tries inhabited by Greeks. In his Areopagiticus he urges Athens to adopt, as her only safeguard, the ancient democracy of Solon. In his Panegyricus he is equally warm in his exhortations to all the Greeks to unite against the barbarians. In his "Philip," an oration addressed to the king of Macedon, he entreats the king to unite with the Greeks, and lead them against the Persians. But Isocrates was not a practical statesman, and he was unconsciously urging Philip to become the ruler of the Grecian states, an object which Philip was then himself secretly planning. His Panathenaicus, a eulogy on Athens, was written when he was 94 years of age. After the victory of the Macedonians over his countrymen at Chæronea, he was unwilling to survive the destruction of their liberties, and destroyed himself. The writings of Isocrates were all carefully studied and elaborated; he is said indeed to have taken over 10 years to write his Panegyricus. They are remarkable for their flowing elegance and melody, the precisely turned sentences and periods making the style almost monotonous. Of 28 genuine orations of his, 21 have come down to us, 8 of which were written for judicial cases, and were intended to serve as models for forensic writing. Beside these, there are titles and fragments of 27 others, and also 10 letters, some of which are undoubtedly not genuine. His works have been translated into English by Sadlier, by Dinsdale, and by Gillies, and also into French, but unsuccessfully. The best text is Bekker's.

ISOMERISM (Gr. toos, equal, and μepos, part), a term used in chemistry to express the relation existing between those substances which, while they possess the same ultimate composition, exhibit essentially different chemical and physical properties. The term isomeric is often applied indiscriminately to several classes of bodies which will here be separately considered. Strictly speaking, it ought perhaps now to be confined to those cases in which the elements composing the dissimilar substances are both quantitatively and qualitatively the same. Difference of properties can here be readily explained by admitting that the elements of the different bodies are arranged or grouped in different ways. For example: two chemically distinct compounds of mercury, oxygen, and sulphur are known, each of which contains in 100 parts equal quantities of these several ingredients, viz. of mercury 80.65, of sulphur 6.45, and of oxygen 12.90 parts. Both have therefore the empirical formula Hg, SO4. From the manner in which they comport themselves toward other substances, however, it has been ascertained that one must be regarded as basic sulphite of the protoxide of mercury, the rational formula of which is (HgO)2, SO2; the other a sulphate of the suboxide of mercury, of which the rational formula is Hg2 O, SO.. Similar instances abound, especially among organic bodies. Thus both formiate of ethyle and acetate of methyle-very different substances-have the

ultimate composition expressed by the formula C. H. O. They are however proximately composed of

Formic acid.......C, H O, | Acetic acid.....
Oxide of ethyle....C, H, O
....C, H, O,
Oxide of methyle..C, H, O

C. He 04 That such compounds really contain different proximate constituents is proved by the fact that they afford different products when decomposed under similar conditions. Hence they are regarded as distinct chemical substances, and not as modifications of one and the same body. The different grouping of the elements of these compounds has been compared to that of letters in words like ate, eat, tea, &c., on the arrangement of which the meaning of the word is entirely dependent. All bodies which are thus isomeric with each other, all that have absolutely the same ultimate composition, must of course possess identical equivalent weights. Such substances are often called metameric (Gr. μera, indicating change or alteration), in contradistinction to polymeric substances (Gr. rolus, many), which are composed of similar elements united in the same relative proportion in each case, but in different absolute quantities; the equivalent weights in which these substances combine with other bodies being unlike. This distinguishes them from members of the preceding class, in which both the relative and absolute number of equivalents are the same. Very many polymeric substances are known, whole series of organic compounds being formed of them. As an example, olefiant gas and cetene both contain 85.7 per cent. of carbon and 14.3 per cent. of hydrogen. If nothing were known of their chemical comportment, the empirical formula CH would be applicable to both; but by studying their properties it has been found that 40 and 4H have united to form olefiant gas, the rational formula of which is therefore C. H. and its combining equivalent 28; while 320 and 321I have united to form cetene, which has consequently the rational formula C3, H.2, and the equivalent number 224. Between these two bodies there are 14 others polymeric with them and with each other. Differences like this have been compared to those between words like ma, mamma, tar, tartar, &c., which contain the same letters arranged in the same way, but in different quantities. The arrangement of the elements in polymeric substances is not however of necessity the same; thus, the ether of wood spirit is polymeric with common alcohol; yet the rational formula of the former is C, H, O; of the latter, C, H, O, HO.—Until a comparatively recent period it was the prevalent opinion among chemists that bodies of similar composition must of necessity possess similar properties. Any observations tending to throw doubt upon the correctness of this belief were considered erroneous. Even the discovery, by Wöhler and Liebig, that cyanic and fulminic acids are of like percentage composition although they possess very different properties, was attributed to

