it becomes red. The same is the case if they be added to other vegetable colors, as violet, &c. Hence these colors are employed as tests of acids, that is, to ascertain when they exist in any substance. We may add the infusion to the fluid in which we are trying to detect an acid, but a more convenient method is, to spread it on paper, and allow it to dry. If a strip of this be put into a fluid in which there is an acid, it instantly becomes red. Some acids appear only in a fluid state, either gaseous, as carbonic acid, or liquid, as sulphuric acid; others appear in a solid form, or crystallized, as benzoic acid, boracic acid, &c. All acids are compound bodies, and are sometimes divided into four classes, the three first of which are compounded with oxygen; the fourth class consists of those which, at least according to some modern chemists, have no oxygen; e. g. sulphuretted hydrogen. The first class consists of acids compounded with oxygen and one other body; the second class comprises the acids compounded of carbon, hydrogen and oxygen; the third class consists of those acids which contain nitrogen, in addition to the three substances abovementioned. The ancient chemists were acquainted with but few of the acids now known; they divided them, according to the kingdoms of nature, into mineral, vegetable and animal acids. This division, however, cannot now be retained, as there are some acids which appear in all the kingdoms; e. g. phosphoric acid. If the same radical be compounded with different proportions of the acidifying principle, forming different acids, the most powerful acid receives a name from the radical, terminating in ic; the weaker, a name formed in the same manner in ous; e. g. sulphurous acid and sulphuric acid, nitrous and nitric acid; and, where there - are intermediate compounds, the term hypo is occasionally added to the compound next above it in point of acidity. Thus hyposulphuric acid signifies an intermediate acid between sulphurous and -sulphuric acids; hypophosphorous acid, an acid containing less oxygen than the phosphorous acid. (For Prussic acid, Pyroligneous acid, &c. see Prussic, Pyroligneous, &c.)

ACIRS; hurricanes of snow which prevail among the Cevennes, in the south of France. Villages are sometimes so rapidly covered, that the inhabitants have no means of communication, but by cutting passages under the snow.

ACKERMANN, Rudolph, was born in

1764, at Schneeberg, in Saxony, where his father was a saddler. He received his education at the Latin school of his native city, and, after learning the trade of his father, travelled through the country as a journeyman, according to the custom of Germany. After residing for some time at Paris and Brussels, he went to London. He there became acquainted with Facius, a German, who had undertaken to conduct a journal of fashions, (Journal des Modes,) and met with tolerable success. A. soon afterwards published, in the same way, drawings of coaches and curricles, invented, drawn and painted by himself. The novelty and elegance of the forms excited universal attention, and he received orders for drawings from all quarters. This laid the foundation of a trade in works of art, which his activity, attention and precision in business so much enlarged in a short time, that he was enabled to marry an English woman, became a citizen of London, and founded an establishment called Repository of Arts, in the Strand, in the centre of London. It is one of the curiosities of the British capital, and gives employment to several hundred men. An account of every thing new has appeared for 8 years in A.'s splendid journal, Repository of Arts, Literature and Fashion, the first series of which, in 14 volumes, costs £18; and the new series already amounts to more than 40 numbers. Every number contains three or four elegant, colored copperplates. For 8 years he has also been engaged in a series of topographical works, exhibiting all the splendor of British aquatinta, which already constitute a small library, and, for truth of design and elegance of execution, are hardly surpassed by any similar undertaking in any country. He now has the most instructive books of the English and other languages translated into Spanish, (principally by the well-known Blanco White,) and sends them to America, where his eldest son is engaged, in Mexico, in extensive dealings in books and works of art. For some years he has also published the first souvenir in England, called the Forget me not. When the association was formed, in 1813, for the relief of those who had been plunged into misery by the war in Germany, Ă. showed himself an active philanthropist. A. is now the best lithographer in London. He employs in the summer 600 men, every day, in and around London.

