these properties, yet they all possess a sufficient number of them to distinguish them from other substances. The form under which acids most commonly appear, is that of a transparent liquor, in which case they are generally combined with a greater or less quantity of water; several of them; however, are found in a solid state, as Benzoic acid, or the flowers of Benzoin; and some exist in a state of gas: as carbonic acid gas. Acids are divided into four classes, according to their bases or radicals. First, those with simple radicals, of different kinds. Secondly, those with double radicals, viz. carbon and hydrogen, in different proportions. Thirdly, those with triple radicals, carbon, hydrogen, and azote. And fourthly, those with unknown radicals. The old chemists were only acquainted with the three mineral acids, as they are called, the sulphuric, the nitric, and the muriatic, besides the acetous acid, or vinegar.

ACIDULATED WATERS, generally called Acidula, a species of mineral waters, which contain a considerable quantity of carbonic acid, and which are known by the poignancy of their taste, the sparkling appearance which they assume when shaken or poured from one vessel into another, and the facility with which they boil.

Acis, in fabulous history, the son of Faunus and Simethus, was a beautiful shepherd of Sicily, who being beloved by Galatea, Polyphemus the giant was so enraged, that he dashed out his brains against a rock; after which Galatea turned him into a river, which was called by his name.

ACOUSTICS is the science which instructs us in the nature of sound. It is usually divided into

two parts, viz. 'diacoustics,' which explains the properties of those sounds that come directly from the sonorous body to the ear; and 'catacoustics,' which treats of reflected sounds. Almost all sounds that affect us are conveyed to the ear by means of the air; but water is a good conductor of sound; so also are timber and flannel.

A bell rung under water returns a tone as distinct as if rung in the air.

Stop one ear with the finger, and press the other to one end of a long stick, or piece of deal wood, and if a watch be held at the other end of the wood, the ticking will be heard be the wood or stick ever so long.

Tie a poker on to the middle of a strip of flannel, two or three feet long, and press with the thumbs or fingers the ends of the flannel into your ears, while you swing the poker against an iron fender, and you will hear a sound like that of a very heavy church bell. These experiments prove that water, wood, and flannel are good conductors of sound, for the sound from the bell, the watch, and the fender pass through the water, and along the deal and flannel to the ear.

It must be observed, that a body, while in the act of sounding, is in a state of vibration, which it communicates to the surrounding air, the undulations of the air affect the ear, and excite in us the sense of sound. Sound, of all kinds, it is ascertained, travels at the rate of thirteen miles in a minute: the softest whisper travels as fast as the most tremendous thunder. The knowledge of this fact has been applied to the measurement of distances,

Suppose a ship in distress fire a gun, the light of which is seen on shore, or by another vessel, 20 seconds before the report is heard, it is known to be at the distance of 20 times 1,142 feet, or little more than four miles and a half.

Again, if I see a vivid flash of lightning, and in two seconds hear a tremendous clap of thunder, I know that the thunder cloud is not more than 760 yards from the place where I am, and should instantly retire from any exposed situation.

The pulse of a healthy person beats about 76 times in a minute; if, therefore, between a flash of lightning and the thunder, I can feel 1, 2, 3, 4, &c. beats of my pulse, I know the cloud is 900, 1,800, 2,700, &c. feet from me.

Sound, like light, after it has been reflected from several places, may be collected into one point as a focus, where it will be more audible than in any other part on this principle whispering galleries are constructed. Suppose fig. 1, plate 1, to represent the concave hemisphere of such a gallery, if a sound be uttered at A, its vibrations expanding every way on the points D, D, &c. will be reflected to E, E, &c., thence to F, F, G, G, &c. till they all meet in C, the opposite part of the gallery, where the sound is distinctly heard.

Speaking trumpets, and those intended to assist the hearing of deaf persons, depend on the reflection of sound from the sides of the trumpet, and also upon its being confined and prevented from spreading in every direction. A speaking trumpet, to have its full effect, must be directed in a line towards the hearer. The report of a gun is much louder when fired towards a person, than one

placed in a contrary direction. Thus, in fig. 2, the sound is received in the open part A C, and is eventually reflected from all sides into the line D B, by which it is sent to the hearer.

An echo is the reflection of sound striking against a surface adapted to the purpose, as the side of a hill, house, wall, &c. Thus fig. 3, if a person stand at m, and the bell a be struck, the sound along a x will proceed to the wall and be reflected from c to m, so that a person standing at m will hear the direct sound coming along a m, and afterwards, the reflected sound or echo, along c m.— See MUSIC.

ACQUITTAL, a discharge, deliverance, or setting free of a person from the guilt or suspicion of an offence. Acquittal is of two kinds; in law, and in fact. When two are appealed or indicted of felony, one as principal, the other as accessary, the prin cipal being discharged, the accessary is, by consequence, also freed: in which case, as the accessary is acquitted by law, so is the principal in fact.

ACRE, a measure of land very general in name, but differing almost in every two places as to the extent which it is intended to denote. A statuteacre in England contains four square roods; a rood, containing forty perches or poles, of sixteen feet and a half each; but, in different countries, the length of the pole varies, from the statute measure of sixteen feet and a half, to twenty-eight: these several lengths are called customary measures. In Scotland, the acre is larger than in England. The French acre contains one English and a half. The Strasburgh acre is about half an English acre. The Welsh acre commonly contains two English.

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The Irish acre is equal to one acre, two roods, 1927 perches English.

ACRONYCHAL, in astronomy, is applied to a star or planet, when it is opposite to the sun. It is from the Greek, signifying the point or extremity of night, because the star rose at sun set, or the beginning of night, and set at sun-rise, or the end of night; and so it shone all the night. The acronychal is one of the three Greek poetic risings and settings of the stars; and stands distinguished from cosmical and heliacal. By means of which, for want of accurate instruments, and other observations, they might regulate the length of their year.

ACROSTIC, a poem, the lines of which are so contrived, that the first letters of each, taken together, will make a proper name or other word.

ACT. Act, in the universities, is the delivery of orations, or other exercises, in proof of the proficiency of a student who is to take a degree. At Oxford, the time when masters or doctors complete their degrees, is called the act. At Cambridge, the same period is called the commencement.

Act of Faith, or auto-da-fe. In dark and barbarous countries where the Spanish inquisition had power, the act of faith was a solemn murder of infidels and heretics, usually performed on some great festival, and always on a Sunday.

Act of Parliament is a positive law, consisting of two parts, the words of the act, and its true sense and meaning; which, being joined, make the law. The words of the acts of parliament are to be taken in a lawful sense. Cases of the same nature are within the intention, though without the letter, of the act; and some acts extend, by equity, to cases not mentioned therein.