IS 15. J On Iodine. 213 



We shall endeavour to answer this question hereafter. We shall 

 terminate what concerns the action of the oxides on iodine by 

 treating of those in which the oxygen is very little condensed, as 

 the oxides of mercury, gold, and silver. 



M. Colin has observed, that when we expose to a heat between 

 140° and 212° a mixture of water, iodine, and peroxide of mercury, 

 super-iodate of mercury is formed, which is held in solution in the 

 water and an iodate with excess of base, which is insoluble, and 

 remains mixed with the red ioduret which is formed at the same 

 time. Oxide of gold treated in the same manner does not appear to 

 form an ioduret; for after a great number of washings, metallic 

 gold remains, and the water holds an acid iodate in solution. We 

 may suppose thai the water has been decomposed, and that the 

 oxides of mercury and gold act upon iodine like the alkalies ; but 

 if we consider that the oxide of zinc does not form an iodate, it 

 will become exceedingly probable that the iodic acid has been 

 formed at the expense of the oxygen of a part of the oxide. If we 

 recapitulate the action of the oxides on iodine, it will appear, 



1 . That the alkaline oxides in which the oxygen is very much 

 condensed, and which completely neutralize the acids, occasion 

 with iodine the decomposition of water, and produce iodates and 

 hydriodates. 



2. That the metallic oxides in which the oxygen is still very 

 much condensed, though less than in the preceding, and which do 

 not completely neutralize the acids, do not produce with iodine a 

 force sufficiently great to decompose water and form iodates. 



8. That the oxides in which the oxygen is weakly condensed 

 cannot concur with iodine in decomposing water; but that they 

 convert iodine into an acid by yielding to it a portion of their 

 oxygen. 



Such are the general results of the action of iodine on the 

 oxides. We shall afterwards treat in detail of some saline combina- 

 tions of iodine ; but at present we propose to state the properties of 

 iodic acid. 



VVe have seen that the acid is only formed by the concurrence of 

 different forces, and that it is always combined with some base, 

 from which, of course, we must separate it. At first sigiit it seems 

 tliat we might treat with hydro-sulphuric acid the iodates oF easily 

 reducible metals ; but the iodic acid would be at tlie same time 

 decomposed by this acid, because its elements are very little con- 

 densed.* After various trials, 1 adopted the following process: — 

 Upon iodate of barytcs I pour sulpliuric acid diluted witli twice its 

 weight of water, and heat the mixture. The iodic acid quickly 

 abiiiidons a portion of its base, and combines with the water, liut 

 ahhougli a quantity of sulphuric acid was employed not suflicient to 



• II>ilro-iul|>liurir acid may be employed to tiecoinposc iihosphnte of lead an«l 

 •biain plid'-phuric acid. 



