This substance has all the interest of a newly discovered gas, and derives a further interest from being the second coloured gas known in chemistry ; colour being a proper ty possessed by none previously known, excepting the oxymuriatic or chlorine gas. A volume of it of conFidera ble diameter exhibits a beautiful and rich violet appear ance.
In the solid state, it has the form of flat octahedral crys tals, the axes of which are in the proportion of two, three, and four. The form which most frequently appears in a section parallel to the plane of the greatest and smallest axis is a rhombic plate, bevelled at each of its edges by two narrow planes, inclined to one another at an angle of about According to the experiments of Dr. Brewster, iodine possesses the property of polarizing, in two opposite planes, the light which it reflects, a property which is pe culiar to metallic bodies.
Its fusing temperature, when pure, is 225°, and it is vo talized at 347° or 356° ; though it may be distilled over with water at Its odour resembles that of oxymu riatic gas, though weaker. It is soft and friable, and may be rubbed into a fine powder. Its taste is acrid, and its action on the stomach poisonous. It gives a deeft brown stain to the skin, which, however, is soon removed. Like the oxymuriatic acid, it destroys vegetable colours, though with less energy. It is soluble in 4000 times its weight of water, and communicates to it an orange yellow colour. Its specific gravity at 621° is 4.948.
Iodine is not combustible, and cannot be made to com bine by any direct method, with oxygen.
It has the property of combining with almost all the metals. In thii, and some other properties, it resembles the oxymuriatic or chlorine gas ; but much less heat is evolved than by the combination of the metal with this last mentioned substance. One probable reason ol this is, that iodine is previously in a solid state. From this property, however, Dr. Thomson classes it with chlorine and oxy gen, under the title of supporters of combustion. In this particular, it is to be observed, it also resembles sulphur, which combines with metals with an evolution of caloric.
Potassium, during its union with iodine, emits a pale blue flame. The result of the union is a white compound,
fusible at a red heat, soluble in water, of an acrid taste, and re-yielding the iodine when treated with sulphuric acid.
With iron, mercury, tin, zinc, and lead, it forms com pounds, fusible at a moderate heat, and generally of bright colours. Most of these are capable of forming com pounds with potass, from which sulphuric acid evolves the iodine.
Under water, zinc and iodine combine on the applica tion of a moderate heat. 100 parts of iodine combine with about 25.225 of the metals. These metallic compounds are called iodurets. Iron is acted on in the same manner. The ioduret of iron is brown. and fusible at a red heat. Its solution in water is of a light green, like the muriate, or what Gay-Lussac called the chloruret of iron, from supposing that it is not a combination of a metallic oxide with muriatic acid, but of the pure metal with chlorine.
The ioduret of tin is very fusible. When in powder it has a dark orange colour. It was found by Sir H. Davy to possess the characters of an acid. It combined w ith the alkalies without deposi ing any oxide. The ioduret of antimony is, like the preceding, fusible. and resembles it when in the state of powder. The iodurets of lead, cop per, bismuth, silver, and mercury, are insoluble in water, a property in which they differ from those of the more oxi dable metals.
Thcre are two iodurets of mercury, the yellow and the red, depending on differences of proportion in the com binations. The yellow contains one half less le dine than the red.
The iodurcts of the metals arc decomposed by chlorine, which unites with the metals, and expels the iodine. When ioduret of potassium is heated in contact with chlorine, chloruret of potassium ( he substance called muriate of potass) is formed, the violet gas appears, but soon unites with chlorine, and they form by their union a peculiar acid compound ; but towards the cod of the process, as the pro portion of chlorine diminishes, the violet coloured gas again appears. loduret of silver gives similar phenomena. Iodine, however, appears to possess a stonger attraction for most of the metals than oxygen.