errors of observation, and generally discredited. Faraday's investigation of several isomeric hydrocarbons in 1825 first proved the fallacy of this supposed law. Its exceptions, being now more carefully observed, were found to be very numerous. In 1830 Berzelius proposed that they should be classified as isomeric substances. It was however soon perceived that the doctrine of isomerism could not with propriety be employed to explain the cause of all the differences which had been observed; least of all, to explain those which occur among the elements themselves-bodies which, from his inability to decompose them, the chemist is forced to regard as simple. In 1840 Berzelius suggested that these peculiarities might depend upon some absolute difference of quality in the different varieties of a substance, and not upon any dissimilarity in the arrangement or number of its molecules. He proposed the term allotropism (Gr. aλλoтρoños, of a different nature) to express this idea, which has ever since steadily gained favor, although directly opposed to the doctrine of the immutability of matter, one of the principal tenets on which the chemistry of the first half of the present century was based. Allotropism is of special interest from the fact that several of the most common and best known elements may occur in two or more allotropic states. Thus, pure charcoal (lampblack), graphite, and the diamond are essentially identical chemical substances. They differ widely from each other, it is true, not only in their physical properties, but also in their chemical behavior toward other bodies. They can however be mutually converted into each other, excepting our inability to make artificial diamonds; the equivalent weight in which they unite with other substances is always the same;* and as a general rule, to which however many exceptions may yet be found, the bodies formed by such combination exhibit identical properties. It is admitted therefore that they are mere modifications of one and the same chemical substance, carbon. Again, the element phosphorus, as it commonly occurs, is a soft, waxy, yellowish white, exceedingly inflammable, and very poisonous substance, with a strong odor and taste, luminous in the dark, and readily soluble in bisulphide of carbon. It may easily be transformed, however, into another allotropic state, in which it is of a dark red, nearly black, color; is hard and brittle, devoid of taste or smell, and, so far as is known, of poisonous properties; is not luminous, and is completely insoluble in bisulphide of carbon. It differs moreover from ordinary phosphorus in specific gravity, and entirely in its affinity for other substances. Indeed, it is not known that it is itself combustible; for it may be heated without undergoing change to about 500° F., at which temperature it is reconverted into ordinary phosphorus. These two conditions of phosphorus are so ut

Recent experiments of Brodie ("Quarterly Journal of

the Chemical Society of London," Oct. 1859, page 261) cast serious doubt upon this commonly received notion.

