Dry iodine, when presented to chlorine gas, rapidly ab sorbs it in a quantity less than one-third by weight, pro ducing a compound, which in some parts has a fine orange vellow colour ; in others an orange red. The yellow parts contain the largest proportion of chlorine, and arc the most volatile. The yellow compound is called, by Gay-Lussac, the chloruret, and the red the sub-chloruret of iodine. Both of them speedily deliquesce in the air. The solution of the chloruret is colourless, when the excess of chlorine is driven off, and then the mutual saturation of the two con stituents appears to be complete. These solutions are very acid, and destroy the colour of a soiution of indigo in sulphuric acid. The chloruret receives from Sir H. Davy the name of chlorionic acid. When a solution °lit is satu rated with an alkali, the chlorine requires hydrogen, and the iodine oxygen, so that a muriate and an oxiodate of the alkali are obtained. Such are the relative tendencies of these two substances That of combining with hydro gen is greatest in chlorine, and that of combining with oxy gen is greatest in iodine. Heat disengages part of the chlo rine, and reduces the solution to a sub-chloruret. The solution of the sub-chloruret is more stable, and capable of being volatilized without decomposition.
We shall now describe the properties of the neutral salts formed by combinations of the acids of iodine with salifiable bases. First the hydriodates, and then the oxio dates.
The hydriodates may, in general, be obtained by com bining hydriodic acid with the bases. But those of potass, soda, harytes, strontian, and lime, may also be prepared, by treating iodine t% ith these bases, and employing the methods already described for separating these salts from the oxiodates, which are formed at the same time. The hydriodates of zinc and iron, and in general of all the metals that decompose water, are obtained by dissolving the iodurets of those metals in water, and applying heat.
The hydriodates arc not changed by sulphurous or mu tiatic acid, or sulphuretcd hydrogen. Chlorine, nitric acid, and concentrated sulphuric acid, constantly decom pose them, and separate the iodine.
When a solution of hydriodate of potass is made to crystallize, the oxygen of the potass and the hydrogen of the acid unite, according to Gay-Lussac, to form water, i and crystals of ioduret of potassium are formed ; these are considered by that chemist as completely analogous to the compound of chlorine and sodium, his chloruret of so dium, the sodanc of Sir H. Davy, and the muriate of soda of hornier chemists. That there is no alkali in these com pounds, and that the appearance of alkaline properties, neutralized by an acid, arises from the chlorine supplying the place both of oxygen and acid, though it contains no oxygen and possesses no acid qualities, is one of the strik ing paradoxes which the adherents of the old school find most difficulty in admitting, and which, in determining the plausibility of the two theories, seems to form a suffi cient counterpoise to the curious coincidencies in some other particulars by which the new doctrines are support ed. Ilydriodate of potass is composed of
Ilydriodate of soda may be obtained in flat rhomboidal prisms of considerable size. These unite to form larger ones, terminated in echelon, and striated like crystals of sulphate of soda. They contain a large proportion of wa ter of ci ystallization, and are very deliquescent. Heat drives off the water, melts the salt, and then renders it slightly alkaline. It does not sublime so easily as hydrio date of potass. At the temperature of 57, 100 parts of this salt are soluble in a little more than 57 parts of water. When dried, Gay-Lussac considers it as an ioduret of so dium, conformably to the doctrine already mentioned, which makes the salt called muriate of soda a chloruret of sodium. Ilydriodate of soda is composed of Ilydriodate of barytes crystallizes in fine prisms, like those of mut kite of strumites. These are very gradually decomposed by exposure to the air, and iodine is evolved. They are faintly deliquescent, but have great solubility in water. They bear, without change, a vet y strong heat ; and heat neither melts this salt, nor alters its state of neutraliza tion ; but, if oxygen is made to play on its surface when thus heated, vapours of iodine arc evolved, and the salt be comes alkaline. A red heat, according to the opinion of the chemist just mentioned, converts it into an ioduret of barium. Hydriodate of barytes is composed of The hydriodates of lime and strontitcs are very soluble. That of lime is also deliquescent, and has a bitter taste, similar to that of the muriate of lime.
Hydriodate of ammonia may be formed, either by com bining equal volumes of ammoniacal and hydriodic gas, or by saturating the liquid acid with ammonia. It possesses nearly the volatility of 'initiate of ammonia, but it is inure soluble and more deliquescent. It ci ystallizes in cubes.
The hydriodate of magnesia, formed by uniting its con stituents, is deliquescent, and crystallizes wtth difficulty. When heated to redness, the acid abandons the magnesia, in the same way as takes place with the muriate of this earth.
The hydriodate of zinc is obtained by putting iodine in water with an excess of zinc, and applying heat. This salt is extremely deliquescent, and scarcely crystallizable by evaporation. Heat deprives it of its water, then melts it, and sublimes it in fine prisms. If this is performed in close vessels, the salt is not decomposed ; but air, when admitted, disengages iodine, and oxide of zinc is left be hind. This hydriodate is composed of Acid 100.