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ISO 



water strata in the regular coal 

 formation, and in the coal strata of 

 the oolites of Yorkshire, and 

 among the clay and sandstone 

 strata in the wealds of Kent. Few 

 geologists have attempted to ex- 

 plain the formation of ironstone. 

 The manufacture of iron was 

 formerly carried on to a consider- 

 able extent in the county of 

 Sussex ; Fuller in his "Worthies 

 observes, " it is almost incredible 

 how many great guns were made 

 of the iron of this county." 



IRON FLINT. A mineral, thus named 

 by Jameson. 



PEON-GLANCE. Rhombohedral iron- 

 ore. A peroxide of iron, of a dark 

 steel-gray colour. There are sev- 

 eral varieties ; the red varieties are 

 called red iron ore, and the fibrous, 

 hematite. 



I'RON SAND. A variety of octohedral 

 iron-ore, in grains. 



ISCHIA'TIC. (ischiadicus, Lat. ItrxiaSi- 

 /co9, Gr.) Pertaining to the ischium, 

 as the ischiatic notch, &c. 



I'SCHIUM. (ischium, Lat. iffx l v > ^ r O 

 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 acetabulum 

 or cup which receives the head of 

 the thigh bone. 



I'SEEIN. | (from eisen, Germ.) A 



I'SERINE. j mineral of an iron- 

 black colour, from which it derives 

 its name. It consists of 48 per 

 cent, of oxide of titanium, an equal 

 proportion of oxide of iron and four 

 per cent, of uranium. It occurs in 

 small obtuse angular grains, being 

 a kind of metallic sand. It appears 

 to differ but little from menachinite. 



ISOCA'RDIA. A heart-shaped shell, 

 with separated involuted and di- 

 verging beaks. The hinge formed 

 by two flattened cardinal inserted 

 teeth, and an isolated lateral tooth 

 under the cartilage slope. 



ISOCHEI'MAL. (from r<ro* ; equal, and 



winter, Gr.) Of the same 

 winter temperature : lines drawn 

 through places having the same 

 winter temperature are denomi- 

 nated isocheimal lines. 



ISO'CHRONAL } (isochrone, Fr. from 



ISO'CHRONOTJS. ) iffo<i y and %/>oVo9, 

 Gr.) Having equal times ; uniform 

 in time. The isochronal vibrations 

 of a pendulum are such as are 

 performed in the same space of 

 time ; as all the swings or vibra- 

 tions of the same pendulum are, 

 whether the arches it describes are 

 longer or shorter. 



PSOGEOTHE'RMAL LINES. (from 10-09, 

 777, and 0e/yto9, Gr.) Certain lines 

 or divisions in the earth's crust 

 possessing an equal degree of mean 

 annual temperature. If we draw 

 lines through all the points which 

 have the same terrestrial tempera- 

 ture, these isogeothermal lines re- 

 semble the isothermal, as they are 

 parallel to the equator, but diverge 

 from it in several points. 



ISOME'RIC. (from "ao?, equal, and 

 ytte/309, a part, Gr.) A term applied 

 to substances which consist of the 

 same ingredients in the same pro- 

 portion, and yet differ essentially 

 in their properties. 



ISOMO'EPHISM. (from i'aos and /AO/?^, 

 Gr.) That quality which a sub- 

 stance possesses of replacing some 

 other substance in a compound 

 body, without any alteration of its 

 primitive form. 



ISOMO'RPHOITS. That has the property 

 of retaining its primitive form when 

 united with other substances in a 

 compound body. 



ISOPERIME'TRICAL, (from fo-os, vepi, 

 and fieTpov, Gr.) Such figures as 

 have equal perimeters or circum- 

 ferences, of which the circle is the 

 greatest. 



ISO'PODA. (from "ao? and TTOV?, Gr.) 

 An order of crustaceans, thus named 

 from the formation of their feet 

 which are fourteen in number. 



