ISI 



402 



ISO 



applied tc 

 of which 



cilc gravity and hardness, in both of 

 which it exceeds ebony. The species ge- 

 nerally inhabit the southern parts of 

 Africa : one occurs in New Holland. 



IRRA'DIATION. In physics and astronomy, 

 the apparent enlargement of an object 

 Strongly illuminated. 



IRRA'TIOSAL. In algebra and arithmetic 

 to numbers or qualities, the roots 

 rhich are incommensurable with 

 unity, and therefore cannot be accurately 

 extracted. Thus ^/2 is irrational, as it 

 cannot be expressed by a finite number. 



IRREGULAR CADENCE. In music, one 

 which does not end upon the essential 

 chord of the mode in which a piece is 

 composed. 



IRRITABI'LITT, from irrito, to provoke. 

 A property peculiar to muscles, by which 

 they contract upon the application of cer- 

 tain stimuli, without a consciousness of 

 action. Even when the body is dead, and 

 the nervous sensibility gone, the irrita- 

 bility of the muscles remains till the or- 

 ganisation begins to be dissolved. This 

 muscular irritability descends even to 

 the vegetable kingdom, and gives a pe- 

 culiarity to what are called sensitive 

 plants. The muscular parts of the body 

 have all the irritability of the system, 

 with but little feeling, and that little 

 owing to the nerves which enter into 

 their substance, while nerves have all the 

 sensibility of the system with no motion. 



IS'AOON, from iirof, equal, and '/mice,, 

 an angle. A figure whose angles are 

 equal. 



IS'ATIS. 1. In botany, a genus of herba- 

 ceous plants. Tetradynamia Silicttlosa. 

 The I. tinctoria, an indigenous biennial, 

 affords the dye-stuff called woad, now well 



nigh being superseded by indigo. 2. In 



zoology, the arctic fox or Cams lagopus, 

 Lin. 



ISBRAN'IKI. In ecclesiastical history, a 

 name of the Russian sect of Kaskolniks. 



ISCHIA'GRA, Lat. from nr%iov, the 

 haunch, and a-yea, a seizure. A name 

 for hip-gout, and also for neuralgia of the 

 sacro-sciatic nerve. 



IS'CHIAS, i<r%iat;, from icr%tov, the hip. 

 A rheumatic affection of the hip-joint; 

 the hip-gout. 



IS'CHIUM, Lat. from nr%t; , the loin. A 

 bone of the pelvis of the fetus, and a 

 part of the os innominatum of the adult. 

 See INNOMIHATCM, Os. 



ISCH'CRT, from itr^u, to retain, and 

 evftv, the urine. A retention of the urine ; 

 distinct from dysury. 



I'SINOLASS, for ice-glass, fish-elue. A 

 manufacture almost peculiar to Russia. 

 It is gelatine, nf.rly pure. Prepared 

 from the air-bladders, or sounds of stur- 

 geons, especially the great sturgeon (Ac- 



cipenser puso, Lin.) fished on the shores 

 of the Caspian Sea, and in the rivers which 

 flow Into it, almost entirely for the sake 

 of its swim-bladder. The best is that 

 made into little ringlets, &c. ; the second 

 is laid together like the leaves of a book ; 

 the third put up without care. 



IS'LAMIS, ) A term used by Mohamme- 



IS'LAMISM. j dans to signify the true 

 faith : Mohammedanism. 



ISMAE'LIANS, a Mohammedan sect, from 

 whom the famous assassins sprung in the 

 10th century. 



ISOCHEI'MAL, from iirof, equal, and 

 Xiifjux, winter. Of the same winter tem- 

 perature. Isocheimal lines are those drawn 

 through places having the same winter 

 temperature. 



ISOCHROMA'TIC, tfof, and %ouu,, co- 

 lour. Having the same colours. 



ISOCH'HOXAL, ) from ttroi, equal, and 



ISOCH'RONOCS, I z?0 vef, time. Equal 

 time, performed in equal times. The vi- 

 brations of a pendulum are isochronal. 



ISOD'OMUM, tiros and ., structure. 

 In ancient architecture, a species of wall- 

 ing, in which all the courses were of the 

 same height. 



ISOGEOTHER'MAL, from iaro(, equal, yr, t 

 the earth, and Oig/tAo;, warmth. Of equal 

 terrestrial temperature. Isogeothermal 

 lines are those divisions in the earth's 

 crust which have an equal degree of mean 

 annual temperature. 



IS'OLATED, ) from Ital. isola, an isle. 



I.N'SCLATED, / An isolated harbour, is one 

 of refuge built independently of the coast, 

 and connected to it by a bridge, under 

 which the shingle is allowed to "pass. 



ISOM'ERISM, i<rog , equal, and /u.i%o; , part. 

 The condition of compounds which contain 

 the same elements in the same propor- 

 tion, and yet possess distinct chemical 

 qualities. They are said to be isomeric. 



ISOMORPHISM, from i/rot, like, and 

 fju>%/pq, form. That quality of certain bo- 

 dies, by which they are capable of re- 

 placing each other in compounds, without 

 change of the primitive form. 



ISOPERIM'ETRT, from urc*, equal, met, 

 about, and ^jr^ov, measure. A branch 

 of the higher geometry, which treats of 

 the properties of isoperimetrical figures, 

 i.e. surfaces contained under equal peri- 

 meters or boundaries. 



ISOP'ODA, isopods. An order of Crusta- 

 ceans characterised by having uniformly 

 fourteen equal feet (uro;, equal, and -rev? , 

 a foot), unguiculated, and without any 

 vesicular appendage at their base. Most 

 isopods are aquatic, and those which are 

 terrestrial require a certain degree of hu- 

 midity to enable them to breathe. Lir.nxuf 

 comprises then? ill in hi.> genus 6mt 



