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the tuberofity of the os pubis, and it is often- covered in part by a continuation of the epiphyfes of the tuberofity. Thefe three parts of the Ifchiwn taken together, form a large opening, which makes the greateft part of the foramen ovale. Three other notches are remarkable in this bone ; one po- fterior between the tuberofity and the fpine, for the paflage of the internal obturator mtifcle, which is a little cartila- ginous, and divided into three or four fuperficial channels ; one lateral between the tuberofity and the acetabulum, for the paflage of the external obturator mufcle ; and one ante- rior at the edge of the acetabulum for ligaments. JVinJloiv'~ Anatomy, p. 71.
ISCHNAMBLUCIS, in natural hiftory, the name of a genus of foffils of the clafs of the felenitas, but one of thofc which are of a columnar form, not of the common rhomboidal one. See Tab. of Foflils, Clafs 2. The word is derived from the Greek, XayjU, thin, «/*6 blunt or obtufe, and neWj a column ; and exprefles a body in form of a thin or flatted column, with obtufe ends. The characters of this genus are, that the bodies of it are of a flatted columnar form, and octohedral in figure, coniifting of fix long planes, and two abrupt or broken ends. The whole being of a flatted figure. The top and bottom planes are much broader than the reft ; the four other planes, called the fides, are narrower than thefe, but are ufually of very riearly the fame breadth with one another, as are alfo thefe tops and bottoms, fo that the whole figure comes very near an hexhedral prifm. Hill, '.Hift. of Foff. p. 121. The bodies of this genus very frequently have a long crack reaching their whole length, and clay often getting into this, fpreads itfelf into the form of an ear of fome of the grafles, and has been miftaken for a real ear of grafs. Of this genus there are only four known fpecies. 1. Aflat, broad, and pellucid kind, found in Northamptonfhire, Lei- cefterfhire, and Yorkfhire, at eonfiderably great depths in blue clay. 2. A dull rough -fur faced and thicker kind, found in many parts of Kent, and in great plenty in the cliffs of Sheppey ifland. 3. A dull longitudinally ftriated kind, found in the clay-pits of Yorkfhire and elfewhere, and frequently marked in the middle with the figure of an ear of grafs. And 4; A thick, rough, and fcaly kind, frequent on the fhores of Sheppey ifland, and both in the clay-pits and on the fhores of Yorkfhire. This alfo has frequently the reprefentation of an ear of grafs. Ibid. p. 136, 137, 139.
ISCHNOPHONIA, a word ufed by fome authors to exprefs that weaknefs and fhrilnefs of the voice which attends perfons in fome difeafes ; more ufually, however, it is underflood to exprefs a ftammering or imperfection in the fpeech.
ISCHURIA {CycL) — See the article Suppression of Urine.
ISCUSTOS, a word ufed by the writers of the middle ages as the name of a gem, but feeming to be only a corrupt way of fpelling afbeftos. Albertus fays it was of'a yellowilh colour, and found in Spain, and that its fibrous texture made it capa- ble of being wrought into cloth, which when thrown into the fire, was not confumed, but only cleaned by burning. See the article Asbestos.
ISING-GLASS (Cycl.)— A ftrong glue is made of lfmg-glaf$ diflblved in fpirit of wine. See the articles Glue and Ich- thyocolla, Cycl. and Suppl.
Isinglas Ftjh\ the Englifti name of the fiffl from which the drug called khthyocolla or ijinglafs is made. It is a fpecies of the accipenjer or fturgeon, and is diftinguifhed by not having any of the tubercles which the body of the common
, flurgeon has. See the article Accipenser.
ISIR, orlxiR, a name ufed by fome of the chemical writers for an elixir.
ISIS, in botany, a term ufed by Linnaeus to exprefs a genus of fea-plants, comprehending all thofc which are branched and incruftated, and which have a Ample furface.
ISKA, a name given by the antients to a light kind of agaric growing to the old flumps of the oak, hazel, and other trees. This being a very light kind of fuel, was ufed by the antients in the manner of a cautery, as we have of late learned from the Indians to ufe moxa or the down of mugwort leaves.
ISLAND {Cycl.) — Kircher tells us of a very remarkable change of the face of the earth in the eaflern parts of the. world, by the coalition of ninety-nine IJles into one larger one, under the fame extent of furface which they had while divided by the fea. But we are to obferve, that no abfolute dependance is to be given to what he affirms of the miraculous things of China. His ftory of the leaves of trees turning into black birds, is a proof that he took many abfurd things, on the credit of people whom a philofopher ought not to have be- liev'd. Kircher, China illuftrata. See the article Black- Birds.