ACOLYTHI, or ACOLYTES; servants of the church, who appeared in the Latin

church as early as the 3d century; but in the Greek, not till the 5th. Their office was to light the candles, thence they were called accensores; to carry the tapers in the festal processions, thence ceroferarii; to present the wine and water at the supper; and, in general, to assist the bishops and priests in the performance of the ceremonies. They belonged to the clergy, and had a rank immediately below the subdeacons. In the Roman church, the consecration of an acolythus is the highest of the lower kinds of ordination. The person ordained receives a candlestick and chalice, in token of his ancient employment. The duties, however, formerly appertaining to this office, have been performed since the 7th century by menials and boys taken from the laity, who are improperly called acolythi, in the books of the liturgy of the Catholic church. The modern Greek church no longer retains even the name.

ACONITA; a vegetable poison, recently extracted from aconitum napellus, or wolf's-bans, (properly alkaline,) by Mr. Brande. The analysis has not yet been made known.

ACOUSTICS. One of our most important connexions with external objects is maintained through the sense of hearing; that is, by an affection which certain actions or motions, in those objects, produce on the mind, by being communicated to it through the ear. The peculiar excitation or motion perceptible by the ear is called sound; and the consideration of this motion, its qualities and transmission, forms the science of acoustics. Philosophers make a distinction between sound and noise: thus those actions which are confined to a single shock upon the ear, or a set of actions circumscribed within such limits as not to produce a continued sensation, are called a noise; while a succession of actions which produce a continued sensation are called a sound. It is evident from the mechanism of the ear, so far as it is understood, that it is a refined contrivance for conveying a motion from the medium which surrounds it to the auditory nerve; and that this nerve must receive every motion excited in the tympanum. Every motion thus excited, however, does not produce the sensation of sound. That motions may be audible, it is necessary that they impress themselves upon the medium which surrounds the ear with velocities comprised within certain limits. These motions are commonly produced by disturbing the equilibrium which exists be

tween the parts of a body. Thus, for example, if we strike a bell, the part which receives the first impulse of the blow is driven nearer to the surrounding parts; but, the impulse having ceased, it is urged back by a force of repulsion which exists in the metal, and made to pass beyond its former position. By the operation of another property of the metal, namely, cohesive attraction, it is then made to return in the direction of its first motion, again, beyond its position of repose. Each of these agitations inflùences the adjacent parts, which, in turn, influence those beyond them, until the whole mass assumes a tremulous motion; that is, certain parts approach to and recede from each other; and it only recovers its former state of repose, after having performed a number of these sonorous vibrations. It is evident that such vibrations as are here described must result from the combined operation of attraction and repulsion, which, together, constitute the elasticity of solid bodies. When fluids, whose elasticity is confined to repulsion, emit sounds, a force equivalent to that of attraction in solids is supplied to them by external pressure. The sonorous vibrations of bodies are exceedingly curious, and the more difficult to be understood from our habits of measuring changes or motions by the sight; but these motions affect very sensibly another organ, while they are almost imperceptible to the eye; and, as we are without the means of converting the ideas derived from one sense into those derived from another, the sensation of the motion of sound does not assist us to understand its precise nature, as compared with visible motions. Thus, the ear at once perceives the difference between a grave and an acute sound; but it is only from attentive observation by the eye, that we discover the different rapidity of succession in the vibrations which produce them. The vibrations of a great many bodies, as strings, bells and membranes, when emitting sounds, may, however, be distinctly seen, and even felt; but they may often be rendered more sensible to the eye by a little artifice, such as sprinkling the vibrating body with sand, or some light, granular substance. Sound may be produced without vibrations or alternations; thus, if we pass the nail quickly over the teeth of a comb, the rapid succession of single shocks or noises produces all the effect of vibrations. It must be evident that the rapid motions here described, whether

