quences, there is one very striking; namely, that the picture after degenerating into a sign or character, will be associated by memory with the oral character, or name, or correspondent word. An immediate step after this, must be that characters associated with monosyllabic words will be frequently put together to form polysyllabic words, in which the picture is left out of the consideration, and the sound alone forms the subject of the record, (as if the characters for man and eye were united to form the word many, or multitudinous.) And lastly, habit must in fact have given a preference in the composition of these polysyllabic words, to such simple sounds and their characters as were found to be most extensively useful. That is to say, an unintentional process of analysis must have thus given rise to the alphabet.

The sounds of language are modified by articulation, which depends on certain gross, and in general obvious changes in the figure of the organs; and by accent or mere intensity; and by intonation or music. The first of these, as used in discourse, is much more capable of having its variations marked by characters than the others; and from this circumstance it is found that the alphabet can deliver with correctness the words of such languages as communicate chiefly by articulation: but in languages where the same articulated monosyllable denotes a great variety of things according to the accent or intonation, there will be comparatively few instances of depicted sound, and the system of writing will continue to be hieroglyphic or rather symbolic in all its improvements. This system is for the reason here mentioned in use in China, and does not seem inferior to the alphabet, but in some respects more advantageous.

ALPHABET is also used for a cypher, or table of the usual letters of the alphabet, with the corresponding secret characters, and other blank symbols intended to render the writing more difficult to be decyphered. See the article DECYPHERING.

nandria Monogynia class of plants, the corolla whereof is monopetalous, unequal, and as it were double; the exterior one is trifid, the upper segment is hollow, the two side ones flat, and it has a tube; the interior is short, its edge is trifid, and the lower segment of the three hangs out beyond the lateral parts of the exterior corolla, the other two are emarginated, and the base is ventricose; the fruit is a fleshy capsule, of an ovated figure, composed of three valves, and containing three cells; the seeds are numerous, of an ovated figure, with a prominent but truncated apex, and a caudated base. There are seven species.

ALSINE, chick-weed, in botany, a genus of the Pentandria Trigynia class and order, and of the natural order of Caryophyllei: its characters are, that the calyx is a fiveleaved perianthium, leaflets concave, oblong and acuminate; the corolla has five equal petals, longer than the calyx; the stamina consist of capillary filaments, the anthers roundish; the pistillum has a subovate germ, styles filiform, and stigmas obtuse; the pericarpium is an ovate, one-celled, three-valved capsule, covered with the calyx; the seeds are very many and roundish. There are five species, of which the following is the principal. A. media, common chick-weed, with petals bipartite, and leaves ovate cordate. The number of stamens in the flower of the common chickweed is uncertain, from three to ten. This species in different soils and situations assumes different appearances; but it is distinguished from the cerastiums, which it most resembles, by the number of pistils, and by having the petals shorter than the leaves of the calyx, and from all the plants related to it, and particularly the stellaria nemorum, by having the stalk alternately hairy on one side only. Dr. Withering refers it to the stellaria, with which genus it agrees in various respects, and especially in the capsules opening with six valves. He observes, that it grows almost in all situations from damp and almost boggy woods, to the driest gravel walks in gardens; but in these various states its appearances are very different, so that those who have only taken notice of it as garden chicken-weed would hardly know it in woods, where it sometimes exceeds half a yard in height, and has leaves near two inches long, and more than one inch broad. In its truly wild state, he says, in damp woods, and hedge bottoms with a northern aspect, it

soils and more sunny exposures, the stamens are usually five or three. When the flowers first open, the peduncles are up right; as the flowers go off, they hang down; and when the seeds ripen, they again become upright. Dr. Withering observes, that the flowers are upright, and open from nine in the morning till noon; but if it rains, they do not open. After rain they become pendent; but in the course of a few days, rise again. In gardens or dunghills, chick-weed sheds abundance of seeds, which are round, compressed, yellow, and rough, with little tubercles; and thus becomes a troublesome weed; but if it be not suffered to seed, it may be destroyed, as it is annual, without much trouble. This species is a remarkable instance of the sleep of plants; for every night the leaves approach in pairs, including with their upper surfaces the tender rudiments of the new shoots; and the uppermost pair but one, at the end of the stalk, is furnished with longer leaf-stalks than the others, so that they can close upon the terminating pair, and protect the end of the branch. The young shoots and leaves, when boiled, can scarcely be distinguished from spring spinach, and are equally wholesome. Swine are very fond of it, cows and horses eat it; sheep are indifferent to it; and goats refuse it. This plant is found wild in most parts of the world. It is annual, and flowers almost through the whole year.

