I R I 



jlluftration of this order ; fee ExsAT.i:.— Its characters are 

 thus given, the character of the clafs being monocotyledonous, 

 va'tth Jlamem :njlrl:d into the calyx o>- corolla. 



Corolla, which Jufiieu terms calyx, fuperior, coloured, tu- 

 bular at tlic bale, its limb in fix more or lefs deep fegments, 

 equal or unequal. Stamens three, infertcd into the tube, 

 oppofite to three alternate fegments of the flower, their 

 filaments dilliaft, or rarely united into a tube embracing the 

 ftyle. Gcrmcn inferior ; with a fingle ftyie and three-fold 

 ftigma. Capfule inferior, of three cells, and three valves, 

 with many, generally roundifli, feeds. 



Root either fibrous, tuberous, or bulbous. Stem moftly 

 herbaceous, leafy, rarely almolt wanting. Leaves alternate, 

 flieatliing, moftly fword-fliaped. Flowers growing out of 

 iheaths, either folitary, or many together, which (lieaths often 

 conilil of two valves. 



Tiie leading genera in Linnaeus aiid Juflieu are Sifyrir.chwm, 

 Ins. Fx'ia, Glni/ioliii, ^nlbolyza, Crocus, &c. uh.icli Mr. Ker 

 has greatly fubdivided. See litsPEiMNTHA and Gla- 



IRIDIUM, 1:1 Chemijlry, is a fimple fubftance, and a 

 metal. It was difcovered by Mr. Smithfon Tennant in 

 1803, and an accou-t of it was pubiiffied in the Phil. Tranf. 

 for 1S04. i*^ appears that, previous to its publication, the 

 fame difcovcry was made, to a certain d^gre;-, by Defcotils, 

 x-jhicii was afterwards taken up and enlarged upon by 

 Fourcroy and Vanquelin. 



Mr. Tennant found that the fmall black fcales exifting in 

 crude platina, and which are left after the nlatina has been 

 diflblved, contained- two metals wliich had not been pre- 

 vioufly noticed. (See Pl.atina.) He fubjccted tlie above 

 fiibllancc to the alternate aClion of potalh and muriatic acid, 

 by which means lie obtained tv.o fohitions. The black fcales 

 ar? heated to rcdnefs in a f;lver crucible, with their own 

 weight of potaih. When the heated mafs has water added 

 to It, the potafli dilTolves in combination v.-ith one of the 

 metals, the folution being of an or^.nge colour. The refi- 

 duum being treated with muriatic acid, a folution is 

 obtained, which at liril is blue, then becomes olive green, 

 and ultimately of a red colour, confiiling of the acid united 

 to the oxyd of tiie other metal. By tliis alternate treatment, 

 the oxydof ofmium, the other metal alluded to, is diffoived in 

 the potaih, and the o.^;yd of iridium in the muriatic acid. The 

 acid folution, on being evaporated to drvnefs, affords cryllals 

 of an octahedral form, which is, no doubt, a pure muriat of iri- 

 dium. The folution of thefe cryftals is of a deep red colour. 

 When a plate of zinc, or any other metal, having a greater 

 alTmity for oxygen than itfelf, is placed in the lolution, a 

 black powder is precipitated, which, on being heated, be- 

 jcomcs v.-liite, and puts on metallic lu(tre. Tliis metal, like 

 gold and platina, has fo little attraction for oxygen, that it 

 may be obtained by cxpofing the cryll.ils of its muriat to a 

 ftrong heat. In this ftate it is deemed pure iridium. It is 

 a white metal, having the appearance of platina. Owijig 

 to its great infufibilitv, we are not acquainted with fo:ne of 

 it's phjdical properties, fuch as its fpecilic gravity and mal- 

 leability. It has not been fufed by Mr. Tennant nor tiie 

 French chemifcs. It has, however, fince been made to 

 afiume the liquid form, b)' means of the very intenfe heat 

 produced by the large Galvanic batterj- lately exhibited at 

 the Royal Inilitution by Mr. Davy. This heat was fo 

 ■great, that at the fame time the earths alumine and zirconia 

 were fufed, and charcoal was volatilized. 



It forms malleable alloys with feveral of the metals, a 

 •prefumptive proof that it is itfelf malleable. Mr. Tennant 

 found that it'did not combine with ari'cnic, but that it eafily 

 ■united with lead ; from which, like the uoble-mcta!', it may 

 ^ Vol. XIX. 



