Charcoal has no action on iodine, either at high or low temperatures.
With sulphur. iodine forms a weak compound of a grey ish black colour, radiated like sulphuret of antimony, and iodine is separated from it when distilled with water.
With phosphorus it combines in different proportions with the nisengagement of heat without light. One part of phosphorus and eight of iodine brin a compound of a red orange brown colour, fusible about 212°, and volatile, at a higher temperature.
With 16 parts of iodine to 1 ol phosphorus, a greyish black crystallized compound is formed, fusible at 84°. With 24 parts a black compound is ontained, partially fu sible at 115".
The iodurets of phosphorus exhale, when moistened, acid vapours. These are an acid suOstance, bearing the same relation to iodine that moriatic acid dues to chlorine; and those who still maimain that chlorine is a compound of muriatic acid with oxygen, and that muriatic acid is ob tai led from it by separating the oxygen, may consider io dine as a compound containing oxygen, amid tne acid pro duced from it as comparatively a simple substance, being deprived of the oxygen contained in the iodine. 13ut su this case, as in the former, the presence of hydrogen is essential to the formation of acid l•om the alleged com pound. This most commonly happens from the presence of water ; and for this, among other reasons, Gay-Lussac and Sir 11 Davy consider the chemical change induced as consisting in the union of a simple substance, chlorine in the one case, and iodine in the other, with hydrogen. Though fully aware of the difficulty of deciding this tion in the present state of science, we shall adopt the phraseology and nomenclature which depend on the latter of these theories, and which are in conformity with the doctrines of chlorine and muriatic acid, as stated in our articleCH EMISTRY. We shall keep in siew that an acid is obtained from iodine by combining it with hydrogen, and this we shall call the hydriodic acid This indeed is a matter of fact, whether iodine is regarded as a simple or a compound body.
Hydrogen, whether dry or moist, has no action on iodine at the ordinary temperature ; but when a mixture of it with iodine is exposed to a red heat in a tube, they unite, and hydriodic acid is produced, which shcws its presence by giving a reddish brown colour to water. The cause of this colour will afterwards appear. In this process, 100 parts of iodine absorb only .849 by weight of hydrogen.
Hydrioche acid is produced most easily by means of io duret of phosphorus. In order to procure it free from any admixture of phosphuretted hydrogen, it is necessary to employ a compound in which the phosphorus does not ex ceed a ninth part.. This ioduret was. in the experiments
of Gay-Lussac, moistened with pure water, or, what is better, water containing some hydriodic acid previously formed. Another method is to put the iodine into a small tube, which is then to be reversed in mercury, and the air expelled from it, by introducing a glass rod which nearly fills it ; after which the phosphorus is brought in contact with the iodine, by being made to rise through the mercury ; the substances immediately combine, and the hydriodic gas is disengaged, and may he collected, by putting the open extremity of the tube under a glass jar standing in the mercury. The gas, however, begins to be decomposed as soon as it comes in contact with the mercury. The iodine combines with the mercury, form ing a greenish yellow ioduret. By agitating the mercury, this decomposition may be made to extend to the whole gas present, and a volume of hydrogen is left exactly equal to one half of the hydriodic gas. The contact of zinc and potassium produces the same result.
Hydriodic acid gas is colourless. and has an ndour simi lar to that of muriatic acid. Ls taste is intensely acid. It saturates its own bulk of annmoniucal gas. Chlorine de prives it of its hydrogen, and reduces it to the state of io dine, while assumes the form of muriatic acid. This acid is denominated by Gay-Lussac hydrochloric, being formed of chlorine and hydrogen, and this name the analogy subsisting between it and the hydrio•ic. Tne specific gravity of hydriodic gas is to that of air as 41-43 to 1000. Iodine and hydrogen combine in equal volumes to produce this gas, and, as the vapour of iodine is the heaviest we are acquainted with, being to that of oxygen as 15.6'21 to 1, and to that of hydrogen as 117.71 m I, it does not take one blindreth part of its weight of the gas to be converted into an acid. On this part of the sub ject, Gay•Lussac lakes occasion to illustrate his principle on the ratio of saturation, as being determined by the re lative densities of bodies in the state of vapour. The facts, as applied to the well-kn•i•n gaseous substances, were first pointed out by that philosopher, and the doc trine extended by him to all other bodies, though of course it can only he determined with regard to those which we find capable of being raised to the state of vapour in our experiments, and not to all of these, as the high tempera ture at which many of them are volatilized renders all manipulations on their vapour, for the purpose of ascer taining their specific gravity, unmanageable.