IODINE. 387 



drop of sulphuric acid to the liquid containing an iodide, in a 

 cylindrical vessel, allows the vapour only from the chlorine- 

 water bottle to fall upon the solution, and not the chlorine- 

 water itself. In this way, the danger of adding an excess of 

 chlorine is easily avoided, and the test indicates in a sensible 

 manner an exceedingly minute quantity of iodine. The iodide 

 of starch, in water, becomes colourless when heated, but re- 

 covers its blue colour if immediately cooled. The soluble 

 iodides give, with nitrate of silver, an insoluble iodide of silver, 

 of a pale yellow colour, insoluble in ammonia ; with salts of 

 lead, an iodide of a rich yellow colour, and with corrosive subli- 

 mate, a fine scarlet iodide of mercury. 



In ascertaining the quantity of iodine in the mixed chlorides, 

 and iodides of mineral waters and other solutions, Rose recom- 

 mends the addition of nitrate of silver, which throws down a 

 mixture of chloride and iodide of silver, which is fused and 

 weighed. This is afterwards heated in a tube and chlorine passed 

 over it, by which the iodine is expelled, and the whole becomes 

 chloride of silver. It is weighed again, and a loss is found to 

 have occurred, owing to the equivalent of the replacing chlorine 

 being less than that of the replaced iodine. This loss, multi- 

 plied by 1.389, gives the quantity of iodine originally present, 

 which has been expelled by the chlorine.* Dr. Schweitzer em- 

 ploys a similar method in estimating the quantity of iodine when 

 mixed with bromine, heating the iodide and bromide of silver 

 in an atmosphere of bromine. The difference in weight multi- 

 plied by 2.627 gives the proportion of iodine, and multiplied by 

 1.627 the proportion of bromine-t 



Uses. Iodine is employed in the laboratory for many chemi- 

 cal preparations, and as a test of starch. It was first introduced 

 into medicine by Coindet of Geneva, who employed it with suc- 

 cess, in the treatment of goitre, dissolved in alcohol, in solution 

 of iodide of potassium, or as iodide of sodium ; and since that 

 application, most mineral waters to which the virtue of curing 

 goitre was ascribed, have been found to contain iodine. 

 M. Boussingault has adduced striking confirmations of the 

 efficacy of iodine in that disease, in his interesting memoir 



* Handbuch der analytischen Chemie von Heinrich Rose, B. 2, p. 577, Ber- 

 lin, 1838. 



| Phil. Mag. 3rd. series, v. 15, p. 57. 



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