THE HALOGENS 507 



easy decoinposability of hydrobromic acid, and especially of hydriodic 

 acid, clearly distinguish these acids from hydrochloric acid. For this 

 reason, hydriodic acid acts in a number of cases as a deoxidiser -or 

 reducer, and frequently even serves as a means for the transference of 

 hydrogen. Thus Berthelot, Baeyer, "Wreden, and others, by heating 

 unsaturated. hydrocarbons in a solution of hydriodic acid, obtained their 

 compounds with hydrogen nearer to the limit C n H 2 , l+2 or even the 

 saturated compounds. For example, benzene, C C H 6 , when heated in a 

 closed tube with a strong solution of hydriodic acid, gives hexylene,. 

 C 6 H 12 . The easy decomposability of hydriodic acid accounts for the 

 fact that iodine does not act by metalepsis on hydrocarbons, for the 

 hydrogen iodide liberated with the product of metalepsis, RI, formed, 

 gives iodine and the hydrogen compound, RH, back again. And there- 

 fore, to obtain the products of iodine substitution, either iodic acid, HI0 3 

 (Kekule), or mercury oxide, HgO (Weselsky), is added, as they imme- 

 diately react on the hydrogen iodide, thus : HIO 3 + 5HI = 3H 2 + 3I 2 , 

 or, HgO 4- 2HI =. HgI 2 + H 2 O. From these considerations it will 

 be readily understood that iodine acts like chlorine (or bromine) 

 on ammonia and sodium hydroxide, for in these cases the hydriodic 

 acid produced forms NH 4 I and Nal. With tincture of iodine 

 or even the solid element, a solution of ammonia immediately forms 

 a highly-explosive solid black product of metalepsis, NHI 2 , generally 

 known as iodide of nitroaen, although it still contains hydrogen 

 (this was proved beyond doubt by Szuhay 1893), which may be 

 replaced by silver (with the formation of NAgI 2 ) : 3NH 3 + 2I 2 

 = 2NH 4 I +KHI 2 . However, the composition of 'the last product is 

 variable, and with an excess of water NI 3 seems to-be formed. Iodide 

 of nitrogen is just as explosive as nitrogen chloride. 78 bis In the 



78 bis iodide of nitrogen, NHI 2 is obtained as a. brown pulverulent precipitate on adding 

 a solution of iodine (in alcohol, for instance) to a solution of ammonia. If it be collected 

 on a filter-paper, it does not Decompose so long as the precipitate is moist; but t when dry 

 it explodes violently, so that it can only be experimented upon in small quantities. 

 Usually the filter-paper is torn into bits while moist, and the pieces laid upon a brick ; 

 on drying an explosion proceeds .not only from friction or a blow, but even spontaneously. 

 The. more dilute the solution of ammonia, the greater is the amount of iodine required 

 for the formation of the precipitate of NHI 2 . A low temperature facilitates its formation. 

 NHI 2 dissolves in ammonia water, and when heated the solution forms HIO 3 and iodine. 

 With KI, iodide of nitrogen gives iodine, NH 3 and KHO. These reactions (Selivanoff) 

 are explained by the formation of HIO from NHI 2 + 2H 2 O^NH 5 f 2HIO and then 

 Kt + HIO = !,+ KHO. Selivanoff (see Note 29) usually observed 'a temporary for- 

 mation of hypoiodous acid, HIO, in the reaction of ammonia upon iodine, so that 

 here the formation of NHI 2 is preceded by that of HIO i.e. first I 2 + H 2 O = HIO + HI, 

 and then not only the HI combines with NH 3 , but also 2HIO f NH 3 ==NHI 2 + 2H 2 0. 

 With dilute sulphuric acid iodide of nitrogen (like NC1 3 ) forms hypoiodous acid, but it 

 immediately passes into iodic acid, as is expressed by the equation 5HIO = 2I 2 + HIO 3 

 + 2H 2 O (first SHIO = HIO 3 + 2HI, and then HI + HIO = I a + H 2 O). Moreover. Selivanoff 



