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I R O 



from marine vegetables. It is of a grey- 

 ish-black colour and shining metallic 

 lustre. 



I'OLITE. (from lov and \i9og, Gr.) A 

 stone, as its name implies, of a violet 

 colour. It is the prismatischer quartz of 

 Mohs, the lolith of Werner, and the lolithe 

 of Haiiy. It is found massive and dis- 

 seminated, and crystallized, in Finland, 

 Norway, Greenland, Switzerland, and 

 Spain ; in gneiss and granite. It occurs 

 in regular six and twelve-sided prisms. 

 Its fracture is conchoidal and uneven. 

 It is of a deep blue colour when seen 

 along the axis, and of a brownish yellow 

 when seen in a direction perpendicular to 

 the axis of the prism. When we look 

 alongthe resultant axes, which are inclined 

 62 50' to one another, we see a system 

 of rings which are pretty distinct when 

 the plate is thin ; but when it is thick, 

 and when the plane passing through the 

 axis is in the plane of primitive polarisa- 

 tion, branches of blue and white light are 

 seen to diverge in the form of a cross 

 from the centre of the system of rings. 

 It consists of silica, nearly 50 per cent, 

 alumina, magnesia, oxide of iron, and 

 oxide of manganese. 



IRIDE'SCENCE. (from iris, Lat. the rain- 

 bow.) The quality of shining with many 

 colours, resembling those of the rainbow. 



IRIDE'SCENT. Shining with the colours of 

 the rainbow. Many membranous shells 

 exhibit, on several parts of their internal 

 surface, a glistening, silvery, or iridescent 

 appearance. This appearance is caused 

 by the peculiar thinness, transparency, 

 and regularity of arrangement, of the 

 outer layers of the membrane, which, in 

 conjunction with the particles of carbo- 

 nate of lime, enter into the formation of 

 that part of the surface of the shell. The 

 surface, which has thus acquired a pearly 

 lustre, was formerly believed to be a 

 peculiar substance, and was termed mo- 

 ther-of-pearl ; Sir David Brewster has, 

 however, satisfactorily proved in the Phi- 

 losophical Transactions, that the iride- 

 scent colours exhibited by these surfaces 

 are wholly the effect of the parallel 

 grooves, consequent upon the regularity of 

 arrangement in the successive deposites 

 of shell. This iridescent property may 

 be communicated to shell-lac, sealing- 

 wax, gum Arabic, balsam of Tolu, or 

 fusible metal, by taking an accurate cast 

 or impression of the surface of mother-of- 

 pearl with any one of these substances. 

 Dr. Roget. 



IRI'DIUM. (from iris, Lat.) An exces- 

 sively infusible metal to which this name 

 has been given from some of its salts 

 having varied tints like those of the rain- 

 bow, and from the variety of colours ex- 



hibited by its solution. It was discovered, 

 by Mr. Tennant, in 1803, who, in ex- 

 amining the black powder left after dis- 

 solving platina, found that it contained 

 two distinct metals, which he named 

 iridium and osmium. It is of a pale 

 steel-grey colour. It occurs in grains, in 

 alluvium, in South America. 

 I'RIS. (iris, Lat. iris, Fr. iride, It. "ipig, 

 Gr.) 



1. The rainbow. 



2. The membrane round the pupil of the 

 eye, deriving its name from its various 

 colours. The colour of the iris corre- 

 sponds in general with that of the hair, 

 being blue or grey where the hair is 

 light, and brown or black where the hair 

 and complexion are of a dark colour. It 

 floats in the aqueous humour, and serves 

 to regulate the quantity of light sent to 

 the bottom of the eye. 



3. A genus of plants ; order Monogynia, 

 class Triandria ; the flag-flower. 



IRON. One of the most generally diffused 

 of all solid minerals. Of all the metals 

 the oxides of which are neither alkalies 

 nor earths, iron, geologically considered, 

 is the most important. " Calculating the 

 mean," says Mr. De La Beche, ' of 

 thirty kinds of rocks, and neglecting iron 

 ores, properly so called, of every kind, 

 iron constitutes, as an oxide, 5'5 of the 

 lowest stratified rocks, amounting to 

 14-72 per cent, in mica slate with gar- 

 nets, and 15*31 per cent, in chlorite slate. 

 It forms 12'62 per cent, in hypersthene 

 rock, and about 20 per cent, in basalts. 

 Oxide of iron constitutes about two and 

 three per cent, of the mass of granites and 

 gneiss, and between three and four per 

 cent, of the mass of greenstone and the 

 more common trappean rocks. When 

 we consider the large amount of iron 

 which exists either in the state of an 

 oxide, a carbonate, a carburet, a silicate, 

 or a sulphuret, therefore including all 

 iron ores of importance, we shall probably 

 not err greatly if we estimate iron as 

 constituting about 2 per cent, of the 

 whole mineral crust of our globe. There 

 is scarcely a rock without iron. Geolo- 

 gical Researches. 



It is to the presence of iron that rocks 

 and stones most frequently owe their 

 colour, earths when pure being white. 

 The specific gravity of all stones or earthy 

 minerals if it much exceed 2'5 may be 

 attributed to the presence of iron. 



In its natural state iron is very unlike 

 what we are hourly accustomed to see it. 

 It presents itself every where only as an 

 earthy mass, a dirty impure rust ; and 

 even when found in the mine with a me- 

 tallic lustre, it is still far from possessing 

 those qualities which are necessary to fit 



