386 IODINE. 



sulphate of iron, which throws down an insoluble subiodide of 

 copper, almost white. 



Properties. Iodine is generally in crystalline scales of a 

 bluish black colour and metallic lustre. It may also be ob- 

 tained, from solution, in the form of elongated octohedrons with 

 a rhomboidal base. The density of iodine is 4.948 ; it fuses at 

 225, and boils at 347 ; but it evaporates at the usual tempera- 

 ture, and more rapidly when damp than when dry, diffusing an 

 odour having considerable resemblance to chlorine, but easily 

 distinguished from it. Iodine stains the skin of a yellow co- 

 lour, which however disappears in a few hours. Its vapour 

 is of a splendid violet colour, which is seen to great advantage 

 when a scruple or two of iodine is thrown at once upon a hot 

 brick. Hence its name, from 'Iwc^s, violet-coloured. The 

 vapour of iodine is the heaviest of gaseous bodies, its density 

 being 87 16 according to the experiment of Dumas, and 8707-7 

 according to calculation from its atomic weight. 



Pure water dissolves about l-7000th of its weight of iodine, 

 and acquires a brown colour ; but when charged with a salt, 

 particularly the nitrate or hydrochlorate of ammonia, water 

 dissolves a considerably greater quantity of iodine. The solu- 

 tion of iodine does not disengage oxygen in the light of the sun, 

 and does not destroy vegetable^ colours, but after a time it 

 becomes colourless, and then contains hydriodic and iodic acids. 

 In other respects, iodine generally comports itself like chlorine, 

 but its affinities are much less powerful. Iodine is soluble in 

 alcohol and ether^ with which it forms dark reddish brown 

 liquors. Solutions of iodides, too, all dissolve much iodine. 

 A liquid containing 20 grains of iodine and 30 grains of iodide 

 of potassium in 1 ounce of water, is known as Lugol's solution, 

 and preferred to the tincture in medicine, because the iodine is 

 not precipitated from it by dilution with water. 



A solution of starch forms an insoluble compound with 

 iodine, of a deep blue colour, the production of which is an 

 exceedingly delicate test of iodine. If the iodine be free, starch 

 produces at once the blue precipitate, but if it be in combination 

 as a soluble iodide, no change takes place, till chlorine is added 

 to liberate the iodine. If more chlorine, however, be added 

 than is necessary for that purpose, the iodine is withdrawn from 

 the starch, chloride of iodine formed, and the blue compound 

 destroyed. Dr. A. T. Thomson, after adding the starch with a 



