The action of iodine on the oxides takes place both with and without the presence of water, but exhibits dif ferent phenomena, according to this difference of circum stances. When brought in contact with the oxide of pot assium produced by combustion, it combines with the pot assium to Corm an ioduret, and the oxygen is disengaged.
Bar) tes, strontites, and lime, unite with iodine, without the evolution of any gas. The oxides of zinc and iron un dergo no change. The iodurets of barytes, strontites, and lime, show strong alkaline properties when dissolved in water, and are considered by Gay-Lussac as subiodu rets.
\Vhen iodine and oxides are made to act on one another in contact with water, the water is decomposed, its hydro gen forms hydriodic acid with a portion of the iodine, and its oxygen combines with another portion to form oxiodic acid. This takes place with potass, soda, barytes, stron tites, lime, and magnesia.
Iodine, when made to act on a concentrated solution of potass, is rapidly dissolved, and forms, as one of the re sults, a granular white precipitate, which is shown to con sist of oxiodic acid, combining to form an oxiodatc with potass ; for this substance dcflagrates on burning coals like nitre, giving out oxygen, and becoming ioduret of' potass. The solution consists of hydriodate of potass, con taining iodine. It is of an orange yellow, but, when satu rated with iodine, it is of a deep reddish-brown. When it is evaporated, and heated to redness, an ioduret of pot assium is formed. In the first part of this process, the oxygen, according to Sir H. Davy, is expelled from one part of the potass, to form the ioduret of potassium. and is united to another to form the oxiodate of potass. But Gay Lussac's view of it, that water is decomposed, its oxygen serving to form the oxiodate, and its hydrogen the hydri odate, seems more completely conformable to the facts. Sir H. Davy, from his tirst experiments, conceived that iodine formed substances analogous to alkalis, by combin ing with the alkaline metalloids; for the compound pro duced by its action on solution of potass, even when it was in great excess, reddened turmeric paper, and rendered the colouring matter of violets green ; but he afterwards found that this was owing to a small qu unity of supercar bonate of potass which existed in the hydrate ; and when the compound is treated with hydriodic acid, and heated to redness, so as to produce an ioduret, it loses this property, and acquires the taste of a neutral rather than an alkali.
Concentrated soda produces similar phenomena ; a della grating oxiodate is precipitated, and a hydriodate retained in solution. The case is the same with barytes, lime, and strontites, and their oxiodates have little solubility. Hence these salts are obtained in a state of considerable purity.
\Vhen peroxide of mercury is exposed to a heat between 140° and 212° in water and iodine, a super-oxiodate of mer cury is formed, which is held in solution, and an oxiodatc which is insoluble, and remains mixed with the red iodu ret, which is at the same time formed.
The oxiodic acid, in order that its properties may be ascertained, ought to be obtained in a state of separation from the bases with which it is combined in its formation. For this purpose Gay-Lussac adopted the following pro cess : Upon oxiodate of barytes, he poured sulphuric acid, diluted with twice its own quantity of water, and heated the mixture. The oxiodic acid in this case, according to him. abandoned its earthy base, and combined with the water. A small quantity of the sulphuric acid continued mixed with it, in consequence of the strong affinity of ox iodic. acid for barytes. Sir H. Davy, however, found reason to be dissatisfied with these results, as disguising the real Properties of this compound : and he succeeded in produc ing one in a pure state, by bringing together iodine and cuchlorine, (hyperoxymuriatic acid,) it, the form of gas, at the ordinary temperature of the atmosphere. The result ing compound, when the other ingredients were driven off by heat, was a white semitransparent solid, of an astrin gent and acid taste, and sufficiently heavy to sink in sul phuric acid It is deliquescent : its aqueous solution may he evaporated to the consistence of a syrup, and, by the continuance of heat, may be reduced to the original solid compound. It detonates when heated in mixture with in flammable substances. The oxiodic acid obtained by Gay Lussac differed from this, by containing a portion of sul phuric acid, which prevented that chemist from being able to procure it in a solid form, and led him to believe that water was essential to its constitution. This acid cannot be procured by the direct union of iodine with oxygenous gas.