ALSTONIA, in botany, a genus of the Polyandria Monogynia class and order. Essen. char. corol. one-petalled, eight or ten-cleft: clefts alternated. There is but one species, a shrub found in South America. It is very smooth, and has the air of the bohea-tea, in the leaves, calyxes, and situations of the flowers. The dried leaves taste like those of Chinese tea.

ALSTROEMERIA, in botany, a genus of the Hexandria Monogynia class and order: cor, six-petalled, somewhat two-lipped; the lower petals tubular at the base: stamina declined. There are six species, all found in South America.

ALT in music, a term applied to that part of the great scale of sounds which lies between F above the treble-cleft note and G in altissimo.

ALTAR, a place upon which sacrifices were antiently offered to some deity.

The heathens at first made their altars only of turf; in following times they were made of stone, of marble, of wood, and even af horn, as that of Apollo in Delos. Altars VOL L

differed in figure as well as in materials. Some were round, others square, and others oval. All of them were turned towards the east, and stood lower than the statues of the gods, and were generally adorned with sculpture, inscriptions, and the leaves and flowers of the particular tree consecrated to the deity. Thus, the altars of Jupiter were decked with oak, those of Apollo with laurel, those of Venus with myrtle, and those of Minerva with olive,

The height of altars also differed according to the different gods to whom they sacrificed. Those of the celestial gods were raised to a great height above the ground; those appointed for the terrestrial were almost on a level with the surface of the earth; and, on the contrary, they dug a hole for the altars of the infernal gods. According to Servius, the first were called altaria, the second are, and the last crobiculi ; but this distinction is not every where observed, for we find in the best authors, the word ara, as a general word, including the altars of celestial, infernal, and terrestrial gods.

Before temples were in use, altars were erected sometimes in groves, sometimes in the high ways, and sometimes on the tops of mountains; and it was a custom to engrave upon them the name, proper ensign, or character of the deity to whom they were consecrated. Thus, St. Paul observed an altar at Athens, with an inscription To the unknown God.

In the great temples of ancient Rome, there were ordinarily three altars; the first was placed in the sanctuary, at the foot of the statue of the divinity, upon which incense was burnt, and libations offered: the second was before the gate of the temple, and upon it they sacrificed the victims : and the third was a portable altar, upon which were placed the offerings and the sacred vessels.

Besides these uses of the altars, the ancients swore upon them, and swore by them in making alliances, confirming treaties of peace, and on other solemn occasions. Altars also served as a place of refuge and sanctuary to all those who fled to them, whatever crime they had committed.

ALTAR is also used, among christians, for the communion-table.

ALTERNATE, in heraldry, is said in respect of the situation of the quarters.

Thus the first and fourth quarters, and the second and third, are usually of the same nature, and are called alternate quarters,

K

ALTERNATION is used for the differ ent ways which any number of quantities may be changed, or combined. See Coм

BINATION.

ALTHEA, marsh-mallow, in botany, a genus of plants, with a double calyx, the exterior one being divided into nine segments; the fruit consists of numerous capsules, each containing a single seed. It belongs to the Monadelphia Polyandria class and order. There are nine species. The A. officinalis is perennial, and flowers from July to September. It grows plentifully in salt marshes, and on the banks of rivers and ditches, in many parts of England, Holland, France, Italy, Siberia, &c. It has been in great request in every country where medicine has been regularly cultivated. All its parts abound with a glutinous juice with scarcely any smell or peculiar taste.

ALTIMETRY, denotes the art of measuring altitudes or heights. See MENSU

RATION.

ALTITUDE, in geometry, one of the three dimensions of body; being the same with what is otherwise called height.

Altitude of a figure is the distance of its vertex from its base, or the length of a perpendicular let fall from the vertex to the base.

Thales is supposed to have been the first person who applied the principles of geometry to the mensuration of altitude: by means of the staff he measured the height of the pyramids of Egypt, making the altitude of the staff and pyramid proportional to the length of the shadows.

ALTITUDE, in optics, is the height of an object above a line, drawn parallel to the horizon from the eye of the observer.

ALTITUDE of the eye, in perspective, is its perpendicular height above the geometrical plane.

ALTITUDE of a star, &c. in astronomy, is an arch of a vertical circle, intercepted between the stars and the horizon.

This altitude is either true or apparent, according as it is reckoned from the rational or sensible horizon, and the difference between these is what is called by astronomers the parallax of altitude. Near the horizon, this altitude is always increased by means of refraction.

ALTITUDE of the mercury, in the barorometer and thermometer, is marked by degrees, or equal divisions, placed by the side of the tube of those instruments. The altitude of the mercury in the barometer in and about the metropolis is usually com

prised between 28.4 and 30.6 inches: in the course of the last seven years it has not varied from these limits more than twice. During the same period, the thermometer in the shade has been rarely higher than 82° or 83°, and this seldom more than three or four times in a whole summer, nor often lower than about 8° or 10° below the freezing point. This degree of cold is not common. ALTO-RELIEVO. See RELIEVO.

