1815.] On Iodine. 301 



at a red heat; but the muriate of lime through which the gas after- 

 wards passed, did not increase sensibly in weight. Hence every 

 thing leads us to consider the hydriodates that have been melted or 

 dried, as converted into metallic iodurets. The hydriodate of lime 

 made with hydriodic acid, may be dried in the air without being 

 decomposed. On the contrary, what has been made with iodine 

 and lime becomes deep coloured as it is concentrated, though we 

 evaporate at a very moderate heat. The reason is, that this last 

 hydriodate holds in solution a certain quantify of iodate, and these 

 two salts have the property of 'decomposing each other, when 

 brought to a certain degree of concentration. The hydrogen of 

 the hydriodic acid and the Oxygen of the iodic acid form water, 

 and the .iodine which is thus disengaged is dissolved in the unde- 

 composed portion of hydriodate, and gives it a reddish brown 

 colour. The whole hydriodate is not destroyed, because there is 

 only a small quantity of iodate present ; and when calcined in close 

 vessels, it is completely freed from colour.* 



Hydriodate of ammonia results from the combination of equal 

 volumes of ammoniacal gas and hydriodic gas. It is usually pre- 

 pared by saturating the liquid acid with ammonia. It is nearly as 

 volatile as the hydrochlorate of ammonia ; but it is more soluble 

 and more deliquescent. I have obtained it crystallized in cubes. 

 When heated in close vessels only a very small portion of it is de- 

 composed. What sublimes is greyish black. If it be sublimed in 

 contact of air, a much greater proportion of it is decomposed, and 

 it becomes more or less coloured. It may be deprived of its colour 

 by adding a little ammonia, or by exposing it to dry air. In the 

 last case, tiie iodine to which it owes its colour is gradually 

 volatilized. 



The hydriodate of magnesia, formed by uniting its constituents 

 together, is deliquescent, and crystallizes with difficulty. When 

 heated to redness in close vessels, the magnesia abandons the acid 

 in the same way as it abandons the hydrochloric acid. Having 

 heated together iodine, magnesia, and water, to ascertain whether 

 hydriodate and iodate was formed, as liappens with the other alka- 

 lies, I obtained a flocky compound, which exactly resembled well 

 prepared kermes. Tlie liquid which coovcred it was scarcely 

 coloured, and I ascertained in it the presence of hydriodate and 

 iodate of magnesia, but in very small quantity. When evaporated, 

 a flea-coloured matter is deposited on the sides of the vessel, 

 quite similar to that of wiiich 1 have spoken ; and towards the end 

 of the process, the liquid acquires a deep colour. This pheno- 

 menon is owing (as in the case of lime) to the mutual decompo- 



• To evapnrntf or ralciuc the hyilriodales without (he contact of air, I put 

 th«in into a rolort, to (he l»p.ik of which I fit n tube, which, alter having re- 

 ceived ific form priipcr for colli-cling piics, liscs al its exlr»iiiit^' p.nallcl to the 

 descending tirumli, and a,suciie!i nearly (In; sliap<; ( f llie Icder U. When the 

 • qucou» vaponr lia- ex|)ellcil all the uii of the retort, I place the ascending; brsncli 

 under a gUjijar iilltd with hy<lrogcnous a2utic gas, above the level of the water. 



