THE HALOGENS 503 



It crystallises from solutions as a hydrate containing 2H 2 O (corre- 

 sponding with HC1O 4 ,2H 2 O), but as there are salts containing up to 5 

 atoms of metals, this water must be counted as water of constitution. 

 Therefore IO(OH) 5 = HIO 4 ,2H 2 O corresponds with the higher form 

 of halogen compounds, IX 7 . 85 In. decomposing (at 200) or acting as an 

 oxidiser, periodic acid first gives iodic acid, but it may also be ultimately 

 decomposed, with the formation of hydriodic acid. 



Thus both bromine and iodine present a great analogy to chlorine 

 in their behaviour with various substances, but nevertheless a series of 

 qualitative distinctions characterise each element. The formation of a 

 compound between chlorine and iodine must be classed among these 

 distinctions. 86 These elements combine directly together with the 

 evolution of heat, and form iodine monochloride, IC1, or iodine tri- 

 chloride, IC1 3 . 87 As water reacts on these substances, forming iodic 



salts, which correspond with the type of the meso-salts, as pyrophosphoric salts corre- 

 spond with orthophosphoric salts i.e. 2H 2 IO5 H 2 O = H 4 IoO9. 



85 Periodic acid, discovered by Magnus and Ammermtiller, and whose salts were 

 afterwards studied by Langlois, Rammelsberg, and many others, presents an example of 

 hydrates in which it is evident that there is not that distinction between the water of 

 hydration and of crystallisation which was at first considered to be so clear. In HC1O,2H 2 O 

 the water, 2H 2 O, is not displaced by bases and must be regarded as water of crystallisa- 

 tion, whilst in HIO 4 ,2H 2 O it must be regarded as water of hydration. We shall after- 

 wards see that the system of the elements obliges one to consider the halogens as 

 substances giving a highest saline type, GX 7 , if G signify a halogen, and X oxygen 

 (O = -X" 2 ), OH, and other like elements. The hydrate IO(OH) 5 corresponding with many 

 of the salts of periodic acid (for example, the salts of barium, strontium, mercury) does 

 not exhaust all the possible forms. It is evident that various other pyro-, meta-, &c., forms 

 are possible by the loss of water, as will be more fully explained in speaking of phosphoric 

 acid, and as was pointed out in the preceding note. 



86 With respect to hydrogen, oxygen, chlorine, and other elements, bromine occupies 

 an intermediate position between chlorine and iodine, and therefore there is no particular 

 need for lingering over the compounds of bromine. This is the great advantage of a natural 

 grouping of the elements. 



87 They were both obtained by Gay-Lussac and many others. Eecent verified data 

 respecting iodine monochloride, Id, entirely confirm the numerous observations of Trapp 

 (1854), and even confirm his statement as to the existence of two isomeric (liquid and 

 crystalline) forms (Stortenbeker). With a small excess of iodine, iodine monochloride 

 remains liquid, but in the presence of traces of iodine trichloride it easily crystallises. 

 Schiitzenberger amplified the data concerning the action of water on the chlorides (Note 88) 

 and Christomanos gave the fullest data regarding the trichloride. 



After being kept for some time, the liquid monochloride of iodine yields red deliques- 

 cent octahedra, having the composition IC1 4 , which are therefore formed from the mono- 

 chloride with the liberation of free iodine, which dissolves in the remaining quantity of the 

 monochloride. This substance, however, judging by certain observations, is impure iodine 

 trichloride. If 1 part of iodine be stirred up in 20 parts of water, and chlorine be passed 

 through the liquid, then all the iodine is dissolved, and a colourless liquid is ultimately 

 obtained which contains a certain proportion of chlorine, because this compound gives a 

 metallic chloride and iodate with alkalis without evolving any free iodine : IC1 5 + 6KHO 

 = 5KC1 + KIO 3 + 8H 2 O. The existence of a pentachloride IC1 5 is, however, denied, 

 because this substance has not been obtained in a free state. 