originating in vibrations, or a succession of concussions, must be communicated from the body, in which they are excited, to the sheet of air, or whatever else be in contact with it, and from this again to another sheet beyond the first; thus diffusing the motion in every direction. The agitation of the sounding body must thus be communicated to the surrounding medium to a great distance, and impressed upon any body situated within this distance; if this body be the ear, the tremor excited in it by these agitations will be perceived by the mind. The necessity of some medium for the transmission of sound is proved by experiment. If a bell be rung in an exhausted receiver, the sound will be hardly perceptible, while the tones will become clear and distinct, on re-admitting the air. Having thus given a general outline of the source and propagation of sound, we shall proceed to consider, with as much minuteness as the limits of this work will permit, some of the more important facts connected with them.-The most obvious characteristics, by which we distinguish different sounds, consist of differences in their degrees of what we call loudness, and acuteness, or pitch. We can produce, at pleasure, sounds having different degrees of loudness, from the same sonorous body, by making the concussions upon it more or less violent; disturbing in a greater or less degree the arrangement of its parts. So two bodies of like substance and figure, but unlike mass, when subjected to the same shock, emit sounds unlike in loudness; and, again, bodies of like mass and figure, but unlike substance, form sounds more or less loud, when subjected to the same shock. In this latter case, the loudness has a relation to the quantity of elasticity possessed by the bodies; and in all cases, when the disturbance of the parts is carried beyond the elastic power of the body, so as to produce a permanent change of figure, no increase of loudness is induced. From a consideration of the preceding facts, we may conclude, that loudness depends upon the quantity of motion, or sonorous vibration, in which it originates. The other principal characteristic of sound, its acuteness or pitch, depends upon the frequency with which the concussions or vibrations of the sonorous body succeed each other. That sounds may be audible to a common ear, it is necessary that the concussions upon the medium, which communicates them, should follow each other in such succes

sion, that not more than 8192, nor less than 32, distinct concussions shall be made upon the medium during the lapse of one second. Some ears, however, can perceive sounds emanating from vibrations a little beyond the extremes to which the perceptions of other ears are confined. We should be careful not to confound the frequency of vibrations with the velocity of vibratory motion. A string may vibrate with a greater or less velocity, as it passes its axis to a greater or less distance; yet the times of its vibrations may be all equal. The difference of velocity, affecting the quantity of motion only, would produce no change, except in the loudness of the sound. To those sounds which proceed from infrequent vibrations, we give the name of grave or low; those from frequent vibrations we call sharp or acute. When vibrations succeed each other in equal times, their sound excites a pleasant sensation, and they are called musical. When two bodies are made to sound together, if their vibrations are performed in equal times, the sounds are said to be in unison. When the vibrations are performed in unequal times, so that some of those of the one are not accompanied by those of the other, the ear perceives a degree of dissonance in the sounds. If, however, the vibrations meet after short and regular intervals, the dissonance is not easily detected, and the sounds are said to accord. During the continuance of most primary sounds, however excited, we perceive other and more acute sounds co-existing with them. These are called their harmonics. They are supposed to originate in a series of secondary vibrations, more short and frequent than the principal vibration. Thus a sounding string, for example, may be supposed not to pass its axis in a simple curve, but to resolve itself into a tortuous line, formed by a number of smaller curves, each of which vibrates across its own axis, thus producing its harmonics. It is perhaps some combination of the harmonics with the primary sound, that characterizes the sound of different instruments, though of the same loudness and pitch, so that we can distinguish one from another. The air, being the common medium which surrounds the ear, is that by which sounds are usually transmitted. This transmission is performed with a velocity of about 1130 feet in a second. All other bodies, however, are capable of transmitting sound. It may be done perfectly, even by the solid parts of the head. If, for example, we hold the stem

