ISMAIL PASHA 207 ISOMORPHISM ISMAIL PASHA (iz'mal or iz-ma-el), a Khedive of Egypt, son of Ibrahim Pasha and grandson of Mehemet Ali; born in Cairo, in 1830. He was made commandant of the army in 1862, and succeeded Said Pasha as viceroy in 1863. In 1867 he acquired from the Porte the title of Khedive. His lavish expendi- tures not only greatly encumbered his own estate, but embarrassed all the peo- ple of Egypt. He took an active inter- est in the building of the Suez Canal, but under his mismanagement the country became so involved in debt that he was forced to abdicate. This he did, his son taking his place. Not being allowed by the Sultan to go to Constan- tinople, he made his home at Naples and various other places for many years, but finally died in Constantinople, March 2, 1895. ISNIK, a town of Asia Minor in which, in 325, the first oecumenical coun- cil was held. ISOBARIC LINES (is-6-bar'ik) , lines drawn on a map or globe through all places where the barometer is at the same height at a certain time. Tele- graphic communication enables these lines to be drawn with some immediate- ness. ISOCHROMATIC LINES, in optics, colored rings appearing when a pencil of polarized light is transmitted along the axis of a crystal, as of mica or niter, and, after passing through a plate of tourmaline, finally reaches the eye. ISOCHRONISM (is-ok'ron-ism), a property appertaining to all systems in equilibrium, by which, when slightly dis- turbed more or less, the oscillations re- sulting are all performed in the same time, or so nearly in the same time that any retardation or acceleration is imperceptible. When a pendulum, for instance, is allowed to vibrate till it rests, it will be found that no percepti- ble difference exists between the vibra- tions of longer or shorter extent, the same number of vibrations being made in the same length of time. Again, in the sound produced by a musical string, the finest ear cannot detect any difference in the pitch of a note made by a smart blow on the pianoforte key and a gen- tle touch; yet a very small difference in the number of vibrations per second would be perceptible to the ear. ISOCRATES (i-sok'ra-tes), a Greek orator and rhetorician; born in Athens in 436 B. c. He was carefully educated, Socrates having been of the number of his preceptors; and at an early age was celebrated for the facility with which he used his native tongue, though the weakness of his voice precluded any hope he may once have entertained of distinction in public life. He therefore opened a school of oratory, the fame of which soon filled all Greece, in conse- quence of the exceptional attainments of its graduates. The ages have spared to us 21 of his compositions, rhetorical and espistolary. He is best represented by the discourses knov/n as the "Areopagi- ticus" and the "Panegyricus." ISODYNAMIC, ISOCLINIC, and ISO- GONIC LINES, lines of equal force, equal inclination, and equal declination, are three systems of lines which being laid down on maps represent the mag- netism of the globe as exhibited at the earth's surface in three classes of phe- nomena, the varying intensity of the force, the varying dip or inclination of the needle, and its varying declination from the true meridian. ISOGEOTHERMIC LINES, a term in- troduced by Kupffer for lines drawn on a globe or map across those places in which the mean temperature of the soil is the same. ISOMERISM (i-som'er-ism) , in chem- istry, a term applied to those bodies which are composed of the same elements, in the same proportions, but which diffej either in their physical characteristics or in their chemical properties. They may be divided into three distinct classes : isomeric, metameric, and polymeric bod- ies. Isomeric bodies or isomerides are those which show analogous decompositions and changes, when heated, or when treated with reagents, but differ in physical properties. Metameric bodies, or metamerides, are those which exhibit dissimilar trans- formations when heated, or when acted on by reagents. Polymeric bodies, or polymerides, con- tain the same elements in the same pro- portions, but have different molecular weights. ISOMORPHISM (I-so-mor'fizm), a general law, discovered in 1819 by Pro- fessor Mitscherlich, of Berlin, by which the variation of minerals is governed. It is that the ingredients of any single species of mineral are not absolutely fixed as to their kind and quality, but one ingredient may be replaced by an equivalent portion of some analogous in- gredient. Thus in augite the lime may ' be in part replaced by portions of perox- ide of iron, or of manganese, while the , form of the crystal and the angle of the '"
