greeks, Romans, and other ancient nations. But from this charge must be excepted the Thebans, who had an express law by which it was made capital to expose children, and at the same time ordained that such as were not in a condition to educate their offspring should take them to the magistrates, that they might be brought up at the public expense. Among the other Greeks, when a child was born, it was laid on the ground; and if the father designed to educate it, he immediately took it up; but if he forbore to do so, the child was carried away and exposed. The Lacedemonians indeed had a different custom: with them all new-born children were brought before certain persons, considered as among the wisest in their own tribes, by whom the infants were carefully examined; and if the latter were found strong and well formed, orders were given for their education, with a certain proportion of land for their maintenance; but if weakly or deformed, they were cast into a deep cavern in the earth, near Mount Taygetus, as it was thought neither for the good of the infants themselves nor for the public interest that defective children should be reared. Many children were exposed only because their parents were not in a condition to educate them; and it was the unhappy fate of daughters especially to be thus treated, as their education and settlement in life involved a greater expense than those of sons. The parents frequently tied jewels and rings to the children they exposed or any other thing by which they might afterwards discover them, if they should survive; or to encourage those that might find them to nourish and educate them; or, if found dead, to give them human burial. It was usual to expose children in such places as were most frequented, in order that they might be found and compassionated by persons who were in circumstances to defray the expense of their education. As foundlings became the property of those who brought them up, instances were not wanting of great cruelties exercised towards these unfortunate individuals; such as, for instance, mutilating their persons and exhibiting them in the streets, in order to derive an infamous livelihood from the alms bestowed on them by compassionate passengers. By a law passed at Rome A.D. 374, the exposure of children was made a punishable offence; but it was not till the year 530 that their slavery was abolished, when Justinian published an edict to that effect.
**Ex Post Facto**, a law phrase used to denote something made to apply to a thing already done. Thus, for example, a law is said to be made *ex post facto* when it renders an offence punishable in a manner in which it was not punishable at the time it was committed.
**Expressed Oils**, are such as are obtained from bodies by pressure only. See Oils.
**Extent**, in *Law*, is used in a double sense, denoting either a writ of execution (called also an *extendi facias*) directed to the sheriff against the person, lands, and goods, or the lands only, of a debtor; or the act of the sheriff or officer upon this writ. These writs are of two kinds, to both of which the sovereign is by ancient prerogative entitled, for the purpose of obtaining satisfaction of debts due or assigned to the crown. The writ of *extent in chief* is a proceeding by the sovereign for the recovery of his own debt, and in which he is the real plaintiff. The writ of *extent in aid* is also sued out at the instance and for the benefit of the crown against the debtor of a crown debtor. In this last proceeding the sovereign is only the nominal plaintiff.
**Extract**, in *Pharmacy*, a preparation of vegetable principles obtained by digesting vegetable substances in water or in alcohol, and evaporating them to a pasty or solid consistence; or by boiling down the expressed juices of fresh plants. According to the mode of preparation, they are termed either aqueous or spirituous extracts.
The name *extractive principle*, or *extractive*, is given by chemists to a peculiar principle which is supposed to form the basis of all vegetable extracts.
**Extract of Lead**, the name given to the impure acetate of lead obtained by boiling litharge in vinegar. It is commonly known as *Goulard's extract*.
**Extraction of Roots**, in *Algebra* and *Arithmetic*, the method of finding the roots of given numbers or quantities. See *Algebra* and *Arithmetic*.
**Extraordinarii**, in the ancient Roman army, a select body of men consisting of the third part of the foreign cavalry and a fifth of the infantry. These were carefully separated from the other forces borrowed from the confederate states, in order to prevent any treacherous coalition between them. From among the *extraordinarii* a more choice body of men were drawn, under the name of *allecti*.
**Extravagantes**, a name given to certain decretal epistles, or constitutions of the popes, which were published from the Clementines, but were not at first digested or arranged with the other papal constitutions. Hence the name which they continued to retain, even after their insertion in the body of the canon law. See *Canon Law*, vol. vi., p. 190.
**Extravasation** (*extra* beyond, and *vas* a vessel), in *Medicine*, the escape of blood, or other fluid, from its natural receptacle into a neighbouring part of the body. The fluid thus thrown out is said to be *extravasated*.