terly unlike in all their properties, excepting the weight of their equivalent, that were it not in the power of chemists to prove their identity by converting them one into the other, they would without hesitation be considered distinct elements. Similar instances occur among gases. For example, ordinary oxygen gas may be converted into an allotropic modification called ozone, which possesses properties entirely dif ferent from those of the original oxygen. Chlorine gas also, according to Prof. J. W. Draper of New York, after exposure to strong sunlight, possesses the power of combining with hydrogen even in the dark, and exhibits other prop erties unlike those of chlorine which has been kept from the light. Several other elements are known to be capable of existing in two or more allotropic states; and a considerable number of compound bodies occur under different modifications, which, it is not unlikely, may yet be found to depend upon the allotropism of one or more of their elements. Indeed, these instances are so common that some chemists have been led to believe that most if not all of the ele ments may exist in distinct allotropic states. It has not as yet, however, been well ascertained to how great an extent the peculiar state of an ele ment can influence the properties of the com pounds it may form by uniting with other bodies. Schönbein, the discoverer of ozone, is confident that it exists, as such, chemically combined in several oxides. Other chemists have referred the dissimilar varieties of certain compounds of phosphorus, arsenic, &c., to the allotropism of their elements. Berzelius long ago pointed out that the different states of sulphide of mercury, iodide of mercury, &c., were probably to be attributed to a similar cause. Recently, Berthe lot has advanced the opinion that the allotropic modifications of sulphur are intimately connect ed with, if not directly dependent upon, the electrical relation which this substance bears to the elements with which it is or has been united. When separated, by agents which are without action upon it, from those compounds in which it acts as an electro-positive body, as in sulphurous acid, it is amorphous and insoluble in bisulphide of carbon and other neutral solvents. On the contrary, when obtained from compounds in which it plays the part of an electro-negative element, as in sulphuretted by drogen, it is susceptible of crystallization, and is soluble in bisulphide of carbon, &c. Berthe lot also states that the modifications of selenium exhibit a similar comportment, and has suggest ed that the different states of phosphorus may in like manner represent respectively electronegative (ordinary phosphorus) and electropositive (red phosphorus) conditions. It is worthy of remark that these views, which are of prime importance in their bearing upon the theory of substitutions, are almost identi cally the same with those concerning chlorine published some years since by Prof. Draper. Although the correctness of the observations of both these chemists has been called in question

phosphorus crystalline, the other condition amorphous, and refer all differences of properties to this difference of form. Diversity of crystalline structure, or its entire absence, is however evidently only one of the many differences of properties incidental to allotropism; in many cases it must be regarded as a consequence of the latter, by no means as its cause. At all events, the cases of allotropism which occur among gases cannot be explained by this theory. Others, without paying special attention to crystalline form, have supposed that all cases of isomerism, taken in its widest meaning, depend upon variations in the grouping of the molecules of bodies. They even refer the instances which have here been classed under allotropism to differences in the arrangement of the particles of matter of which the elements themselves are composed. But few, however, now hold this opinion, the doctrine of allotropism being generally admitted. Although the mere term allotropism conveys no definite idea of the different conditions of matter which it indicates, and is, strictly speaking, nothing more than a convenient name for a class of phenomena as yet inexplicable, the fact which it denotes -that an element can exhibit the properties of two different substances-is of preeminent importance. At the present moment it is generally regarded as the greatest chemical discovery which has been made for years. Indeed, several important theories in this science have been materially changed by its recognition. ISOMETRIČ PROJECTION (Gr. toos, equal, and μerpov, measure), a species of drawing, used chiefly by engineers, in which the perspective plane of the paper must be imagined as making equal angles with the three principal dimensions of the figure, and the eye at an infinite distance. Thus lines in the three principal directions will be drawn on the same scale, and that scale the same for all parts of each line.

ISOMORPHISM (Gr. toos, equal, and μopon, form), in chemistry, the property possessed by certain bodies, either elements, bases, or acids, of replacing each other in compounds without causing in these an essential change of crystalline form. The bodies that thus replace each other possess themselves similar forms, and are said to be isomorphous. Familiar examples of this mutual replacement in minerals are of the protoxides of iron and manganese, and of lime and magnesia. Chlorine, bromine, and iodine possess this relation toward each other; also arsenic and phosphorus, and the acids of these elements.

