from ἴσος, equal, and μορφή, shape or form. A vast number of substances are distinguished by crystalline forms, which have been arranged into six classes or systems founded upon the relation of their axes of symmetry to each other, the study of which belongs to the science of Crystallography. (See Mineralogical Science.) Certain elementary substances, which are analogous to each other in chemical properties, have the power of assuming a similar crystalline form, and may be substituted for each other in a compound without producing any change in its crystalline form. Such bodies are said to be isomorphous. Thus, magnesia, oxide of copper, protoxide of iron, and oxide of nickel, are isomorphous: when they enter into combination with the same acid, and with similar proportions of water of crystallization, they furnish crystals which are identical in form, and if of the same colour cannot be distinguished by the eye. Common alum is a sulphate of alumina and potash; but the alumina may be displaced by peroxide of iron, the potash by ammonia, or by soda, and yet the shape of the crystal remain the same. Mixtures of isomorphous salts cannot be separated by the method of crystallization, unless of very different degrees of solubility.

ISOHERMAL LINES. See Climate.

ISPANAH or ISPAHAN, formerly the capital of the Persian empire, now of the province of Irak-Adjemi, is situated on the River Zendaroond, N. Lat. 32° 42', E. Long. 51° 46', about 220 miles S. of Tehran, and 230 miles N. of Schiray. It is the centre point from which all the important roads of the country radiate. The present city is in a very dilapidated condition. The streets are empty, palaces in ruins, the gardens, once so famous for their luxurious beauty, strewn with stones and lime from the neighbouring ruins, and the bazars deserted, though formerly the resort of merchants from all quarters of the East. The area of the city is extensive, and covers both sides of the river; on the one side is the town proper, on the other the suburb called Julfa devoted to the Christians. The present in- habitants confine themselves to certain districts where the streets are more commodious and accessible; the rest of the town being quite tenantless. The suburb Julfa is occupied by Armenians, and forms the residence of the European embassies when the Court is at Isfahan. It is connected with the city proper by three ruinous bridges, only one of which can be used. This bridge is composed of thirty-three arches, above each of which are three smaller ones, and has, besides a causeway for horsemen and cattle, a covered footpath for passengers. The principal features of Isfahan are the mosques, palaces, and gardens, that vie with each other in displaying tokens of departed greatness. In the centre of the town is the Maidan, or Square of Shah Abbas, said to be 2000 feet long by 700 feet in breadth, each of its sides being occupied by arcades of double ranges of arches. In the N.W. side of the square is the gate of entrance to the Shah Abbas Bazaar, on which the celebrated clock of Isfahan formerly stood. This bazaar, built by the Shah whose name it bears, is described by travellers as one of great extent, but is now occupied by only a few tradesmen. Opposite its gate, on the south-eastern side of the Maidan, stands the mosque of Shah Meshed, the most richly adorned of any in Isfahan, and having a gate of great magnificence covered with gold and silver ornaments. Christians are not admitted to the interior. The Loot Ullah Mosque and Ali Gate front each other on the S.E. and S.W. sides of the square, and both are remarkable specimens of Persian architecture. But the building which strikes the eye of the stranger most in this city is the palace of forty columns, or Chehel Sitoon. Its interior is covered with gilding, mirrors, and paintings. Some of the pictures represent scenes which occurred two or three hundred years ago, and are said to be well executed. The Shah holds his audiences here when in Isfahan. Besides these buildings there are several smaller mosques, colleges, and a considerable number of caravanserais and public baths. The interest attached to this city is, however, chiefly derived from the historical descriptions of its former wealth and grandeur. It was under the caliphs of Bagdad that Isfahan began to increase in wealth, population, and trade, and rose to be capital of Irak. The invasion of Timor, however, gave a severe blow to its prosperity. The city was taken by him in 1387, and great atrocities were perpetrated on the inhabitants. At the beginning of the seventeenth century the town began to rise again into notice. Abbas Shah, of the Safi dynasty, removed the seat of government from Kazwin to Isfahan, and used every means to advance its prosperity, and increase trade. He carried off the inhabitants of Julfa, a town on Aras River, in Armenia, and allotted them the suburb of Isfahan, now called Julfa. These were famous for their industrious habits and their knowledge of the arts. Under this monarch the city rose to great eminence as a place of commerce and wealth, and is said to have contained a population of more than a million souls, but after his death it again declined, and at the end of the eighteenth century the seat of government was transferred to Técheran, the present capital of Persia. Isfahan is again beginning to acquire importance, and to attain somewhat of that commercial position which, from her natural advantages, she is entitled to occupy. The inhabitants are reputed the most intelligent in the empire. At present the chief manufactures are silk and gold brocade, and the general trade embraces all those articles belonging to the East, such as silk, tea, coffee, &c., that are brought through Isfahan on their road westwards. The population is variously estimated from 50,000 to 120,000. See also Persia.