TlIK HALOCKNS 491 



separates as a precipitate. To obtain pure iodine it is enough to distil 

 it, and neglect the first and last portions of the distillate, the middle 

 portion only being collected. Iodine passes directly from a state of 

 vapour into a crystalline form, and settles on the cool portions of the 

 apparatus in tabular crystals, having a black grey colour and metallic 

 lustre. 63 



The specific gravity of the crystals of iodine is 4-95. It melts at 

 114 and boils at 18-4. Its vapour is formed at a much lower tempera- 

 ture, and is of a violet colour, from whence iodine receives its name. 

 The smell of iodine calls to mind the characteristic smell of hypochlorous 

 acid ; it has a sharp sour taste. It destroys the skin and organs of the 

 body, and is therefore frequently employed for cauterising and as an 

 irritant for the skin. In small quantities it turns the skin brown, but 

 the coloration disappears after a certain time, partly owing to the 

 volatility of the iodine. Water dissolves only -,,, 1 ,,-,, part of iodine. A 

 "brown solution is thus obtained, which bleaches, but much more feebly 

 than bromine and chlorine. Water which contains salts, and especially 

 iodides, in solution dissolves iodine in considerable quantities, and the 

 resultant solution is of a dark brown colour. Pure alcohol dissolves a 

 small amount of iodine, and in so doing acquires a brown colour, but 

 the solubility of iodine is considerably increased by the presence of a 

 small quantity of an iodine compound for instance, ethyl iodide in the 

 alcohol.*' 3 Ether dissolves a larger amount of iodine than alcohol ; but 



winch corresponds to the higher stage SO-, is able to act thus, forming the lower oxide 

 S(_).j. The liberation of iodine from hydriodic acid proceeds with still greater ease under 

 the action of substances capable of disengaging oxygen. In practice, many methods 

 are employed for liberating iodine from acid liquids containing, for example, sulphuric 

 acid and hydriodic acid. The higher oxides of nitrogen are most commonly used; they 

 then pass into nitric oxide. Iodine may even be disengaged from hydriodic acid by the 

 vet ion of iodic acid, ivc. But there is a limit in these reactions of the oxidation of hydri- 

 odic acid because, under certain conditions, especially in dilute solutions, the iodine set 

 free is itself able to act as an oxidising agent that is, it evinces in itself the character 

 of chlorine, and of the halogens in general, to which we shall again have occasion to 

 refer. 



52 For the ultimate purification of iodine, Stag dissolves it in a strong solution of 

 potassium iodide, and precipitates it by the addition of water (see Note 58). 



' '" The solubility of iodine in solutions containing oxides, and compounds of iodine in 

 general, may serve, on the one hand, as an indication that solution is called forth by 

 analogy (p. 70), and, on the other hand, as an indirect proof of that view as to solutions 

 which was cited in Chapter I., because in many instances unstable highly-iodised com- 

 pounds, resembling crystallo-hydrates, have been obtained from such solutions. Thus 

 iodide of tetramethylammonium N(CH-) 4 I, combines with I 2 and I 4 . Even a solution of 

 iodine in a saturated solution of potassium iodide presents traces of the formation of a 

 definite compound KI 3 . Thus, an alcoholic solution of KI 3 does not give up iodine to 

 carbon bisulphide, although this solvent takes up iodine from an alcoholic solution of 

 iodine itself (Girault, Jiirgensen, and others). The instability of these compounds resem- 

 bles the instability of many crystallo-hydrates, for instance of HC1,2H 2 O. 