of a watch between the teeth, and cover the ears with the hands, the beats are heard more distinctly than when the instrument is held at an equal distance in the air. The rubbing together of two stones under water may be heard, by an ear in the same medium, at the distance of half a mile. When the air, or any other body of indefinite extent, is disturbed, in a point situated within it, by a sonorous vibration, it forms a wave which passes from the disturbed point, as a centre, in every direction. It follows that as the wave extends itself, the mass to be put in motion increases until the original motion is rendered insensible from the magnitude of the mass to which it has communicated itself. The velocity with which waves, thus formed, move through any homogeneous elastic medium, is always equal to that which a heavy body would acquire by falling through half the height of the modulus of elasticity. In applying this law to the transmission of sound by the air, it was for a long time found not to give the same results as were obtained by experiment. The discrepancy, however, has been most ingeniously reconciled by a small correction for the latent heat made sensible by the compression; the effect of this being to increase the height of the modulus of elasticity. We ought, therefore, to find that liquids, and more especially some of the solids, should transmit sound much more rapidly than air; and this agrees most perfectly with various experiments. Cast-iron, for example, has been found to transmit sound with a velocity 10 times greater than air. Sound does not readily pass from one medium to another; a sound made in the air is not easily distinguished under water, although the distance be very small. It is from this, probably, that cork and all soft cellular bodies are bad conductors of sound, as in these the sound must, in passing through the walls of the cells and the air contained in them, change successively from one medium to another. All sounds, whatever be their loudness or pitch, are transmitted with the same velocity; a fact most completely proved by every musical performance. Were it otherwise, indeed, this beautiful art could not exist. To make this apparent, it is only necessary to consider, that harmony is a combination of different sounds arranged with certain relations of time and pitch. Now, if one sound were transmitted with *The height of the modulus of elasticity of air is 4

27,800 feet.

VOL. I.

greater velocity than another, these relations would differ at different distances, or be confounded, except at a single given point. Nay, further; melody, which is a succession of single sounds, would not reach different ears with the same relations of time, if the different notes were not transmitted with equal velocities. Some observations on sound, in very high latitudes, seem to contradict the above law of transmission. The seeming anomaly, however, is sufficiently reconciled by supposing the different strata of air, through which the sounds, in those instances, were transmitted, in very dif ferent hygrometrical or thermometrical states; which would make corresponding differences in their modulus of elasticity. When a wave of sound meets an elastic surface, it is partly transmitted and partly reflected. This reflection, when it returns back perpendicularly, is called an echo. That an echo may be distinctly heard, it is necessary that the reflecting surface be at such a distance that the original sound shall have ceased before the reflected one returns to the ear; otherwise they will be blended, and the echo not perceived.-Hitherto we have considered the propagation of sounds in an unconfined medium, particularly the air, in which the wave of sound can diffuse itself in every direction. When this diffision is prevented by enclosing the medium in a surface capable of reflecting the wave so that the sound shall be confined to one direction, the transmission from one point to another is much more perfect. Experiments have been made in this way, in which a hollow cylinder, about half a mile long, was formed by castiron pipes. The sound was transmitted by the air, in this cylinder, with wonderful distinctness. The least whisper, at one end of the cylinder, was distinctly heard at the other end. So perfect, indeed, was the transmission, “that, not to hear, it was absolutely necessary not to speak." Captain Parry and lieutenant Foster made several experiments, during the northern expeditions, to ascertain the velocity of sound. A table of them is given in a number of the Edinburgh Philosophical Journal. These experiments were made at Port Bowen, by means of a brass six-pounder, over a range of 12,892.89 feet. The results given are the mean of four shots in one case, of five in another, and, in the rest, of six shots by each observer. The mean results varied from 12",7617 to 11",7387 and 11",5311 for the time in which the range of 12,892.89

