IODINE This word is of Greek etymology, and signifies a violet colour. It has been recently introduced into the French and English languages, for the purpose of being applied as a name for a chemical substance re cently discovered. The description of that substance and its properties will now holej a prominent place in all systems of chemistry ; but it is one with which the sci ence has been enriched since our article Chemistry was published. What applications it may admit of in the use ful arts we do riot as yet know ; but those properties which it has been found to possess render it extremely interest ingto chemistry as a science, by affording scope for ex perimental investigation and scientific reasoning ; and it so happens that it gives an additional importan c, by its curious analogies, to some of those doctrines which have been in modern times most keenly contested, and in which the contending parties, V/ ith all the advantages of patient experiment and consummate acumen, aided by the zeal which a certain degree of opposition ol opinion excites, have scarcely been able to devise experiments of a de cisive kind.
The discoverer of iodine was M. Courtois, a manufac turer of nitre in Paris, whose attention was directed to the subject, in consequence of a corrosion to which the metal lic vessels employed in the process for procuring soda from the ashes of sea-weeds were the cause of which he investigated, and in the investigation discovered this substance. The leading investigators of its chemical properties have been M. Gay-Lussac and Sir Humphry Davy ; both of whom have mingled their experimental inquiries with their views of the more recondite laws of chemical change. In the present article, it is our inten tion to give a plain and perspicuous account of the leading facts, without omitting to notice the most important and least intricate of the arguments connected with them.
M. Courtois discovered iodine in the year 1811 ; and the discovery was announced to the Institute by M. Clem ent about two years after, viz. on the 29th of November 18i3. Tile chief reason of this long delay seems to have been, that these two gentlemen had wished to ascertain previously the qualities of this new substance, but had been prevented by the attention which their other pursuits demanded, from executing their design so speedily or so extensively as they hoped.
This substance is procured from kelp in the following manner. The kelp is treated with hot water till all its soluble parts are dissolved. The solution is then evapo rated, and from the dry salt the substance in question is separated by sulphuric acid. The salt is put in a tubula ted retort, deep, but not large. The beak of the retort, which ought to be short. is introduced into a large globular glass receiver, which has an opening to allow the air to escape. Concentrated sulphuric acid is poured on the salt, through the opening, which is then closed with a stop per. An active effervescence ensues, and a violet-colour ed gas is driven off, which crystallizes on the inside of the receiver. This is the iodine. When raised to its vapo rific point of temperature, it is a violet coloured gas. Be low that point, it has the form of brownish black shining plates.
It was at first erroneously reported that the iodine was obtained from the insoluble parts of kelp. It cannot be obtained from pounded kelp The solution and crystalli zation seem to be indispensable pre-requisites to the ope ration.
The following method, recommended by Dr. Wollas ton, yields it more abundantly. After concentrating the aqueous solution of the soluble parts of kelp, and separat ing from it all the crystals that can be obtained. the re maining liquid is to be poured into a clean vessel, and mix ed with an excess of sulphuric acid. This is boiled for some time, during which process sulphur is precipitated, rind muriatic acid is driven off. The clear liquid is then do canted off, and strained through wool It is.next put into a small flask, with a quantity of black oxide of man gavese, equal to that of the sulphuric acid first employed. A glass tube, closed at one end, is fixed to the top of the flask, and heat applied to the bottom. The iodine is sub limed and crystallized within the tube. Soaper's black ashes yield it in large quantity.