Theophrastus became his successor. The number of his pupils from all parts of Greece was at one time 2,000. His popularity and influence on the public affairs of Greece excited a party spirit against him, and being brought before the Areopagus on a charge of impiety, he pleaded his own cause, and was acquitted. After this he taught in tranquillity until 305, when Sophocles, son of Amphiclides, carried a law which prohibited all philosophers, under pain of death, from giving any public instruction without the permission of the state. Theophrastus left Athens; but in the next year the law was abolished, and he returned, and continued his labors without interruption until his death. He developed no new system of philosophy, but expounded that of his master Aristotle. He wrote works on politics, laws, legislators, and oratory, which are lost, and "A Dissertation on the Senses and the Imagination," a work on "Metaphysics," "Characters," and two works on botany, "The History of Plants" and "The Causes of Plants," which are extant in whole or in part. The two last are among the earliest works on botany written with any scientific precision, and contain much valuable matter. The book of "Characters" consists of 30 sketches of the general vices of humanity as developed in individuals. All the remains of Theophrastus are contained in the edition of Aristotle printed by Aldus (5 vols. fol., Venice, 1495-18); the best edition of his works is that of J. G. Schneider (5 vols., Leipsic, 1818-21). His "Characters" were translated into French and prefixed to his own by La Bruyère, and into English, among others, by Francis Howell, with notes and illustrations and the original text (8vo., London, 1824).

THERA (now Santorin), a Grecian island of the Egean sea, now belonging to the government of the Cyclades, in the kingdom of Greece, in lat. 36° 20′ 45′′ N., long. 25° 32′ 58′′ E.; length about 9 m. from N. to S., breadth about 3 m.; pop. in 1852, 21,827. It is crescentshaped, with the concavity on the W. side forming a bay or roadstead partially protected by the small islands of Therasia and Aspronisi. As no bottom is found, vessels make fast to the abrupt and rocky shores in this roadstead. The soil of the island is volcanic and inclined to dryness, but very fertile. The annual production of wine is over 1,000,000 gallons. Ship building is the only considerable manufacture carried on. Though an ancient Lacedæmonian colony, Thera is only of historic importance as having sent a colony to found the city of Cyrene in Africa, 631 B. C. It possesses much interest to the physical geographer, however, from the volcanic changes which have occurred in it within the historic period. The concave W. side of the island has been proved to be a part of the inner wall of an immense volcanic crater, of which the two islands of Therasia and Aspronisi form a continuation. Soundings made by command of the English admiralty show that this crater is from 1,200 to 2,449

feet in depth, and that it forms a complete bowl except at the northern point between Therasia and Thera, where there is a perpendicular slit a mile in width and 1,170 feet deep midway between the two islands. In the centre of this bowl 4 different islands have risen during volcanic eruptions within the historic period, 3 of which still exist, and are called the Cammenis. The first, then called Hiera (Holy), now Palma (Old) Cammeni, burst out of the sea with terrible flame and noise in 197 B. C. A second island, which has since disappeared, rose near the first in 67 B. C. A third appeared in A. D. 46, and is supposed to be that now known as Mikia Cammeni (Little Cammeni). Numerous other eruptions and changes occurred down to 1707, when another island, the Nea Cammeni (New Cammeni), was formed, at first composed of white pumice, but subsequently receiving additions of brown trachytic rock, to which the name of Black island was given. The eruption did not wholly cease or the island assume its present form till 1712, since which there has been no volcanic action.

THERAMENES, a native of Cos, who was a political leader at Athens toward the end of the 5th century B. C. In 411 he became a member of the council of 400; but seeing that the downfall of this government was near at hand, he deserted it and became one of the leading agents in its overthrow. In 410 he commanded a portion of the Athenian fleet, which was engaged in cruising about and exacting money from the islands, and finally joined the fleet under Thrasybulus, and took part in the battle of Cyzicus, in which he commanded one of the 3 divisions of the Athenian force. He also served with Alcibiades, and in 408 participated in the siege of Chalcedon and the capture of Byzantium. He was one of the inferior generals at the battle of Arginusæ in 406. In the trial of the generals for not saving the crews of the ships after the result of the battle was known, Theramenes came forward as the principal accuser of his colleagues, and it was chiefly owing to his influence that they were convicted and sentenced to death. After the battle of Ægospotami, and during the siege of Athens by the Spartan general Lysander, when the city was reduced to great extremity, Theramenes offered himself to the people as a suitable envoy to the Lacedæmonians, declaring that he could detect the real intention of the ephors in regard to Athens, and that also he had influence to obtain for them more favorable terms than any other. He was accordingly sent to inquire and report, but remained 3 months with Lysander, who he pretended detained him that length of time without informing him that the ephors only had power to grant peace; and upon his return to the city, which was now suffering under a terrible famine, he was sent to make peace on any terms. The hard conditions imposed by the Lacedæmonians were assented to (see GREECE, vol. viii. p. 448), and in 404 Theramenes, who had during his 3 months' stay