I R I 



be feparated by cupellation. It forms malleable alloys witk 

 copper and filver. Its alloy, with gold, differs htlle from 

 pure gold, even when the iridium is in confiderable quantity. 

 It appears to Hand next above gold and platina in its affinity 

 for oxygen. It may be inferred, from the change of colour 

 in the acid folution, that it admits of at lead tv.o (iages of 

 oxydation. When the metal is in its pure ftate, it is not 

 aitted upon by any acid except the nitro-muriatic, and even 

 this acid, according to Fourcroy and Vauquelin, dilfolves 

 ou!y ^^^dlhs its weight. Its cmibir.ations with the other 

 acids, and with combullible bodies, have not been attended 

 to. See O.s.Miv.M. 



IRIJU, in Geography, a river of Guiana, which runs 

 into the Atlantic, N. lat. o c8'. W. long. 51 .Jo'. 



JRINON. See Iki.s. 



IRIS, in Anatomy. See Eye. 



Iki.s, Prolapftis, or Procidentia of, in Surgny, denotes a 

 protruhon of a piece of the iris through a wound, or ulcer- 

 ated opening in the cornea, after the efcapc of the aqueous 

 humour. To this dileafe feveral writers have applied the 

 name of ftaphyloma; thougli, in all probability, the influence 

 of Scarpa will in future confine the latter term to a morbid 

 thickening and projection of the cornea itfelf, a cafe which 

 will be defcribed in its proper place. See Staphyloma. 



The prolapfns of the iris is p.-oduced by fu h wounds and 

 ulcers of the cornea as penetrate the anterior chamber of the 

 aqueous humour, and fometimes by blows, which have been 

 fo violent as to rupture the tranfparent membrane invelling 

 the fore-part of the eye. If the lips of a wound of the cor- 

 nea (as, for inftance, thofe of the iiicifion practifcd in the 

 extraction of the c.ataraft, or in opening an hypopium) arc 

 not immediately put into reciprocal contact, fo as to keep 

 the aqueous hu:riour from running out of the anterior cham- 

 ber, as fail as it is fecreted, the iris, drawn by the current 

 of this fluid towards the wound in the cornea, is urged be- 

 tween the lips of the iiiciiion, and, becoming elongated, 

 projects upon the preceding membrane in the form of a 

 fmall tumour. The fame accident happens, when the globe 

 of the eye receives a blow, or is too much compreffed with 

 a bandage, while a difunited wound of the- cornea exifts. 

 In fuch a cafe, likewifc, the prolapfus of the iris might be 

 produced by a violent fpafm of tlie mufcles of the eye, ex- 

 ceffive and long continued voiniting, or repeated coughing, 

 A prolapfus of the iris arifes even more frequently from 

 ulcers which extend through the cornea, than trom wounds 

 of that membrane ; and Scarpa alhgns as a reafon for this 

 fact, the lofs of fubllaiice always attending the former cafes. 

 The little fwelling mull of courfe be of tl.e fame colour as 

 the iris, namely, brown or grey ; and its bafe is furrounded 

 by a fmall opaque circle, formed by the cornea that has been 

 for fome time uLerated or divided. 



U feldotn happens that more than one prolapfus of the 

 iris prefents itfelf in the fame eye, fince it does not often 

 occur that the cornea is ulccraied or wounded in more than 

 one place. However, we nuiil not imagine that the iris is 

 never protruded in feveral places at once ; Scarpa has feen 

 three diftinct procidentix upon the fame cornea, in confc- 

 quence of three feparate ulcers of that membrane. 



Small as the tKmour may be, (and, indeed, it is fome- 

 times not larger than a fly's head,) it is neverthelefs apt to 

 give rife to very feverc (ymptoms. Nor are wc to wonder 

 at this circumltance, as the iris is fupplied with numerous 

 blood-velTcls and nervous filaments ; and that part of it 

 which is protruded mull fuft'er excelTive irritation, as well 

 from the friction of the eye-lids, as from the aitioii of the 

 air, tears, and other matter upon it. Cefides, it is a fa£t, 

 that the pnyecting piece of the iris inflames and fwells foon 

 3 H after 



