lOS On Iodine. [Feb. 



soluble in water, and rapidly deliquesces in the air. It dissolves in 

 water witiiout the evolution of any gas. The solution is slightly 

 acid, and does not crystallize. The alkalies precipitate from it the 

 white oxide of zinc, concentrated sulphuric acid disengages hy- 

 driodic acid and iodine, because sulphurous acid is produced. VVe 

 may conceive that water dissolves the ioduret of zinc without under- 

 going decomposition ; but as the slightest force would afterwards 

 decompose it, and besides, as the solution has exactly the same 

 characters as the hydriodate of zinc obtained by combining the 

 oxide of zinc with hydriodic acid, we have the same motives for 

 admitting that the water is decomposed during the solution of the 

 ioduret, as for admitting that it is formed when hydriodic acid dis- 

 solves the oxide. We may, however, adopt either supposition. 

 For the sake of greater simplicity I shall adopt the latter, in order 

 to determine the ratio of iodine to oxygen and hydrogen. 



When iodine and zinc are made to act on each other under 

 water in vessels hermetically sealed, on the application of a slight 

 heat the water assumes a deep reddish brown colour, because as 

 soon as hydriodic acid is produced it dissolves iodine in abundance. 

 But by degrees, the zinc which I suppose in excess, combines 

 with the whole iodine, and the solution becomes colourless like 

 water. In three experiments, which differed little from each other, 

 and of which 1 have taken the mean, I found that 100 iodine 

 combined with 26'225 zinc. But 26*225 zinc combine with 

 6'102 oxygen, which saturate 0849 hydrogen; consequently, the 

 ratio of oxygen to iodine is 6-402 to 100, or 10 to 156-21, and the 

 ratio of hydrogen to iodine is 0-845) to 100, or 1-3268 to 156-21. 

 Thus if we represent the number for oxygen with Dr. WoUaston 

 by 10, the number for iodine will be 156-21 ; the ratio which I 

 assigned in my first experiments, as well as that of Davy, was very 

 inaccurate. 



Iron is acted upon by iodine in the same way as zinc. The 

 ioduret of iron is brown, and fusible at a red heat. It dissolves in 

 water, and the colour of the solution is a light green, like that of 

 the chloruret of iron. 



Iodine and potassium combine with a great deal of heat, and 

 with the disengagement of a light which appears violet through the 

 vapour of iodine. The compound melts, and sublimes at a red 

 heat. On cooling it assumes a pearly and crystalline appearance. 

 Its solution in water is perfectly neutral. It is easy to determine 

 tiie proportion of these iodurets from those of ioduret of zinc. If 

 we attend to this circumstance, that the quantities of iodine which 

 combine with each metal, are proportional to the quantities of 

 oxygen with vviiich it combines; of course 100 potassium, which 

 requires 20-4 25 oxygen to convert it into potash, combine with 

 319-06 of iodine. 



The ioduret of tin is very fusible. When in powder, its colour 

 is a dirty orango yellow, not unlike that of glass of antimony. 

 When put into ii considerable quantity of water it is completely 



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