ALUM, in chemistry and the arts, is denominated the sulphate of alumina, though it is not merely a combination of alumina with the sulphuric acid. It possesses the following characters: 1. It has a sweetish astringent taste. 2. It is soluble in warm water, and the solution reddens vegetable colours, which proves the acid to be in excess. 3. When mixed with a solution of carbonate of potash, an effervescence is produced by the uncombined acid, which prevents the first portions of alkali, that are added to the solution of sulphate of alumina, from occasioning any precipitate. 4. When sulphate of alumina is heated, it swells up, loses its regular form, and becomes a dry, spongy mass; but according to the experiments of Vauquelin, the whole of its acid cannot be thus expelled. 5. The combination of sulphuric acid with alumina is incapable of crystallizing without an addition of potash, which makes a constituent part of all the alum of commerce. 6. It is decomposed by charcoal, which combines with the oxygen of the acid, and leaves the sulphur attached to the alumina.

Dr. Thomson says there are four varieties of alum, all of which are triple salts; two neutral, and two he calls super-salts. These are thus denominated:

1. Sulphate of alumina and potash. 2. Sulphate of alumina and ammonia. 3. Super-sulphate of alumina and potash. 4. Super-sulphate of alumina and ammonia.

The discovery of alum was made in Asia, from whence it continued to be imported till the end of the fifteenth century, when a number of alum works were established in Italy. In the sixteenth century it was manufactured in Germany and Spain; and during Queen Elizabeth's reign an alum manufactory was established in England. The alum of commerce is usually obtained from native mixtures of pyrites and clay, or sulphuric' acid and clay. Bergman has published a ver complete dissertation on the process usually followed. The earth from which it is procured is usually called aluminons shis

tus, because it is slaty. Its colour is blackish, because it contains some bitumen. In most cases it is necessary to burn it before it can be employed: this is done by means of a slow smothered fire. Sometimes long exposure to the weather is sufficient to produce an efflorescence of alum on the surface. It is then lixiviated, and the water concentrated by evaporation, and mixed with putrid urine, or muriate of potash; crystals of alum and of sulphate of iron usually form together. The composition of alum has been but lately understood with accuracy. It has been long known, indeed, that one of its ingredients is sulphuric acid; and the experiments of Pott and Margraff proved incontestibly that alumina is another ingredient. But sulphuric acid and alumina are incapable of forming alum, Manufacturers knew that the addition of a quantity of potash or of ammonia, or of some substance containing these al kalies, is almost always necessary, and it was proved, that in every case in which such additions are unnecessary, the earth from which the alum is obtained contains already a quantity of potash. Various conjectures were made about the part which potash acts in this case; but Vauquelin and Chaptal appear to have been the first chemists that ascertained by decisive experiments that alum is a triple salt, composed of sulphuric acid, alumina, and potash or ammonia united together. Alum crystallizes in regular octahedrons, consisting of two foursided pyramids applied base to base. The sides are equilateral triangles. The form of its integrant particles, according to Hauy, is the regular tetrahedron. Its taste is, as we have observed, astringent. It always reddens vegetable blues. Its specific gravity is 1.7109. At the temperature of 60% it is soluble in from 15 to 20 parts of water and in ths of its weight of boiling water. When exposed to the air, it effloresces slightly. When exposed to a gentle heat it undergoes the watery fusion. A strong heat causes it to swell and foam, and to lose about 44 per cent. of its weight, consisting chiefly of water of crystallization. What remains is called calcined or burnt alum, and is sometimes used as a corrosive. By a violent heat, the greater part of the acid may be driven off. Though the properties of alum are in all cases pretty nearly the same, it has been demonstrated by Vauquelin that three varieties of it occur in commerce. The first is, super-sulphate of alumina and potash; the second, super-sulphate of alumina and ammonia; the third,

When an unusual quantity of potash is added to alum liquor, the salt loses its usual form and crystallizes in cubes. This constitutes a fourth variety of alum, usually distinguished by the name of cubic alum. It contains an excess of alkali. When the potash is still further increased, Chaptal has observed, the salt loses the property of crystallizing altogether, and falls down in flakes. This constitutes a fifth variety of alum, consisting of sulphate of potash combined with a small proportion of alumina. If three parts of alum and one of flour or sugar be melted together in an iron ladle, and the mixture dried till it becomes blackish and ceases to swell; if it be then pounded small, put into a glass phial, and placed in a sandbath till a blue flame issues from the mouth of the phial, and after burning for a minute or two be allowed to coo!, a substance is obtained known by the name of Homberg's pyrophorus, which has the property of catching fire whenever it is exposed to the open air, especially if the air be moist. This substance was accidentally discovered by Homberg about the beginning of the eighteenth century, while he was engaged in his experiments on the human feces. He had distilled a mixture of human faces and alum till he could obtain nothing more from it by means of heat; and four or five days after while he was taking the residuum out of the retort, he was surprised to see it take fire spontaneously. Soon after, Lemery the younger discovered that honey, sugar, flour, or almost any animal or vegetable matter, could be substituted for human fæces; and afterwards, Mr. Lejoy de Suvigny shewed that several other salts containing sulphuric acid may be substituted for alum. Scheele proved that alum deprived of pot ash is incapable of forming pyrophorus, and that sulphate of potash may be substituted

