384 



IODINE. 



and the kelp prepared from this plant may be recognized by 

 the presence of charred portions of the stems ; and as that is a 

 deep sea plant, it is found in largest quantity in the sea-wreck 

 of exposed coasts. A high temperature in the preparation of 

 the kelp, which increases the proportion of alkaline carbonate, 

 diminishes that of the iodine, owing to the volatility of the 

 iodide of sodium at a full red heat. The kelp which contains 

 most iodine, generally contains also most chloride of potassium, 

 and it is for these two products that the substance is now va- 

 lued, more than for its alkali. 



The kelp broken into small pieces is lixiviated in water, to 

 which it yields about half its weight of salts. The solution is 

 evaporated down in an open pan, and when concentrated to a 

 certain point, begins to deposit its soda salts, namely, common 

 salt, carbonate and sulphate of soda, which are removed from 

 the boiling liquor by means of a shovel pierced with holes like 

 a colander. The liquid is afterwards run into a shallow pan to 

 cool, in which it deposits a crop of crystals of chloride of potas- 

 sium - } the same operations are repeated upon the mother-ley of 

 these crystals until it is exhausted. A dense dark-coloured 

 liquid remains, which contains the iodine, in the form, it is 

 believed, of iodide of sodium, but mixed with a large quantity 

 of other salts, and this is called the iodine ley. 



To this ley, sulphuric acid is gradually added in such quan- 

 tity as to leave the liquid very sour, which causes an evolution 



of carbonic acid, sul- 

 phuretted hydrogen, 

 and sulphurous acid 

 gases, with a conside- 

 rable deposition of 

 sulphur. After stand- 

 ing for a day or two, 

 the ley so prepared, 

 is heated with per- 

 oxide of manganese, 

 to separate the iodine. 

 This operation is con- 

 ducted in a leaden 

 retort a (see figure) 

 of a cylindrical form, 

 supported in a sand- 



FIG. 42. 



