sition of these oxides into each 

 other, that occasions the variable 

 tints of iridium. 



I'KIS. (iris, Lat. iris, Er. iride, It. 

 fas, 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 corresponds 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. 



I'EISATED. A mineral is described as 

 irisated which exhibits the pris- 

 matic colours either externally or 

 internally ; the latter is generally 

 the consequence of some injury 

 sustained by the mineral. 



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 De La Beche, " of 

 thirty kinds of rocks, and neglect- 

 ing 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 garnets, and 

 15-31 per cent, in chlorite slate. 

 It forms 12-62 per cent, in hypers- 

 thene rock, and about 20 per cent, 

 in basalts. Oxide of iron consti- 

 tutes about two or 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 



[ 237 ] I E 



carburet, a silicate, or a sulphuret, 

 therein including all iron ores of 

 importance, we shall probably not 

 errr greatly if we estimate iron as 

 constituting about 2 per cent, of 

 the whole mineral crust of our 

 globe. There is scarcely a rock with- 

 out iron. Geological 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 at- 

 tributed to the presence of iron. 



In its natural state iron is very 

 unlike what we are hourly accus- 

 tomed to see it. It presents itself 

 everywhere only as an earthy mass, 

 a dirty impure rust; and even 

 when found in the mine with a 

 metallic lustre, it is still far . from 

 possessing those qualities which 

 are necessary to fit it for the endless 

 uses to which it is applied. Man 

 has only to purify gold, silver, &c. 

 but he has, as it were, to create 

 iron. It does not appear to have 

 been known so early, or wrought 

 so easily, as gold, silver, and copper. 

 For its discovery we must have 

 recourse to the nations of the East. 

 The writings of Moses furnish us 

 with the most ample proof at how 

 early a period it was known in 

 Egypt and Phoenicia. He mentions 

 furnaces for working iron, "and 

 brought you out of the iron fur- 

 nace;" the ores from which iron 

 was extracted, " a land whose 

 stones are iron;" and he states 

 that swords, knives, axes, and tools 

 for cutting stones, were at that 

 time made of iron, "and if he 

 smite him with an instrument of 

 iron, so that he die, he is a mur- 

 derer," "and his hand fetcheth a 

 stroke with the axe to cut down 

 the tree," " thou shalt not lift up 

 any iron tool upon them." The 

 knowledge of iron was brought 

 over from Phrygia to Greece by 



