EXPONENT BYCK. 



127 



which the flame moves is calculated, by Mr Robins, 

 to be no less than 7000 feet in a second, or little less 

 than 70 miles per minute. Hence the , impulse of 

 tiie fluid is inconceivably great, and the obstacles on 

 which it strikes are carried off with vast velocity, 

 though much less than that just mentioned ; for a 

 Cannon-ball, with the greatest charge of powder, does 

 not move at a greater rate tiian 2400 feet per second, 

 or little more than 27 miles per minute. The velocity 

 of the ball again is promoted by the sudden propa- 

 gation of the heat through the whole body of the 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 

 movements of the ball. We may conclude, upon 

 these principles, that the force of an explosion 

 depends, 1. on the quantity of elastic fluid to be 

 expended ; 2. on the velocity it acquires by a certain 

 degree of heat ; and, 3. on the celerity with which 

 the degree of heat affects the whole of the expansile 

 fluid. These three take place hi the greatest perfec- 

 tion where the electric fluid is concerned, as in light- 

 ning, earthquakes, and volcanoes. See Steam. 



EXPONENT, in mathematics, is the index of a 

 root or power. For instance, if a quantity is multi- 

 plied by itself any number of times, instead of repeat- 

 ing the factor so many times, we place over it, on the 

 right, a figure denoting how often the number or 

 magnitude has been multiplied by itself; e. g. 



a* = aaaa = a, a, a, a 

 9 =9x9x9 = 729. 



EX POST FACTO, in law ; something done after 

 another ; thus a law is said to be ex post facto, when 

 it is enacte J to punish an offence committed before 

 the passing of the law a violation of the plainest 

 principles of justice. 



EXPRESSED OILS, in chemistry, are those which 

 are obtained from bodies only by pressing, to distin- 

 guish them from animal and essential oils, which last 

 are, for the most part, obtained by distillation. 



EXTENSION, in philosophy; one of the common 

 and essential properties of body, or that by which it 

 possesses or takes up some part of universal space. 



EXTRACT (extractum). 1. When chemists use 

 this term, they generally mean the product of an 

 aqueous decoction. 2. In pharmacy, it includes all 

 those preparations from vegetables, which are sepa- 

 rated by the agency of various liquids, and afterwards 

 obtained from such solutions, in a solid state, by 

 evaporation of the menstruum. It also includes 

 those substances which are held in solution by the 

 natural juices of fresh plants, as well as those to 

 which some menstruum is added at the time of pre- 

 paration. Now, such soluble matters are various, 

 and mostly complicated, so that chemical accuracy is 

 not to be looked for in the application of the term. 

 Some chemists, however, have affixed this name to 

 one peculiar modification of vegetable matter, which 

 has been called extractive, or extract, or extractive 

 principle ; and, as this forms one constituent part of 

 common extracts, and possesses certain character?, it 

 will be proper to mention such of them as may influ- 

 ence its pharmaceutical relations. The extractive 

 principle has a strong taste, differing in different 

 plants : it is soluble in water, and its solution speedily 

 runs into a state of putrefaction, by which it is 

 destroyed. Repeated evaporations and solutions 

 render it at last insoluble, in consequence of its com- 

 bination with oxygen from the atmosphere. It is 

 soluble in alcohol, but insoluble in ether. It unites 

 with alumine, and, if boiled with neutral salts thereof, 

 precipitates them. It precipitates with strong acids, 

 and with the oxides from solutions of most metallic 

 Kilts, especially muriate of tin. It readily unites 



with alkalies, and forms compounds with them, which 

 are soluble in water. No part, however, of this 

 subject, has been hitherto sufficiently examined. In 

 the preparation of all the extracts, the London Phar- 

 macopoeia requires that the water be evaporated, as 

 speedily as possible, in a broad, shallow dish, by 

 means of a water-bath, until they have acquired a 

 consistence proper for making pills ; and, towards 

 the end of the inspissation, that they should be con- 

 stantly stirred with a wooden rod. These general 

 rules require minute and accurate attention, more 

 particularly in the immediate evaporation of the solu- 

 tion, whether prepared by expression or decoction, 

 in the manner, as well as the degree, of heat by 

 which it is performed, and the promotion of it by 

 changing the surface by constant stirring, when the 

 liquor begins to thicken, and even by directing a 

 strong current of air over its surface, if it can con- 

 veniently be done. It is impossible to regulate the 

 temperature if a naked fire be used ; and, to prevent 

 the extract from burning, the use of a water-bath is, 

 therefore, absolutely necessary. 



EXTRACTOR, in midwifery ; an instrument, or 

 forceps, for extricating children by the head. 



EXTRADOS ; the outside of an arch of a bridge, 

 vault, &c. See Architecture. 



EXTRAVASATION, in contusions, and other 

 accidents of the cranium, is when one or more of the 

 blood-vessels distributed on the dura mater are 

 broken, whereby there is such a discharge of blood 

 as oppresses the brain, frequently bringing on violent 

 pains, and at length death itself, unless the patient 

 is timely relieved. 



EXTREMITIES. This term is applied to the 

 limbs, as distinguishing them from the other divisions 

 of the animal, the head and trunk. The extremities 

 are four in number, divided, in man, into upper and 

 lower ; in other animals, into anterior and posterior. 

 Each extremity is divided into four parts ; the upper 

 into the shoulder, the arm, the fore-arm, and the 

 hand ; the lower into the hip, the thigh, the leg, and 

 the foot. 



EXUVIAE, among naturalists, denotes the cast-off 

 parts or coverings of animals, as the skins of serpents, 

 caterpillars, and other insects. 



EY ; a Scandinavian word, signifying island, and 

 contained in several geographical words, as Anglesey, 

 the island of the Angles. 



EYCK, HUBERT VAN, a Flemish painter, considered 

 as the founder of the Flemish school, was born in 

 1366, at Maeseyk. He was much distinguished by 

 his paintings in distemper ; and, after the introduc- 

 tion of oil painting by his brother, he practised in 

 that with equal success. An admirable piece of his, 

 in conjunction with his brother, representing the 

 adoration of the Lamb, from the Apocalypse, is pre- 

 served in the museum at Paris. It contains three 

 hundred and thirty figures, painted in a hard manner, 

 but with great truth and character. He died in 

 1426. 



EYCK, JOHN VAN (also called Jan van Brugge, or 

 John of Bruges, from Bruges, the place of his resi- 

 dence, as the former was given him from the place 

 of his birth, Maeseyk, in the bishopric of Liege), was 

 the son of a painter, whose family name is not known, 

 and was born, according to some, about 1370 ; accord- 

 ing to others, at the close of the fourteenth century ; 

 an opinion favoured by many circumstances. His 

 elder brother, Hubert van Eyck (born about 1366) 

 who was also a celebrated painter in his time, gave 

 him his first instruction in the principles of the art. 

 The talents of this rare genius were so rapidly and 

 vigorously developed, that he soon surpassed his bro- 

 ther, and became the admiration of his own and suc- 

 ceeding times. Of the history of these brothers we 