Floating Islands. Hiftories are full of accounts of foaling IJlandt ; but the greater part of them are either falfe, or founded on exaggerations. What we generally fee of this kind is no more than the concretion of the lighter and more vifcous matter floating on the furface of water in cakes, and, with the roots of plants, forming congeries of different fixes, which not being fixed to the fhore in any part, are blown about by the winds, and float on the furface. Thefe arc generally found in lakes, where they are confined from
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being carried too far ; and in procefs of time fome of them £@# quire a very coniiderable fize. Seneca tells us of many of thefe foaling IJlandt in Italy; and fome later writers have defcribed not a few of them in other places. But however true the hiftories of thefe might have been at the time whea they were wrote> there remain very few proofs of their truth at this time, thefe Ijlands having either difappeared again, or been fixed to the fides, in fome part, in fuch a manner as to have made a part of the fhore. Pliny tells us of a great ljland which at one time fwam about in the lake cutilia,,in the country of Reatinum, which was difcovered to the old Romans by an oracle ; and Pomponius tells us, that in Lydia there were feveral' Jjtendp fo loofe in their foundations, that every little accident fhouk and removed them. Herodotus fpcaks alfo very largely of the lake Chemnis in iEgypt. Island Cryjlal, a body famous among the writers of optics for its property of a double refraction j but very improperly calr led by that name, as it has none of the diftinguifhing cha- racters of cryftal, and is plainly a body -of another clafs. Dr. Hill has reduced it to its proper clafs, and determined it to be of a genus of fpars, which he has called, from their fl- , gure, parallellopipedia, and of which he has defcribed feveral fpecies, all of which, as well as fome other bodies of a dif- ferent genus, have the fame properties. Bartholine, Huy- gens, and Sir Ifaac Newton, have defcribed the body at large, but have accounted it either a cryftal or a talc f errors which could not have happened, had the cri tenons of foflils -been at that time fixed ; fince Sir Ifaac Newton has recorded its property of making an ebullition with aqua fortis, which alone muft prove that it is neither talc nor cryftal, both thofc bodies being wholly unaffected by that menftmum. It is always found in form of an oblique 'parallellopiped, with fix fides, and is found of various fizes, from a quarter of an inch to three inches or more in diameter. It is pellucid, and not much lefs bright than the pureft cryftal, and its planes are all tolerably fmooth, tho' when nicely viev/d, they arc found to be wived with crooked lines made by the edges of imperfect plates. What appears very Angular in the ftrtt- cture of this body is, that all the furfaces are placed in the fame manner, and coniequently it will fplit off" into thin plates, either horizontally or perpendicularly ; but this is found, on a microfcopic examination, to be owing to the regularity of figure, fmoothnefs of furface, and nice joining of the feveral (mail parallellopiped concretions, of which the whole is corftpofed, and to the fame caufe is probably owing its remarkable property in refraction. Hilly Hift. of Fofl".
P- 333-
It is very foft and eaflly fcratched with the point of a pin j it will not give fire on being ftruck againft ftccl ; and ferments, and is perfectly diflblved in aqua fortis. It is found' in, ljland from whence it has its name ; and in France, Ger- many, and many other places. In England fragments 'of other fpai s are very often miftaken for it, many of them hav- ing, in fome degree, the fame property. Hill, Hift. of Fofl". p. 334, Seethe article Anomgrhomboidia.
Island Fijb, in the fifh trade, a name given to the common cod-fifh when it has been caught and preferred in ljland. Wilhighby, Hift. Pifc. p. 106. See the article Cod-fish.
ISLEBEf ANUS Lapis, in natural hiftory, a name given by authors to a kind of blackifh flate, very heavy, not very hard, and eaflly fplitting into thin flakes. It is dug near Ifleb in Thuringia, whence it has its name, and is much of the nature of that fort of black flate which we have common in England, lying over the coal ftrata.
As our flate of this kind contains ufually leaves of fern and o- ther plants, this foreign kind contains the impremons of feveral kinds of fifhes, fo exactly delineated in fhape, and with the fins, fcales, and all other parts fo perfect, that the fpecies are eaflly known. All thefe imprefltons and delineations are, however, i'o flat and thin, and have taken up lb little room in the flone, that they feem rather to have been the fkins and exuviae of fifh, than the whole bodies of them ; but this is not wonderful when we conflder how eaflly the bodies, and even the bones- of fifhes, may be diflblved ; and that thefe exuvix are lodged in a fubftance which contains a vitriolic fait, very capable of effecting fuch a folution. It is eafy to conceive, that this, and other fuch falts, while in a flate of folution in water, be* fore, the concretion of the matter of this i}one into a hard mafs* might make that water a menftruuni capable of thus diflblv- ins; the bodies and bones of fifties, While the outer ffiliri and, fcales being much tougher, and of a very different nature, might efcape the effects of them. It is well known that the fins and fcales, and other external parts of fifhes, are much more capable of bearing maceration in water, than the inter- nal fubftances of them ; and it is eaiy to conceive that thefe, after having withftood that power in the w T ater, would at length fubfide down among the muddy matter that was to- form fome of the ftrata of this flate : The motion of the wa ter would not fail to expand thefe fkins, tho 5 ever fo thin, into the true and exact fhape of the whole fifh j and when thus laid along upon the furface of the new formed ftratum, the next quantity of the fame matter that fubfided would form another ftratum, which falling evenly upon the former, would cover it, and on the following concretion of both, -the J figure
