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I R I 



PODIJT. I (from ioeifys, ex "ov, violet, 

 I'ODINE. j and eldos, appearance, Gr.) 

 This substance, which was discov- 

 ered by Courtois, a manufacturer 

 of salt-petre, at Paris, in 1812, 

 obtained its name from the colour 

 of its vapour, which is a beautiful 

 violet. Iodine is procured from 

 sea-water and from marine vegeta- 

 bles. It is of a greyish-black 

 colour and shining metallic lustre. 

 It is crystallizable ; the primitive 

 form of the crystals being a rhombic 

 octahedron. Iodine possesses an 

 extensive range of combination, 

 forming acids both with oxygen, 

 hydrogen, and chlorine. Iodine 

 forms one of the simple or element- 

 ary bodies, included in those that 

 are non-metallic. 



POLITE, (from I'ov, violet, and X/flos, 

 a stone, Gr.) A stone of a violet 

 colour. The Prismatischer Quartz 

 of Mohs, the lolith of Werner, the 

 lolithe of Haiiy, the Dichroite of 

 Cordier, and the Cordierite of 

 Leonhard. It is found massive 

 and disseminated, 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 along the 

 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; 

 bnt 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, 

 mangnesia, oxide of iron, and oxide 

 of maganese. It was first brought 



to France by Launoy, from Cape 

 de Gatte, in Spain. Specific 

 gravity 2'560. It scratches glass 

 easily, quartz with difficulty. 



IEIDE'SCENE. (from iris, Lat. the 

 rainbow.) The quality of shining 

 with many colours, resembling 

 those of the rainbow. 



IEIDE'SCENT. Shining with the col- 

 ours of the rainbow. Many mem- 

 branous shells exhibit on several 

 parts of their internal surface, a 

 glistening, silvery, or iridescent ap- 

 pearance. This appearance is caused 

 by the peculiar thinness, transpa- 

 rency, and regularity of arrange- 

 ment, of the outer layers of the 

 membrane, which, in conjunction 

 with the particles of carbonate of 

 lime, enter into the formation of 

 that part of the surface of the shell. 

 The surface, which has thus ac- 

 quired a pearly lustre, was formerly 

 believed to be a peculiar substance, 

 and was termed mother-of-pearl; 

 Sir David Brewster has, however, 

 satisfactorily proved in the Philo- 

 sophical 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 shells. 



IEI'DIUM. (from iris, Lat.) An ex- 

 cessively infusible metal to which 

 this name has been given from some 

 of its salts having varied tints like 

 those of the rainbow, and from the 

 variety of colours exhibited by its 

 solution. It was discovered by Mr. 

 Tennant, in 1 803, who, in examin- 

 ing the black powder left after dis- 

 solving platina, found that it con- 

 tained 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. From the re- 

 searches of Berzelius, who estimates 

 the equivalent of Iridium at 98-8, 

 it appears to have three degrees of 

 oxidation ; and it is the rapid tran- 