ISOTHERMALS, or ISOTHERMAL LINES (Gr. toos, equal, and epun, heat), lines drawn upon the map of the world, connecting points of the same mean temperature-a method of representing to the eye the belts that possess the same amount of heat or cold, either at certain seasons or throughout the year. Humboldt first explained these lines in a paper published in the Mémoires de la société d'Arcueil, a translation of which appeared in the 3d volume of the

"Edinburgh Philosophical Journal" (1820), and in 1848 he published with Dove a map of the world upon which these lines were constructed, Lines along the points of mean annual temperature he called isothermals; those along the points of mean summer temperature, isotherals (Gr. depos, summer); and those along the points of mean winter temperature, isochimenals (Gr. xepov, winter). The singular results exhibited attracted the attention of other philosophers, and the subject has been further elucidated by the researches of Kämtz and Berghaus in Europe, and of Loomis in the United States. Maps containing these lines for the whole year, and for the months of January and July, may conveniently be consulted in Nichols's "Cyclopædia of the Physical Sciences." They are from Dove's more complete maps made for every month. By these lines the surface of the earth is divided into zones, and those of corresponding temperatures on both sides of the equator are known as isothermal zones. The lines exhibit in a most striking manner how little dependent localities are upon their relative distances from the equator for the temperature of their climates. This is especially the case with the winter season in the northern hemisphere, and was perceived in the early settlement of the western coast of the Atlantic, where the cold was found to be equal to that of regions lying from 15° to 20° further N. on the eastern margin of the same ocean. The same thing is still more strikingly exemplified in the mean winter temperature of Edinburgh, which is 384°, and of Kasan in the same latitude in the east of Russia, which is only 2°. But when the mean annual temperature of these two places is compared, the difference is reduced to only 102, that of Edinburgh being only 48°, and that of Kasan about 38°. The lines of different seasons are thus seen to be more parallel with each other than any of them are with the lines of latitude. For all seasons in the northern hemisphere they are greatly contorted, while over the more oceanic southern hemisphere they pass more regularly in an E. and W. direction. The irregular distribution of land and water is the chief cause of these irregularities of temperature. Large bodies of the former in the polar regions, or lakes and seas that are covered dur ing the winter with ice, send a chilly influence over adjoining countries, which spreads far into low latitudes; while oceanic currents flowing from equatorial regions carry with them into arctic latitudes a mild temperature, and render countries inhabitable, and even comfortable, which, situated so near the poles, would under other circumstances be buried in perpetual ice and snow. So in tropical climates high mountainous tracts, reaching to elevations above the snow lines, chill the surrounding atmosphere, and temper the climate of neighboring countries; while low sandy deserts in the same latitudes reflect the full heat of the sun, and give the maximum of atmospheric temperature. Thus it is that in the deserts of Mesopotamia, as

observed by Griffiths, during the land winds the thermometer has been seen to rise to 132° in the shade and to 156° in the sun; and thus it is that the isothermal lines of July circle around the interior portions of Africa, marking a mean temperature for that month of 90°.

ISPAHAN (anc. Aspadana), a city of Persia, of which it was formerly the capital, situated in the province of Irak-Ajemee, 210 m. S. from Teheran, in lat. 32° 39′ 34′′ N., long. 51° 44' 45" E.; pop. variously estimated by European travellers at from 60,000 to 200,000, the Întter number being probably nearest the truth. It stands in the midst of a broad plain watered by the river Zendarood, which is here 600 feet wide. For miles around the city stretch groves, orchards, corn fields, vineyards, and shady avenues, interspersed with the ruins of deserted towns and palaces. On approaching the city from the south, travellers cross the river by three beautiful and massive bridges, which lead into spacious gardens watered by canals, and surrounded by numerous pleasure houses. A broad shaded avenue terminates in the great bazaar of Shah Abbas, an enormous length of building vaulted above to exclude heat but admit air and light. Hundreds of unoccupied shops line the sides of this once crowded mart of commerce, after traversing which for nearly two miles the traveller enters the great square of Ispahan, the magnificent Maidan Shah, an oblong open space 2,600 feet in length and 700 in breadth, with an area of upward of 40 acres. In the centre of two sides of this square are superb mosques, and in the centre of the other sides are great gates leading to the bazaars and to the royal mosque. Around the rest of the square are stately edifices of uniform architecture, once used as apartments for the nobility and officers of the Persian court, but now ruinous and desolate. In the S. part of the city is an extensive pleasure ground, called the Chahar Bagh, consisting of 8 gardens, or "paradises," as they were termed by the ancient Persians, watered by canals, basins, and fountains, adorned with palaces, and enclosed by lofty walls. The most sumptuous of these palaces is the Chehel Sitton, or "Forty Columns." The columns from which the name is derived are in the principal hall, and are inlaid with mirrors so as to resemble pillars of glass. The walls and roof are decorated with the same fragile material, interspersed with flowers of gold, the whole done with much taste, and so as to convey an impression of great magnificence. Behind this hall are many fine apartments, one of which is embellished with large paintings by native artists, representing the achievements of Nadir Shah and other Persian conquerors. Ispahan is distinguished even at the present day for the excellence of its manufactures, which consist of all kinds of woven fabrics, from the most costly gold brocade to the most ordinary calico or coarse cotton; of gold and silver trinkets, paper, pen cases, ornamental book covers, firearms, swords, glass, and earthenware. These

