504 PRINCIPLES OF CHEMISTRY 



acid and iodine, they have to be prepared from dry iodine and chlorine. 88 

 Both substances are formed in a number of reactions ; for example, by 

 the action of aqua regia on iodine, of chlorine on hydriodic acid, of 

 hydrochloric acid on periodic acid, of iodine on potassium chlorate 

 (with the aid of heat, &c.). Trapp obtained iodine monochloride, in 

 beautiful red crystals, by passing a rapid current of chlorine into molten 

 iodine. The monochloride then distils over and solidifies ; it melts at 

 27. By passing chlorine over the crystals of the monochloride, it is 

 easy to obtain iodine trichloride in orange crystals, which melt at 34 

 and volatilise at 47, but in so doing decompose (into C1. 2 and Oil). 

 The chemical properties of these chlorides entirely correspond with the 

 properties of chlorine and iodine, which would be expected, because, in 

 this instance, a combination by similitude took place as in the forma- 

 tion of solutions or alloys. Thus, for instance, the unsaturated hydro- 

 carbons (for example, C 2 H 4 ), which are capable of directly combining 

 with chlorine and iodine, also directly combine with iodine monochloride. 



Stortenbeker (1888) investigated the equilibrium of the system containing the mole- 

 cules I 2 , IC1, IC1 3 , and C1 2 , in the same way that Koozeboom (Chapter X. Note 38) examined 

 the equilibrium of the molecules HC1, HC1,2H 2 O, and H 2 O. He found that iodine 

 monochloride appears in two states, one (the ordinary) is stable and melts at 27'2, whilst 

 the other is obtained by rapid cooling, and melts at 18'9, and easily passes into the 

 first form. Iodine trichloride melts at 101 only in a closed tube under a pressure of 16 

 atmospheres. 



88 By the action of water on iodine monochloride and trichloride a compound IHCL, 

 is obtained, which does not seem to be altered by water. Besides this compound, iodine 

 andiodic acid are always formed, 10IC1 + 3H 2 O = HIO 3 + 5IHCL> + 2L> ; and in this respect 

 iodine trichloride may be regarded as a mixture IC1 + IC1 5 = 2IC1 5 , but IC1 5 + 3H 2 O = 

 IHOj + SHCl; hence iodic acid, iodine, the compound IHCL>, and hydrochloric acid are 

 also formed by the action of water. 