with Lysander made arrangements with the Athenian oligarchical exiles, became one of the thirty tyrants. He warmly supported the first measures of the government in crushing the democracy and putting to death its prominent leaders; but he afterward opposed the violent measures of Critias and his colleagues, who had private hatreds to gratify. His party daily increased; but Critias, after charging him with being a public enemy, caused him to be dragged off to prison by partisans with concealed daggers whom he had brought into the senate house, and compelled him to instantly drink the hemlock. Theramenes was an able though faithless and cunning man; but the heroic manner in which he met his fate, and the fact of his dying in defence of the liberties which he had previously conspired to betray, rendered him a special object of admiration to the ancients.

THERESA, MARIA. See MARIA THERESA. THERESA OF JESUS, SAINT, a nun and mystic writer of Spain, born at Avila in Old Castile in 1515, died Oct. 4, 1582. At the age of 20 she entered the order of Carmelites in a convent of her native town, in which she remained 27 years. She then became the foundress of a reformed branch of the Carmelites (Barefooted Carmelites), sometimes called after her Theresians. During her life 29 convents of the reformed order were established, and in the 18th century it counted about 2,000 members in 6 provinces, in Spain and Spanish America. Theresa described the internal struggles and aspirations of her heart and her frequent mystic visions in ascetic treatises and letters, which, on account of their theological importance, belong among the most memorable documents of the mystic literature of the Roman Catholic church, while their excellence of language and style has secured for them a place in the history of Spanish literature. She wrote her works reluctantly, and only at the command of her confessor. The following five of them are extant: Discurso o relacion de su vida, written in 1562; El camino de la perfeccion, prepared in 1563, as a guide for the nuns of her reformed order; El libro de las fundaciones, an account of the convents founded by her; El castillo interior, o las moradas, written in 1577, and the most celebrated of her mystic works, in which she portrays in glowing colors the gradual progress of the soul to the 7th heaven, the celestial castle of Christ, her spouse; Santos conceptos de amor de Dios, the original of which she burned in compliance with an order of her confessor, but which has been preserved from a copy taken by one of the nuns. The original manuscripts of the first four works are preserved in the library of the Escurial by order of King Philip II. The first complete edition of these works appeared at Salamanca in 1587, and a recent one, edited by Ochoa, at Paris in 1847 (Tesoro de las obras misticas de Santa Teresa de Jesus). A collection of letters of St. Theresa, addressed to different

persons, was first published at Saragossa in 1658, and often since. All her works have been translated into nearly every language of Europe, and still frequently appear in new editions.

THERMAIC GULF. See SALONICA. THERMO-ELECTRICITY (Gr. Sepμn, heat, and eλEKTрov, amber). For the principles relating to electric currents, so called, and the methods of indicating and measuring them, see ELECTRO-DYNAMICS, and ELECTRO-MAGNETISM. Professor Seebeck, of Berlin, was the first to discover, in 1822, that if two metallic bars having different crystalline texture, or unequal conducting powers through any cause, are placed in contact or soldered together, end to end, and heated or cooled at the junction to a temperature different from that of the other parts, and if from the other ends of the bars, at the same time, conducting wires be arranged to complete a circuit, the natural electricity of the metals is disturbed, and an electrical current is set up, which is maintained so long as the parts are kept at an unlike temperature. When the bars are of bismuth and antimony, and the junction is heated, the positive current is at this from the antimony to the bismuth, and along the conducting wires from the latter to the former; upon cooling instead, the current is reversed; and when the temperature of the bars becomes equal throughout, the current ceases. An extremely feeble current, indeed, is obtained by unequally heating a single bar, as of bismuth. The currents obtained under either of these conditions are distinguished as thermo-electric; and experiments have determined a thermo-electric series of metals, of which the following is a part: bismuth, platinum, lead, tin, copper or silver, zinc, iron, antimony. These are here placed in such order that any one before gives with any one later in the series, by heat, a current through the conducting wires from the former to the latter. With a given difference of temperature, the current is more intense as the metals are further apart in the series, being the most intense yet known with bismuth and antimony; the current is constant so long as the difference of temperatures is so, as when the junction is kept at 212°, the further ends at 32°, or the reverse; and with given metals, it is more intense as the difference of the temperatures is greater. With any single pair of bars, it is still comparatively feeble; but when the metals of several pairs are alternately connected, and the temperatures of the alternate junctions kept equidistant, as is readily done by bending each bar at right angles at the ends, soldering, and arranging so as to bring any number of pairs into a square prismatic pile, having the length of a single bar, with non-conducting strips between the bars, and the terminal ones connected by conducting wires, the intensity of the current is then, as in the galvanic battery, precisely multiplied by the number of the pairs. Such an arrangement, devised by MM. Nobili and Melloni, is a thermo-electric