goods are sent to nearly all parts of Asia, Ispahan being a central emporium on the great line of traffic between Afghanistan, India, and China on the east, and Turkey, Egypt, and the Mediterranean on the west. The wine of Ispahan is thought not much inferior to that of Shiraz. The inhabitants are generally educated, so that almost every one can read and write, and even the shopkeepers and artisans are familiar with the works of the principal Persian poets. The merchants, who form a distinct class, are shrewd and enterprising, live in luxurious style in houses externally shabby, and many of them carry on business with large capitals, and on a scale that enables them to affect prices even in the markets of India.-On the S. side of the Zendarood is the Armenian suburb of Julfa, which is connected with Ispahan by a bridge 1,000 feet long, of 34 arches. It was founded about 1603 by Shah Abbas, who transported to it all the inhabitants of the Armenian town of Julfa on the Araxes, and gave them full toleration for their religion, and valuable privileges as merchants. This colony prospered for more than a century, and once contained 30,000 people and 24 churches. It is now greatly decayed, and has not more than 3,000 inhabitants.-Ispahan is a very ancient city, and is mentioned by historians as early as the 3d century. By the caliphs of Bagdad it was made the capital of their Persian provinces. Tamerlane captured it in 1387, massacred 70,000 of the inhabitants, and nearly ruined the city. It recovered at the beginning of the 17th century, and was the favorite abode of the monarchs of the Sooffee dynasty. In the height of its prosperity it was visited in 1673 by the French traveller Chardin, who resided there 4 years, and who describes it as a great city 24 miles in circuit, with 160 mosques, 48 colleges, 1,800 caravansaries, 273 public baths, and a population of 600,000. Other authors state the population at 1,100,000. But in 1722 it was taken by the Afghans after a siege of 8 months, and its buildings defaced and people massacred in frightful numbers. This catastrophe nearly destroyed the city. The seat of government was removed first to Shiraz, and afterward to Teheran, and for a century Ispahan was little more than a mass of ruins. It has been greatly improved within the last 30 years, chiefly by the exertions of Mohammed Hussein Khan, who was for a long time its governor. Still, the traveller rides for miles through deserted streets, ruined buildings, and silent squares. ISRAEL. See JACOB.

ISRAELITES. See HEBREWS.

ISSAQUENA, a W. co. of Miss., bounded W. by the Mississippi river and S. E. by the Yazoo, which is navigable by steamboats; area, 965 sq. m.; pop. in 1850, 4,478, of whom 4,105 were slaves. It is drained by Sunflower river, and has a low and level surface, portions of which are often inundated. The soil is rich. The productions in 1850 were 143,130 bushels of Indian corn, 18,595 of sweet potatoes, and 8,461 bales of cotton. Capital, Tallula.