I R O 



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I S O 



I'RON-GLANCE. Rhombohedral iron-ore; 

 the Fero ligiste of Haiiy ; the Roth eisen- 

 stein of Werner. A peroxide of iron, of 

 a dark steel-gray colour. There are 

 several varieties ; the red varieties are 

 called red iron ore, and the fibrous, hema- 

 tite. 



I'RON PY'RITES. See Pyrites. 



I'RON SAND. A variety of octahedral iron- 

 ore, in grains. 



IRRU'PTED. (from irrumpo, Lat.) Broken 

 violently, and with great force. 



IRRU'PTION. (irruptio, Lat. irruption, 

 Fr.) A violent breaking in; a bursting 

 in ; a violent rushing into. 



ISCHIA'TIC. (ischiadicus, Lat. trr%iaiK:6c, 

 Gr.) Pertaining to the ischium, as the 

 ischiatic notch, &c. 



I'SCHIUM. (ischium, Lat. la^ibv, Gr.) 

 One of the bones of the pelvis, situated 

 in the lowest part thereof, and being that 

 bone upon which we sit. It forms the 

 under, and largest portion of the aceta- 

 bulum, or cup which receives the head of 

 the thigh bone. 



I'SERIN. (from eisen, Germ.) A mineral 

 of an iron-black colour, from which it 

 derives its name. It consists of 48 per 

 cent, of oxide of titanium, an equal pro- 

 portion of oxide of iron, and four per 

 cent, of uranium. It occurs in small 

 obtuse angular grains. 



ISOCA'RDIA. A heart-shaped shell, with 

 separated involuted and diverging beaks. 

 The hinge formed by two flattened car- 

 dinal inserted teeth, and an isolated 

 lateral tooth under the cartilage slope. 

 This genus includes the Chama cor. 

 Some authors divide the genus Chama 

 into five, of which Isocardia is one. 



ISOCHEI'MAL. (from i<n>, equal, and 

 XeT/ia, winter, Gr.) Of the same winter 

 temperature : lines drawn through places 

 having the same winter temperature are 

 denominated isocheimal lines. 



ISO'CHRONAL. (isochrone, Fr. from IrroQ, 

 and xpovoe, Gr.) Having equal times ; 

 uniform in time. The isochronal vibra- 

 tions of a pendulum are such as are per- 

 formed in the same space of time ; as all 

 the swings or vibrations of the same pen- 

 dulum are, whether the arches it describes 

 are longer or shorter. 



ISO'CHRONAL LINE. That in which a 

 heavy body is supposed to descend with- 

 out any acceleration. 



ISO'CHRONOUS. The same as isochronal. 



I'SOGEOTHE'RMAL LINES, (from i<roc, yrj, 

 and Qepfjibc, Gr.) Certain lines or divi- 

 sions in the earth's crust possessing an 

 equal degree of mean annual tempera- 

 ture. This word is not very common, and 

 I have met with it only in Mrs. Somer- 

 ville's Connexion of the Physical Sciences. 

 It is, however, more definite than iso- 



thermal, inasmuch as it applies solely to 

 divisions of the land, whereas isother- 

 mal applies equally to air, land, and 

 water. 



ISOMORPHISM, (from tcrog and //op0/}, 

 Gr.) That quality which a substance 

 possesses of replacing some other sub- 

 stance in a compound body, without any 

 alteration of its primitive form. 



ISOMO'RPHOUS. That has the property of 

 retaining its primitive form when united 

 with other substances in a compound 

 body. 



ISOPERIME'TRICAL. (from Iffog, Trtpi, and 

 H'trpov, Gr. isoperimetro, It.) Such 

 figures as have equal perimeters or cir- 

 cumferences, of which the circle is the 

 greatest. 



ISO'PODA. (from loog and TTOVQ, Gr. ) An 

 order of crustaceans, thus named from 

 the formation of their feet, which are 

 fourteen in number. This order em- 

 braces the genus Oniscus. 



ISO'SCELES. (laoaiceXriQ, Gr. isocele, Fr.) 

 That which hath only two sides equal. 



I'SOPYRE. A mineral of a greyish or 

 black colour. Occurs massive. Found 

 in Cornwall, imbedded in granite. 



ISO'THERAL. (from IGOQ and Oepoc, sum- 

 mer, Gr.) Of the same summer tem- 

 perature : lines drawn through places 

 having the same summer temperature are 

 denominated isotheral lines. 



LSOTHE'RMAL. (from itroc. and Ospfjirj, Gr.) 

 Possessing equal temperature. Lines 

 drawn upon a map through a series of 

 places having the same annual mean tem- 

 perature are termed isothermal lines, or 

 lines of equal temperature. Mr. Lyell 

 observes, " it is now well ascertained that 

 zones of equal warmth, both in the 

 atmosphere and in the waters of the 

 ocean, are neither parallel to the equator 

 nor to each other. It is also discovered 

 that the same mean annual temperature 

 may exist in two places which enjoy very 

 different climates, for the seasons may 

 be nearly equalized or violently con- 

 trasted. Thus the lines of equal winter 

 temperature do not coincide with the 

 lines of equal annual heat, or isothermal 

 lines. If lines be drawn round the globe 

 through all those places which have the 

 same winter temperature, they are found 

 to deviate from the terrestrial parallels 

 much farther than the lines of equal 

 mean annual heat. The lines, for in- 

 stance, of equal winter in Europe,, are 

 often curved so as to reach parallels of 

 latitude 9 or 10 distant from each other, 

 whereas the isothermal lines only differ 

 from 4 to 5." Principles of Geology. 



ISOTHE'RMAL ZONES. As the isothermal 

 lines are as numerous as the places, and 

 as diversified as numerous, geographers 



