Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/947

 • ISA

When it is grown cold, beat it to a very fine powder, and roaft it a fecond time in a ftronger fire till it emits no linell, then talce it out of the fire.

Compofe a flux of three parts of the common white flux, and one part of powdered glafs, and add of fandiver and coal- duft of each one half part. Add of this flux three times the quantity of the roafted ore* and mix the whole very well to- gether ; then talce a good crucible well luted within, and put into it the ore mixed with the flux ; cover it with com- mon fait, and lhut it clofe with a tile, and with lute applied to the joints. Put the common wind furnace upon its bot- tom part, having a bed made of coal-duft ; introduce into the furnace a fmall grate liipported on its iron bars, and a ftone upon it whereon the crucible may ftand ; furround the whole with hard coals, not very large, and light them at top. Make the fire very ltrong, and continue to add frelh fuel, that the veflel may never be naked at top. When the fire has thus been continued in its full ftrength for an hour, take out the veflel, and ftrike on the floor where it ftands, to collect, all the particles of Iron into a body, and when cold break the veflel, and you will find the pure Iron. Cramer's Art of Allaying, p. 338.

-Several attempts have been made to run Iron ore with pit- coal, but we meet with no account of the fuccefs of fuch attempts : However, Mr. Mafon informs us, that at Cole- brook Dale in Shropfhire, Mr. Ford makes Iron brittle or tough as he pleafes, from Iron ore and coal, both got in the fame dale ; there being cannon thus cart, lb foft as to bear turning like wrought Iron. Phil. Tranf. N° 482. Sett. 6.

IRON-T^err, in botany. See the article Sideritis.

IRREDUCIBLE Cafe, in algebra, is ufed for that cafe of cubic equations where the root, according to Cardan's rule, ap- pears under an impoffible or imaginary form, and yet is real. Thus in the equation, xi — go* — 100 = o, the root, ac-

cording to Cardan's rule, will be * = / 50 -|- v' — 24500 -f-

\? 5° — •*/ — 24500, which is an impoffible expreflion, and yet one root is equal to 10 ; and the other two roots of the e- quation arc alio real. Algebraifts, for two centuries, have in vain endeavoured to refolve this cafe, and bring it under a real form ; and the queftioft is not lefs famous among them, than the fquaring of the circle is among geometers. It is to be obferved, that as in fome other cafes of cubic equations, the value of the root, tho' rational, is found un- der an irrational or furd form ; becaufe the root in this cafe is compounded of two equal furds with contrary flgns, which

deftroy each other ; as if x = 5 -f- ^ c _|_ ^ ,/ 5° then * =

10; in like manner, in the irreducible cafe, when the root is rational, there are two equal imaginary quantities, with con- trary figns, joined to real quantities ; fo that the imaginary quantities deftroy each other. Thus the expreflion :

• 50-f-V— 24500 = 5 + /— 5; and ^50— ,/— 24500 = 5~V — 5- B "t5+v/ — 5 + 5 — i/ — 5 = 10 = *, the root of the propofed equation.

Dn Wallis feems to have intended to fhew, that there is no cafe of cubic equations irreducible, or imfraclicable, as he calls it, notwithftanding the common opinion to the con- trary. Thus in the equation r 3 — 63 r = 1 62, where the value of the

root, according to Cardan's Rule, is, ;

=^81+^—2700

1 — 1/— 2700, the doctor fays, that the cubic root of 81 + t/— 2700, may be extracted by another impoffible binornial, MZ. by % -f- 2. / _ 3 ; and in the fame manner, that the cubic root of 8 1 — / — 2700 may be extrafted, and is equal to f — -I ^ — 3 ; from whence he infers, that f + iv' — 3 + ? — i.</ — 3 = 9' is one °f the roots of "the equation propoied. And this is true : But thofe who will confult his Algebra, p. 190, 191, will find that the rule he gives is nothing but a trial, both in determining that part of the root which is without a radical fign, and that part which is within : And if the original equation had been fuch as to have its roots irrational, his trial would never have fucceed- ed. Befides, it is certain, that the extracting the cube root of 81+/— 2700, is of the fame degree of difficulty, as the extracting the root of the original equation r* — 63 r =: 162 ; and that both require the trifeaion of an an.de for a perfect folution. See M. de Moivre in the Appendix to Saunderfon's Algebra, p. 744, fcq. For Cardan's rule. See the article Cubic Equation IRREDUC7IBLE Cap, in algebra. See the article Irreeu- CIBLE.

IRREGULAR Leap, ( Cycl. ) in mufic. See the article Leap.

ISABELLA, in conchyliology, the name given by the French naturahfts to the beautiful pale-brown voluta, fo much elteemed in the Dutch cabinets.

ISAGON, in geometry, is fometimes ufed for a figure con- fining of equal angles.

ISAMBLUCIS, in natural hiftory, the name of a genus of foffils of the clal> of the felenitae, but of the columnar, not the rhomboid al kind, See Tab. of Foffils, Clafs 2.

