CHLORINE, BROMINE, IODINE, AND FLUORINE. 169 



228. Iodine. While chlorine is a constituent of the salt 

 of the sea, iodine is found in many of the sea's products, as 

 sea- weed, sponge, etc., in combination with sodium and other 

 metals. It is commonly obtained by making a lye from the 

 ashes of sea-weeds, called kelp, and separating the iodine 

 from this lye by a chemical process. The lye is evaporated 

 till all the sodium carbonate and other salts in it are crys- 

 tallized, and the remaining liquor, after being treated with 

 sulphuric acid, is heated gently with manganese dioxide in 

 a leaden retort, a b c,Fig. 71, the iodine passing out in va- 



Fig. 71. 



por, and being condensed in the successive receivers, d. The 

 action of the manganese dioxide is the same as in the cor- 

 responding method of preparing chlorine. After the dis- 

 covery of iodine, in 1811, by M. Courtois, of Paris, the prep- 

 aration of kelp became quite a large business on the coast 

 of Scotland. Iodine is chiefly used in the arts in the proc- 

 ess of dyeing and in the making of photographic pictures. 

 It is also used in medicine. 



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