pile; the ends of the bars in the opposite directions form the two "faces" of the pile; laterally this is enveloped in a sheath, protecting all but the faces; and in this sheath, communicating respectively with the free antimony and bismuth bars at the extremities of the pile, are two cups, termed the poles of the pile. The number of pairs may be 30 or more; and as the size of the bars does not influence the strength of the current, these may be very. small, and if desirable no more than an inch in length. Now, when one of the faces is kept at a given temperature, and the other exposed to a source of heat or to cold, even though this be very feeble, yet the multiplying effect of the number of pairs produces a current strong enough to give a very sensible deflection to the needle of a galvanometer with which the poles are connected; and the instrument forms by far the most delicate means known for the detection of feeble degrees of heat, and thus determining the behavior of various substances in reference to radiation, transmission, &c., of this agent. In the complete apparatus, known as the "thermo-multiplier," there are screens for protecting at will either face of the pile; a lamp and reflector for emitting radiant heat; a stand to support the substances to be experimented upon; and a hollow metallic cone, polished within, for concentrating the rays of heat, when required, upon one face of the pile. In Melloni's pile of 30 pairs, the indications of the needle are strictly proportional to the temperature only to 20° of deflection; and beyond this corrections must be introduced. Deflection of 35° corresponds to 'unequal heating of the faces through 6°-8° temperature. For an example of the sensitiveness of the instrument, as well as for the most important results thus far attained by its use, see DIATHERMANCY.

THERMOMETER (Gr. Sepun, heat, and ueTрov, measure), an instrument designed, by means of the visible or mechanical effect of heat upon some part or substance entering into its construction, to show, and sometimes also to register, the temperatures or sensible heats of the bodies or spaces to the influence of which it is exposed. For the general principles upon which the invention and use of the thermometer are based, see EXPANSION, and HEAT (I.); and for the difference between the actual and the apparent or sensible heat of bodies, with the modes of measuring the former, see CALORIMETER, and HEAT (III.).—The first attempt at indicating to the eye differences of temperature, seems to have been by the contrivance variously ascribed to Drebbel of Holland and Sanctorius of Italy, about the beginning of the 17th century, and known as a weather glass. This was very rude and inaccurate, consisting of a glass bulb and tube inverted, opening below into a cup of colored liquid, which, the air of the bulb having been partly expelled by heat, rose in the tube, and stood at different heights according as the air remaining in the bulb was more or less expanded by heat. This,

the origin of the common air thermometer, as improved by Boyle and by the Florentine academicians, became transformed to a smaller bulb with upright stem of somewhat fine bore, the contained liquid being colored spirits of wine; boiling this to expel air, the tube was hermetically sealed, and the whole then affixed to a case. A scale of degrees was also introduced, its fixed points being the cold of snow or ice and the greatest heat known at Florence; it was of necessity very variable in its indications. At this stage in the progress of thermometry, much discussion in regard to the most suitable fixed points for the scale, the best substance for use in the instrument, &c., including that of the question whether water did not freeze at different temperatures in different latitudes, was carried on in England and on the continent. Hooke advocated as the lower fixed point the temperature of freezing water; but Newton seems first to have discovered or taken advantage of the facts, that a thermometer placed in melting snow or ice always indicates the same temperature, and always very nearly one temperature in boiling water. But of oil, which he suggested for the liquid in the bulb, the movements were found to be too sluggish and uncertain. Römer, overcoming a prejudice that seems to have existed in regard to unequal expansion of mercury, first adopted that liquid; and he doubtless devised the instrument and scale usually attributed to Fahrenheit of Amsterdam (1720); the latter constructing and introducing the instrument, so that it became generally known throughout Europe in the first half of the 18th century. Of this thermometer, the lower fixed point, or zero, was taken at 32° below freezing point of water; but whether as the cold obtained by its maker by mixing salt and snow, or as the greatest cold observed in Iceland, and in either case as the supposed point of absolute cold, is not now definitely known; and since Fahrenheit kept his graduation of thermometers a secret, the same must be said respecting the choice of a scale of 180° between the fixed points, though this is supposed to have originated in some theoretical views as to the dilatation of mercury. Celsius of Sweden (1742) introduced a scale of 100° between the fixed points; this was adopted in France at the time of the revolution, and named the thermomètre centigrade; and owing to its convenient decimal division, it has been wholly adopted in several countries of Europe, while it is coming into general use among scientific men throughout the world. An advantage of Fahrenheit's scale, in many instances, is still that the less range of the degree saves the necessity of so often resorting to fractions in the expression of temperatures; its real fault and opprobrium is its needlessly fixing the 0 elsewhere than at the freezing point; and perhaps, could its zero be restored to the true place, and the range from this to boiling point be divided into 160°, the utmost convenience and perfection of