38

feet was traversed by the sound. At the period of the experiment which gave the first of these results, there was a calm; during the second, the wind was light; during the third, a strong wind was blowing. The velocity per second, in feet, was, in the first instance, 1010.28; in the second, 1098.32; in the third, 1118.10. Omitting the last of the ten results (the last above given), on account of the strong wind, the mean of the other nine gives a velocity of 1035.19 feet, at the temperature of 17.72, Fahrenheit.—The mean of a table of velocities formed from observations made at Fort Franklin, by lieutenant Kendall, who accompanied captain Franklin, in his second journey to the shore of the Polar sea, gives a velocity of 1069.28 feet per second, at the temperature of 9.14, Fahrenheit.-The science of acoustics, like the other physical sciences, has been in a constant state of advancement since the revival of learning. It appears that Pythagoras knew the relation between the length of strings and the musical sounds which they produce. Aristotle was not only aware of this relation, but, likewise, that the same relation subsists between the length of pipes and their notes, and that sound was transmitted by the atmosphere. This constituted the sum of ancient learning in this branch of science. These facts were taught by Galileo, and, moreover, that the difference in the acuteness of sounds depends on the different frequency of vibrations, and that the same string, if of uniform thickness and density, must perform its vibrations in equal times. But, without attempting a history of modern discoveries in acoustics, we can only mention, that the names of Taylor, Moreland, Newton, Daniel Bernouilli, D'Alembert, Euler, Robison, Lagrange, Laplace, Chladni, T. Young and Biot are all connected with it. Of these, Newton gave the law of transmission, which we have stated in this article, and the correction for heat was made by Laplace.

ACRE; a measure of land, containing four square roods, or 160 square poles or perches. The statute length of a pole or perch is 5 yards, or 163 feet; but the length of a pole, and, therefore, the size of the acre, varies in different counties in England. The Scottish acre contains also four square roods; one square rood is 40 square falls. The English statute acre is about three roods and six falls, standard measure of Scotland; or the English acre is to the Scottish as 78,694 to 100,000. The French acre, arpent, is

equal to 54,450 square English feet, of which the English contains only 43,560. The Welsh acre contains commonly two English ones. The Irish A. exceeds the English by two roods, 19 perches, The U. S. of A. use the English statute A.

ACRE (Akka, St. Jean d'Acre); in the middle ages, Ptolemais, a city and harbor on the coast of Syria, capital of a Turkish pachalic, between the pachalics of Damascus and Tripoli, which contains 420,000 inhabitants, and 6275 sq. miles. This city, situated at the foot of mount Carmel, is the chief emporium of Syrian cotton, and contains about 16,000 inhabitants; its harbor, though full of sandbanks, is still one of the best on this coast. At the time of the crusades, A. was the principal landing place of the crusaders, and the seat of the order of the knights of St. John as late as 1291; hence the French name, St. Jean d'Acre. The Turks, under Djezzar, pacha of this place, who is famous for his cruelty, sustained, with the assistance of the British commodore Sidney Smith, a siege of 61 days, by the French army under Buonaparte. After a great loss of men on all sides, the French abandoned the siege. (See Egypt, landing of the French in.)

ACRIDOPHAGI (Gr., from azgiç, a locust, and payw, to eat); an ancient Ethiopian people, who are said to have fed on locusts.

ACRISIUS; the father of Danaë. (See Danaë.)

ACROCERAUNIUM; in anc. geogr. a promontory of Epirus, on which are situated the Acroceraunia or montes Ceraunii. They run between the Ionian sea and the Adriatic, where Illyria ends and Epirus begins, and are the modern Monti della Chimera.

ACROCORINTHUS; a steep rock, about 2100 feet high, near the city of Corinth, of a gray color, and picturesque form, crowned with the remains of old Venetian fortifications, repaired a little by the Greeks, since the commencement of their revolution. It was famous, in ancient times, for its citadel, and on its top stood, according to Pausanias, a temple of Venus. At its foot is a fountain, the ancient Pyrene. The shape of the A. is that of a truncated cone. This little fortress has been several times taken and retaken in the war between the Greeks and Turks. The view from the top is one of the most charming in the world. It is thus described in the "Journal of Dr. Lieber," before whom no Christian traveller, in modern times, had probably visited it, as the