Ammonia ...... 10.94.
Oxiodate of barytes is easily obtained by double affini ties, or by putting iodine into barytes water. It precipi tates in the form of a powder, which may be purified by re peated washing. While drying it concretes into lumps, and becomes mealy. It retains water in composition, which it gives out at a temperature of 212°, before being decom posed by heat. It is composed of Oxiodic acid 100 Barytes 46 340 On hot coals it does not deflagrate, but only gives weak intermitting light. It is infusible, and only a small quantity of the salt comes in contact with the fuel, so that the oxygen which is disengaged contributes very little to its combustion.
The oxiodate of strontites has similar properties.
The oxiodate of lime is generally pulverulent, but may be crystallized in the muriate or the hydriodate of lime, which augment its solubility, and it then assumes the form of small quadrangular prisms. It holds about 3 her cent. of water in combination.
The other oxiodates may be obtained by double decom positions. Nitrate of silver gives with the oxiodate of potass a white precipitate, which is the oxiodate of silver, and is very soluble in ammonia.
The oxiodates do not, like the hydriodates, absorb iodine to form iodureted salts. Their solutions absorb no more of this substance than is done by pure water.
Iodine gives a blue colour to starch by combination ; and this is done with such facility, that starch is one of the most delicate tests of this substance. It does not, however, de tect it in the state of acid, unless the iodine is previously evolved by some other acid substance.
On mixing two parts in volume oralcohol with coloured hydriodic acid of the specific gravity of 1.7, and distilling the mixture in a water bath, an alcoholic liquor is produced, colourless and limpid, which, when mixed with water, be comes muddy, and lets fall in small globules a liquid, at first milky, which gradually becomes transparent. This is hydriodic ether. What rer•ins in the retort is hydriodic acid, very dark coloured, from holding in solution all the iodine which had coloured the original quantity. Hydrio die ether, after being washed two or three times with wa ter, (in which it has very little solubility,) is in a pure state. It has a strong and peculiar odour, though analogous to that of the other ethers. After some days it acquires a red colour, from the evolution of a little iodine, which, however, does not afterwards inert ase This colour may he removed by the addition of a little potass or mercury.
Z z which combine with the iodine. The specific gravity of this ether, at 72°, is 1.9206. It boils at 148.1. It is not inflammable—a circumstance very singular in an ether. It only exhales purple vapours when thrown on burning coals. Potassium may be preserved in it without altera tion. Concentrated st.ithurie acid renders it brown. When passed through a tube, it is speedily decomposed, giving an inflammable carbureted gas, very brown hydrto dic acid, and a little charcoal.
The concurrence of properties which we find in iodine, and its vatious states of chemical combination, is curious and singular. " In its specific gravity," says Sir H. Davy, who considers it as an undecompounded body, " in lustre, the high number in which it enters into combination, and in colour, it resembles the metals ; but in all its chemical agencies it is more analogous to oxygen and chlorine : it is a non-conductor of electricity, and possesses, like these bodies, the negative electrical energy with respect to me tals, inflammable and alkaline substances; and hence, when combined with these substances in aqueous solution, and clectrized in the voltaic circuit, it separates at the positive surface : but it has a positive energy with respect to chlo rine; for, when united to this substance to form a com pound acid, it separates from it at the negative surface. It
has a stronger attraction for most of the metals than oxy gen, but oxygen expels it from phosphorus and sulphur. Under a red heat, oxygen converts the iodurct of phos phorus into phosphoric acid, and evolves the iodine. Its saturating powers seem to be greater than those of oxy gen, and less than those of chlorine. It agrees with chlo rine and fluorine in forming an acid with hydrogen; and it agrees with oxygen in forming an acid with chlorine." One of the best tests of iodine is to be found in its action on silver, which has the advantage of manifesting its pre sence in whatever state of combination it may exist. Wa ter, when it contains less than one-thousandth part of its weight of a hydriodate, or oxiodate, tarnishes polished sil ver. This effect may be distinguished from that produced by compounds containing sulphurets, or sulphureted hy drogen, by this circumstance, that the latter, by being boil ed with a little muriatic acid, are deprived of the power of tarnishing the metal, whereas solutions containing iodine, when treated in the same manner, still retain it.
In the experiments made by Sir H. Davy on natural pro ductions which might be supposed to yield iodine, he found that the fuci and ulvie of the Mediterranean afforded it in smaller quantities than the sel de varec ; and it was only in a few cases that lie could find in them any traces of its ex istence. Slight traces of it were found in the Flans car tilaginus, the membranaceus, rubens, and filamentosus, and in the Ulva pavonia and Ulva linza, but none in the ashes of the corallines and sponges. He examined some speci mens of alkali, formed by the combustion of maritime ve getables which are not submarine, but found in them no decided indications of iodine. Yet he failed in his attempts to obtain it from sea-water, though the use of the voltaic battery afforded some obscure results. The products sepa rated at the positive pole were collected in a small cup of gold, covered with cement, except in the interior and lower part; and this, when exposed to the negative pole of a voltaic apparatus, yielded a black powder fixed in the fire, not unlike the compound formed by heating iodine and gold together ; but it was too minute to admit of chemical analysis, in order to ascertain that it was not the same as is obtained by negatively electrifying the oxymuriate of gold.
See the Memoir of Gay-Lussac on Iodine, in the Annalcs de Chimie ; or the English translation of it in the fifth and sixth volumes of Thomson's Annals of Philosophy ; and the two Memoirs of Sir Humphrey Davy on the same sub ject, in the London Philosophical Transactions for 1814, pa. is i. and ii., which are ri•.uolistad in Tilloch's Philoso phical Magazine ; and an arkitional Memoir in the Trans actions I' 1815, part ii (H D )