a thermometric scale would be attained. The scale of Réaumur is 80° between the fixed points; and that of De Lisle, now little used, is 150°, taking the zero at boiling point and reckoning downward. Obviously, the expansive effect of heat upon any convenient solid, liquid, or gas, can be made the measure and indicator of temperatures; and for the purpose each of these three forms of matter is in use. For ordinary use, however, the simple expansion of a solid is too small, and that of a gas too great, and difficult to measure. Liquids have an intermediate range of expansion, and are hence more conveniently managed and observed; and of the different liquids which offer some features of convenience, mercury is readily determined to be the most eligible for general employment, by the nearly equal rate of expansion throughout the range between its solidifying and its subliming in vapor; by the great extent of that range, more than 700° F.; by the rapidity with which it acquires the temperature of the space it is in; by its slight tendency to adhere to the glass tube; and by the readiness with which it is freed of air. In making a mercurial thermometer, a glass tube with bore of capillary size, and this as nearly as may be of uniform capacity throughout, is selected; the approximate uniformity being readily tested by introducing a short column of mercury, impelling it by air through the tube, and observing whether it preserves nearly equal lengths throughout. Cutting the tube to the required length, a bulb is blown upon one end in the usual method of glass-blowing, but, to avoid moisture, with air from a rubber bag. The greater the capacity of the bulb, as compared with that of the tube, the greater the length of tube that will be filled by a given degree of expansion of the mercury, and so the greater the delicacy of the indications, i. e., the power of showing slight changes of temperature. The bulb is more commonly made spherical, and this form best resists any effect of varying pressures of the atmosphere; but for sensibility in point of time, or quickness of response to the acting temperature, that affording, with a given quantity of mercury, the largest surface is best; and this quality, others being disregarded, is secured by the cylindrical bulb, which is usually straight, sometimes coiled. A small cup is now cemented to the open end of the tube, or a paper funnel introduced into it; into either some mercury is put; the bulb is heated with a lamp to expel air, and then left to cool, when the atmosphere forces in mercury, partly filling it. The lamp is applied again, and the mercury boiled some minutes to expel air and moisture; upon subsequent cooling, mercury from the cup or funnel alone enters, completely filling the bulb and tube. Emptying the cup, this is removed by heating with a blowpipe, at the same time drawing the upper end of the tube to a narrow neck; and then, by reheating the instrument to the highest temperature it is expected to show, the excess of mercury over just enough to fill the bulb

and tube at such temperature is expelled; removing the lamp, as soon as the column begins to contract, the flame is directed upon the end of the tube, hermetically sealing it. In contracting upon cooling, the mercury withdraws mainly into the bulb, leaving much of the tube vacuous, save that it may contain an extremely rare mercurial vapor; and the instrument is now complete, except the graduation and mounting.-It will be evident that we know nothing of absolute temperature, except theoretically; that the range of actual and possible temperatures reaches beyond our powers of detection, both upward and downward, and to an indefinite extent, so as to be comparable to an endless chain of which we can discover only so many links at some part of its course; that any temperature we may choose for our fixed point must be simply arbitrary; and yet, that by selecting two such fixed points, dividing the intervening range into a fixed number of degrees, and extending this scale by equal intervals of sensible temperature both above and below the fixed points, we introduce a sufficient and accurate means of measuring and comparing the temperatures we can observe. The only peculiar fitness of the fixed points that have been selected, is in their being the most nearly invariable, and the most convenient in practice. In Fahrenheit's scale, the freezing point of water is called 32°, the boiling point 212°, the intervening range thus being divided into 180°; in the centigrade, the freezing point is 0°, the boiling point 100°; all temperatures above the 0, in either of these scales, being considered, and when needful marked, +, and all below 0, ——. Though the rate of expansion of pure mercury increases slightly with the rise of temperature (see EXPANSION), yet both the amount and the variation are known; and by this amount, 1 part in 55.08 between the fixed points, the capacity of the tube between the heights of column in it showing the freezing and boiling points, will be precisely 3 of the whole capacity of tube and bulb below freezing point. If, now, thermometer tubes of perfectly uniform bore could be had, and the expansion both of the mercury and the glass tube containing it could also be supposed uniform, then the rise of the column in such a thermometer would be through its whole range precisely proportional to the expansion, and precisely equal for the same number of degrees in any part of the scale. All these suppositions, however, fail in the fact. To find the freezing point, the bulb and the tube, to very nearly the height to which such cold will lower the column, are kept immersed in melting pounded ice or snow, long enough to bring the mercury and glass throughout to the freezing temperature; the contraction and relative volumes of both are thus rightly secured, and the point below which the mercury column ceases to fall is marked on the tube with a file. Water boils at temperatures varying not only with the atmospheric pressure, but also with the purity of the water itself,