Periodic acid, discovered by Magnus and Ammermüller, 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 HClO,2H2O the water, 2H2O, is not displaced by bases, and must be regarded as water of crystallisation, whilst in HIO4,2H2O it must be regarded as water of hydration. We shall afterwards see that the system of the elements obliges us to consider the halogens as substances giving a highest saline type, GX7, where G signifies a halogen, and X oxygen (O = X2), 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.
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 considering at length the compounds of bromine. This is the great advantage of a natural grouping of the elements.
They were both obtained by Gay-Lussac and many others. Recent data respecting iodine monochloride, ICl, 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. Tanatar (1893) showed that of the two modifications of ICl, one is stable, and melts at 27°; while the other, which easily passes into the first, and is formed in the absence of ICl3, melts at 14°. Schützenberger amplified the data concerning the action of water on the chlorides (Note 725), and Christomanos gave the fullest data regarding the trichloride. After being kept for some time, the liquid monochloride of iodine yields red deliquescent octahedra, having the composition ICl4, which are therefore formed from the monochloride 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: ICl5 + 6KHO = 5KCl + KIO3 + 3H2O. The existence of a pentachloride ICl5 is, however, denied, because this substance has not been obtained in a free state. Stortenbeker (1888) investigated the equilibrium of the system containing the molecules I2, ICl, ICl3, and Cl2, in the same way that Roozeboom (Chapter X. Note 620) examined the equilibrium of the molecules HCl, HCl,2H2O, and H2O. 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 13°·9, and easily passes into the first form. Iodine trichloride melts at 101° only in a closed tube under a pressure of 16 atmospheres.