or alum. And Mr. Proust has shewn that a number of neutral salts, composed of vegetable acids and earths, when distilled by a strong fire in a retort, leave a residuum which takes fire spontaneously on exposure to the air. These facts have thrown a great deal of light on the nature of Homberg's pyrophorus, and enabled us in some measure to account for its spontaneous inflammation. It has been ascertained, that part of the sulphuric acid is decomposed during the for mation of the pyrophorus, and of course a part of the alkaline base becomes uncombined with acid; and the charcoal, which gives it its black colour, is evidently divided into very minute particles. It has been ascertained, that during the combustion of the pyrophorus a quantity of oxygen is absorbed. The inflammation is probably occasioned by the charcoal; the sulphuret of potash also acts an essential part. Perhaps it produces a sudden increase of temperature by the absorption and solidification of water from the atmosphere.

A new process for making alum is used at some works, for which we are indebted to Mr. Sadler, which is as follows: The boilers are filled with prepared liquor of 10 pennyweights, to which sulphate of potash is added, and boiled together, until it weighs 16 pennyweights, by which time the whole of the superfluous alumina and the oxyde of iron is precipitated. The fluid is then run into a settler, where it remains until clear, after which it is pumped into a second boiler, and evaporated up to 26 pennyweights, let into the coolers, and left to crystallize. By this process, it is said, he gains the whole of the alum at one evaporation, and from the mother liquor remaining there is a product, the sulphate of iron.

ALUMINA, in chemistry, one of the five proper earths. It was discovered by the alchemists that alum was composed of sulphuric acid and an earth, the nature of which was long unknown; but Geoffroy, and afterwards Margraff, found that the earth of alum is an essential ingredient in clays, and gives them their properties, hence it was called argil; but Morveau gave it the name of alumina, because it is obtained in a state of the greatest purity from alum by the following process. Dissolve alum in water, and add to the solution ammonia as long as any precipitate is formed. Decant off the fluid part, and wash the precipitate iin a large quantity of water, and then allow ft to dry. The substance thus obtained is alamina; not however in a state of absolute

purity, for it still retains a portion of the sulphuric acid with which it was combined in the alum. But it may be rendered tolerably pure, by dissolving the newly precipitated earth in muriatic acid, evaporating the solution till a drop of it in cooling deposits small crystals, setting it by to crystallize, separating the crystals, concentrating the liquid a second time, and separating the crystals which are again deposited. By this process, most of the alum which the earth retained will be separated in crystals. If the liquid be now mixed with ammonia as long as any precipitate appears, this precipitate, washed and dried, will be alumina nearly pure. Alumina has little taste : when pure, it has no smell; but if it contains oxyde of iron, which it often does, it emits a peculiar smell when breathed upon, known by the name of earthy smell. This smell is very perceptible in common clays. The specific gravity of alumina is 2.00. When heat is applied to alumina, it gradually loses weight, in consequence of the evaporation of a quantity of water with which, in its usual state, it is combined; at the same time its bulk is considerably diminished. The spongy alumina parts with its moisture very readily; but the gelatinous retains it very strongly. Spongy alumina, when exposed to a red heat, loses (0.58 parts of its weight; gelatinous, only 0.43: spongy alumina loses no more than 0.58 when exposed to a heat of 150° Wedgewood; gelatinous in the same temperature loses but 0.4825. Yet Saussure has shown that both species, after being dried in the temperature of 60°, contain equal proportions of water. Alumina undergoes a diminution of bulk proportional to the heat to which it is exposed. This contraction seems owing, in low temperatures, to the loss of moisture; but in high temperatures it must be owing to a more intimate combination of the earthy particles with each other; for it loses no perceptible weight in any temperature, however high, after being exposed to a heat of 130° Wedgewood.

Mr. Wedgewood took advantage of this property of alumina, and by means of it constructed an instrument for measuring high degrees of heat. It consists of pieces of clay of a determinate size, and an apparatus for measuring their bulk with accuracy: one of these pieces is put into the fire, and the temperature is estimated by the contraction of the piece. The contrac tion of the clay-pieces is measured by means of two brass rules, fixed upon a plate, the