IODINE, thus named on account of the violet colour of its vapour (loeiS-r}s, violet-coloured), one of the so-called halogen elements, has already been partially described (see CHEMISTRY, vol. v. pp. 490-498).
Iodides occur in minute quantity in most mineral waters and in sea water. The ashes of many marine algae are rich in them ; and formerly iodine was chiefly extracted from kelp or varec, the ashes of sea-weed, by distilling the mother liquor remaining after the separation of the less soluble salts by crystallization with manganese dioxide and sulphuric acid. Of late years, however, large quan-tities of iodine have been obtained from crude Chili saltpetre by a similar process.
The chief use of iodine is in the preparation of methyl-iodide, a substance employed in the manufacture of certain of the so-called aniline dyes. In medicine it is frequently applied externally as an irritant. Potassium iodide is also an important medicinal agent; and iodoform, CHI3, a substance prepared by acting on alcohol with iodine in presence of alkali, has latterly been introduced as an agent for external application in certain diseases. Several iodides, especially ammonium, cadmium, and potassium iodide, are largely employed in photography.
Recent investigations have disclosed a number of most remarkable facts regarding the behaviour of iodine, and the allied elements bromine and chlorine, which merit a brief description here. Free chlorine, bromine, and iodine are respectively represented by the formulae Cl2, Br2, and I2; that is to say, their molecules are "diatomic," each consisting of two atoms (comp. vol. v. pp. 467-472). On the other hand, the molecules of which sulphur vapour at a temperature of about 500° C. consists are hexatomic, as expressed by the formula S6; but on raising the tempera-ture these molecules undergo simplification, so that at temperatures above 800° the vapour appears to consist entirely of diatomic molecules such as are indicated by the formula S2. It would seem that the halogens undergo a similar molecular simplification when heated.
Having devised a method of extreme simplicity for the determination of vapour density, V. Meyer was led in the summer of 1879 to determine the density of a number of elementary bodies at much higher temperatures than had previously been employed, and among others chlorine was examined. He was then led (in conjunction with C. Meyer) to the discovery that at high temperatures this gas has a very much lower density than corresponds to the formula Cl2 (Berichte der deutschen chemischen Gesellschaft zu Berlin, 1879, p. 1430; comp. ibid., 1880, p. 1172). Subsequently he extended his observations to bromine and iodine (ibid., 1880, p. 394), and with similar results. Meier and Crafts took up the subject with the object of verifying V. Meyer's statements (ibid., 1880, p. 851); they introduced several refinements in the method of operating, and determined the temperatures at which the experiments were made more accurately; in the main, however, their observations with iodine were confirmatory of V. Meyer's. V. Meyer's original results, and those of Meier and Crafts, are arranged in the following table, where the numbers in TV the column headed indicate the ratio between the ob-
served density and the theoretical density on the air scale corresponding to the formula I2 (8'79).
== TABLE ==
Meier and Crafts were of opinion that the highest tempera-ture they employed was probably as high as that estimated by V. Meyer at 1570°, and the latter chemist subsequently acknowledged the justice of their criticism of his deter-minations of temperature, which were conducted by a calorimetric method, whereas Meier and Crafts employed an air thermometer. V. Meyer has since extended his observations to a still higher temperature, and has obtained the values 4'53, 4-55, 4'57, which are not far removed from the theoretical value 4'39, corresponding to the formula I for the iodine molecule (op, bit., 1880, p. 1010).
An important series of observations by Meier and Crafts (Comptes Rendus, xcii. 39) on the density of iodine at various temperatures under various pressures show that at temperature below 700° and pressures below atmo-spheric pressure the density is constant, and corresponds to the formula I2, and that the density diminishes more rapidly with rise of temperature.
From the earlier results obtained by Meier and Crafts, A. Naumann has calculated the rate of dissociation of iodine, on the assumption that the decomposition is expressed by the equation I2 = I + I, and has shown that it is in accord-ance with the general law of dissociation deduced from the dynamical theory of gases. He points out as especially remarkable that dissociation probably extends over 1200°, since it is only half completed at a temperature of about 1270°, and commences at least 600° lower.
The observations of Meier and Crafts indicate that the density of iodine begins to be abnormal at a temperature between 600° and 700°. The dissociation of bromine apparently does not commence at so low a temperature, and at a temperature at which the ratio of the observed to the theoretical density is -66 for iodine, it is -8 for bromine. Chlorine is much less readily dissociated than bromine. These results are in accordance with the general chemical behaviour of the halogens. It has yet to be proved, however, that the dissociation is of the character indicated above, and that the molecules of the halogens do not undergo a less simple decomposition such as is contemplated in Sir Benjamin Brodie's calculus of chemical operations. (H. B. A.)