IODINE. 



107 



Gay-Lussac and Sir H. Davy. Its history proved 

 singularly interesting in modifying the then prevail- 

 ing theory of chemistry. Sir H. Davy had, a few 

 years previously, promulgated the new theory of 

 chlorine, which was still received with suspicion 

 among chemists. The strong analogies, however, 

 between this substance and chlorine, in their rela- 

 tions to combustibles, both bodies forming com- 

 pounds by uniting with them, similar to acids con- 

 taining oxygen, or oxides, were conceived to give 

 great weignt to the views of Sir H. Davy, and 

 operated completely to overthrow the erroneous 

 hypothesis of oxygenation, invented by Lavoisier. 

 Its investigation, therefore, may be said to have 

 formed a new era in chemistry. The physical pro- 

 perties of iodine are as follow: It is a soft, friable, 

 opaque solid, of a bluish-black colour, with a metallic 

 lustre, usually in scales, but sometimes in distinct 

 crystals of the form of rhomboids or rhomboidal 

 tables, referrible to an octahedron, with arhombic base 

 as their primary form; its specific gravity is 4.946. 

 It possesses an odour somewhat analogous to that of 

 chlorine. It is a non-conductor of electricity, and 

 possesses in an eminent degree the electrical pro- 

 perties of oxygen and chlorine. Iodine enters into 

 fusion at 225 Fahr., and boils at 347; but when 

 moisture is present, it sublimes rapidly at a tempera- 

 ture considerably below 212, and gives rise to a 

 dense vapour of the usual violet hue. It is scarcely 

 at all soluble in water, but is readily taken up by 

 alcohol and ether, to which it imparts a reddish- 

 brown colour. It extinguishes vegetable colours, 

 but with less energy than chlorine. It is not inflam- 

 mable. Its range of affinity for other bodies is very 

 extensive; the most important compounds it forms 

 with these we shall describe after alluding to its 

 natural state and preparation. It exists most abun- 

 dantly in the various species of fucus, which form the 

 greatest part of the sea-weeds of our coast ; it also 

 occurs in the sponge, and in the -coverings of many 

 molluscous animals, and has been found in a great 

 number of mineral waters, as those of Salz in Pied- 

 mont, Saratoga in New York, &c., and more recently 

 has been detected in some silver ores from Mexico, 

 and in an ore of zinc from Upper Silesia. But it is 

 from the incinerated sea-weed or kelp, that the iodine, 

 in large quantities, is obtained. As the soap-manu- 

 facturers are in the habit of obtaining their soda 

 from kelp, iodine may be procured, very economi- 

 cally, from the residuums of their operation, according 

 to the process invented by doctor Ure, which is as 

 follows: The brown iodic liquor of the soap-boiler, or 

 the solution of kelp from which all the crystallizable 

 ingredients have been separated by concentration, is 

 heated to about 230 Fahr., poured into a large 

 stone- ware basin, and saturated with diluted sulphu- 

 ric acid. When cold, the liquor is filtered through 

 woollen cloth; and to every twelve oz. (apothecaries' 

 weight) of it, is added 1000 grains of black oxide 

 of manganese in powder. The mixture is put into a 

 glass globe, or large matrass with a wide neck, over 

 which a glass globe is inverted, and heat is applied, 

 which causes the iodine to sublime copiously, and to 

 condense in the upper vessel. As soon as the balloon 

 becomes warm, another is substituted for it ; and 

 when the second becomes heated, the first is again 

 applied. The iodine is withdrawn from the globes 

 by a little warm water, which dissolves it very spar- 

 ingly; and it is purified by undergoing a second sub- 

 limation. The test made use of for the detection of 

 iodine in any solution, when it is suspected to be pre- 

 sent, is starch, with which iodine has the property 

 of uniting, and of forming with it a compound, in- 

 soluble in cold water, which is recognised with cer- 

 tainty by its deep blue colour. The solution, should 



be cold at the time of adding the starch; and, if the 

 colour does not become apparent simply on the ad- 

 dition of the starch, a few drops of sulphuric acid 

 should be cautiously added, when, if any iodine is 

 present, the blue colour will make its appearance. 

 This test is so exceedingly delicate, that a liquid, 

 containing irofonr of its weight of iodine, receives a 

 blue tinge from a solution of starch. 



Iodine has a powerful affinity for hydrogen, which 

 it takes from animal and vegetable substances, in the 

 same manner as chlorine, and, uniting with it, forms 

 hydriodic acid. The following are the methods for 

 obtaining this acid in the gaseous and in the liquid 

 state: Into a flask, to which a recurved tube is fitted, 

 dipping under a jar of mercury, are introduced eight 

 parts of iodine and one of phosphorus, and to the mix- 

 ture a few drops of water are added; the water is im- 

 mediately decomposed; the phosphorus, seizing its 

 oxygen, forms phosphoric acid, while the hydrogen 

 combines with the iodine. As there is not water 

 present in sufficient quantity to dissolve the hydriodic 

 acid, it passes over in the gaseous state, and is col- 

 lected over the mercury. In contact with air, it 

 smokes, or fumes, like the muriatic acid, and, like 

 it, reddens vegetable blues. It is distinguished, 

 however, from that acid, by the superior affinity 

 possessed by chlorine for hydrogen, in consequence 

 of which, if chlorine and hydriodic acid gases are 

 mingled together, the yellow colour of the former 

 disappears, and the violet vapour of iodine makes its 

 appearance, which proves the decomposition of the 

 hydriodic acid by the chlorine. If the decomposition 

 is complete, the vessel will be wholly occupied by 

 muriatic acid gas. To obtain the hydriodic acid in a 

 liquid state, we have only to conduct the gas through 

 water, until it is fully charged with it; or it may be 

 obtained by transmitting a current of sulphureted 

 hydrogen gas through water in which iodine, in fine 

 powder, is suspended. The iodine, from a greater 

 affinity for hydrogen than the sulphur possesses, 

 decomposes the sulphureted hydrogen; and hence 

 sulphur is set free, and hydriodic acid produced. 

 The constitution of hydriodic acid is, 



Iodine 

 Hydrogen 



By Volume. 

 . 50 . 

 . 50 . 



100 



By Weight. 

 . 124 

 1 



125 



The solution of hydriodic acid is easily decomposed. 

 Thus, on exposure for a few hours to the air, the 

 oxygen of the atmosphere forms water with the 

 hydrogen of the acid, and liberates the iodine. 

 Nitric and sulphuric acids likewise decompose it 

 by yielding oxygen, the former being converted into 

 nitrous and the latter into sulphurous acid. The 

 free iodine becomes obvious on the application of 

 the above-mentioned test. The compounds of hydrio- 

 dic acid with the salifiable bases may be easily formed, 

 either by direct combination, or by acting on the 

 basis in water with iodine. Sulphurous and muriatic 

 acids, as well as sulphureted hydrogen, produce no 

 change on the hydriodates, at the usual temperature 

 of the air ; but chlorine, nitric, and concentrated 

 sulphuric acid, instantly decompose them, and separ- 

 ate the iodine. The hydriodates of potash and soda 

 are the most interesting of their number, because 

 they are the chief sources of iodine in nature, 

 The latter salt is probably the one which affords the 

 iodine obtained from kelp; while it is believed, 

 that it is the hydriodate of potash, which is 

 most generally found in mineral springs. Hence 

 the necessity of adding sulphuric acid to the residual 

 liquor of the soap-boiler, in order to procure iodine, 

 which requires to be separated from its combination 