ISC

The word is derived from the Greek ins, equal, V&.V, obtuie, or blunt ; and *»», a column ; and exprefies a body in form of an obtufe or blunt column, the fides of which are all equal to one another; This diftinguifhes it from the genus of the ifcbnamiiuatj or thin columnar felenita:,- two of the fides of which being broader than the others, make it of a flatted form. The felenita of this genus con-

i a °l  dCS and two obtufe or 3br,i P t ends ' and a11 che  fides^ being very nearly of the fame breadth, they much re- Jem be broken pieces of the columns of fprig cryftal. 1 he bodies of this genus, as well as the reft of the columnar lelemta:, are fubject to a longitudinal crack, which fome- times admitting a fmall quantity of clay, fhapes it into the figure of an ear of graft. Hill's Hift. of Foil', p. 121. See the article Selenites.

Of this genus there are only two known fpecies, 1 A whi- tilh one, very much refemViing a broken fprig of crvftaL found among the white tobacco-pipe clay near Northampton And, 2. A ihort and pellucid one, with flender filaments : this is found in the ftrata of yellow clay in Yorkfhi re, and iometirnes lying on the lurface of the earth. Hill's Hift of Foil p. 139, 140.

ISAT1S, IVoad, in botany, the name of a genus of plants, tbe charaaers of which are thefe : The flower confifts of tour leaves, and is of the cruciform kind ; the piftil arifes from the cup, and afterwards becomes an unicapfular oblong fruit, compreffed at the fides, and containing only one ob? long feed. See Woad.

The fpecies of Woad, enumerated by Mr. Tournefort, are thefe : j. The broad-leaved, or manured Woad. 2. The narrow-leaved, or wild Woad. And, 3. The fmall wild Portuguefe Woad. Toumef. Inft. p. 211:

ISATODES, a word ufed by Hippocrates, and fome other of the old writers, to exprefs a greenifh colour of the bile dis- covered in the ftools, which referable the colour of the herb tjatu or woad. This was efteemed an indication of a highly depraved bile.

ISCM1MON, in botany, a name ufed by many authors for the grafs called gramm manna, manna grafs, or Ruffia feed rark s Theat. p, 78.

Isch jemon was alfo a word ufed by the antients to exprefs any thing given as a remedy in hemorrhages, or other fluxes of blood, whether from wounds or otherwife.

ISCHiEMUM, in botany, the name by which Linnaeus calls the Jcbananth, or camel's hay. This makes a peculiar E e- nus of plants, the characters of which are thefe : It has male and female flowers, diftina from one another. The male flower is very fmall, and always ftands upon the glume of the cup of the female flower. Its cup is a bivalve glume, with no awns, and containing only one flower; the flower is another bivalve glume of the frze of the cup, and without awns. The ftamma are three ihort capillary filaments, and the anthers fimple ; the female flower is larger ; its cup is a large bivalve glume, placed within the common dame, and terminated by a twifted awn, or beard between each valve i he flower is a fmall double glume; the piftil confifts of an oblong germen, and two reflex ftyli ; all the glumes of the cup, flower, &c. continue to furround and enclofe the feed - and they never drop it out till forced from them, each con- ^.taming one feed. Liluuei Gen. Plant, p. 527.

ISCHAS, in botany, a name given by Clufius and fome others, to thole fpecies of tithymal of fpurge which have tuberous or knobby roots, and are called by others apios. See the article Tithymalus.

ISCHENIA, frxma, in antiquity, anniverfary fports celebrated at Ulymphia in memory of Ifchenus the grandl'on of Mercury and Hierea ; who; in a time of famine, devoted himfelf to be a faenfice for his country, and was honoured with a mo- nument near the olympian rtadium. Potter's Archa:ol. Grac. I. 2. c. 20. torn. 1. p. 407.

ISCHIO-CAVERNEUX, in anatomy, a name given by Win- flow, and other of the French anatomifts, to a mufcle of the penis, called alfo by fome the pojierior and collateral penis, but now generally named, from its office, the ereilor. See the article Erector, Cycl. ,-»,'*«*

Ischio-caverneux is alfo applied by the fame authors to a mufcle of the female pudenda, called by fome the mufculus clitoridis, and clitoridis manife/lus mufculus, and by Cowper and Albinus, from its office, the ereilor clitoridis.

ISCHIUM (L}r/.)_This is the loweft portion of the os inm- rmnatum, as alfo of the whole trunk. It is divided into the body, the tuberofity, and the ramus or branch. The body of the os ifchium forms the loweft and grcatcft part of the acetabulum, and fends out an apophyfis back- ward, called the fpine of the ifchium. The tuberofity is very thick, unequal, and turned downward; and it is on this part that the whole body refts when we fit. It appears cartilaginous, becaufe of the dried and hardened remains of the tendons. The whole convex portion of it is originally an appohyfis, of which the marks are obliterated fo'oner in fome fubjeas than in others : three mufcular impreffions may be diftinguifhed in it.

The branch of the Ifchium is a kind of thin flat produaion, or apophyfis, which afcends forward from the curvature of