Having in my present communication rescued another discovery from Mr. Hume's extensive grasp, and restored it to the rightful owner, I expect from him a repetition of the invectives of which he has been so liberal, with such scanty additions as even his wellstored vocabulary can now afford; but having, as I conceive, answered Mr. Hume's arguments, I shall hold myself excused from entering upon a mode of discussion, in which success is inseparable from disgrace.

I remain yours, very respectfully,

29, Poultry, Feb. 22, 1815.

RICHARD Phillips,

ARTICLE IX.

A Memoir on Iodine, By M. Gay-Lussac.

(Continued from p. 214,)

Combination of Iodine with Chlorine.

Dry iodine rapidly absorbs chlorine, producing a heat which rises to 212°. The compound has in some parts a fine orange-yellow colour; in others, an orange-red. The yellow parts contain more chlorine than the red; they are likewise more volatile. Though I passed a great deal of chlorine over the iodine, yet the greatest part was not saturated. We shall see immediately by what characters this point can be determined. To the red compound of iodine and chlorine I give the name of subchloruret of iodine, though it does not appear to me to have fixed proportions. To the yellow compound I give the name of chloruret.

Both of these compounds speedily deliquesce in the air, The solution of the subchloruret is a deeper orange-yellow the more iodine it contains. The solution of the chloruret is colourless when the excess of chlorine is driven off, and then the mutual saturation of the two constituents seems to be complete. Both solutions are very acid, and destroy the colour of the solution of indigo in sulphuric acid. When the solution of the chloruret is saturated with an alkali, it is changed completely into iodate and hydro-chlorate. When too long exposed to the light, it becomes coloured. It dissolves a great quantity of iodine, and then assumes the characters of subchloruret. Heat disengages chlorine from it, and the iodine being then in excess, the liquid assumes the characters of subchloruret. The solution of the subchloruret is volatilized without decomposition. Light does not produce any further alteration upon it. When saturated with an alkali, it gives iodate and hydro-chlorate; but if the alkali be cautiously added, we obtain a precipitate of iodine, which disappears on the addition of more alkali, and then hydriodate and iodate are formed. Thus the subchloruret is

characterized by the precipitation of iodine when an alkali is added, whereas the chloruret gives no such precipitate.

We obtain but little chloruret in a solid state, as I have already remarked; but it may be obtained with facility, and in great quantity, on solution in water. For that purpose nothing more is requisite than to saturate with chlorine a somewhat diluted solution of subchloruret. It is then exposed for some time to the sun till it loses its colour, or it may be put into a large bottle in which the air is continually renewed. By this means we obtain a very acid colourless liquid, having only a slight smell of chlorine, which destroys the colour of solution of indigo in sulphuric acid, though slowly, and gives an abundant precipitate of iodine when ammonia is poured into it. We cannot employ heat to drive off the excess of chlorine unless it be very moderate; for I have just observed that it converts the liquid into a subchloruret. When we wish to saturate a solution of subchloruret with chlorine, the liquid ought to be dilute; because, when concentrated, the process does not succeed. The subchloruret presents itself frequently, and possesses stability, while the other (to make use of the expression) has only an ephemeral existence.

When we pour hydro-chlorate of potash or barytes into a solution of the chloruret or subchloruret, it gives up its base to a portion of iodic acid which we may conceive to be present; but the hydrochloric acid becoming predominant, prevents a complete decomposition.

We have seen that the solution of chloruret is changed into iodate and hydro-chlorate, when' saturated with an alkali. From this fact, and from the characters of the solution, we may suppose it to be a mixture of iodic and hydro-chloric acid. On the other side, as it deprives indigo of its colour, it would seem that the chlorine and iodine in it still preserve their properties entire. We may conceive it likewise to be a peculiar acid, which is decomposed when we saturate it with a base. I adopt the first supposition, because I compose exacly the solution of the chloruret when I mix iodic and hydro-chloric acids together. But I consider their elements as very mobile, and capable of taking a new arrangement according to circumstances. On this supposition the water is decomposed when the chloruret is dissolved in it. Its oxygen combines with the iodine, and its hydrogen with the chlorine. The inverse distribution could not take place, for the iodic and hydro-chloric acids are much more stable than the chloric and hydriodic acids; and it is a general law that, every thing else being equal, the strong compounds are always formed in preference to the weak.

If we take a given quantity of iodine, and act upon by an alkali, it will be divided into two very unequal parts. The sinallest portion forms iodate; the greatest, hydriodate. If we wished to convert it entirely into iodate, we must begin by making it a chloruret; and after having dissolved it in water, we saturate it with the alkali

which we wish to convert into iodate. The iodates of barytes, lime, and strontian, being very little soluble in water, will be obtained pure after some washings. The others must be separated from the hydro-chlorates by repeated crystallizations or by alcohol.

Of the Hydriodates.

In general these salts may be prepared by combining hydriodic acid with the bases; but we may obtain those of potash, soda, barytes, strontian, and lime, directly by treating iodine with these bases, employing the methods above described to separate them from the iodates which are formed at the same time. The hydriodates of zinc, iron, and in general of all the metals that decompose water, are obtained by dissolving the iodurets of these metals in water. We may put together the water, the iodine, and the metal, and by the application of heat the hydriodate is quickly formed. I do not propose to treat of all the hydriodates in detail, but merely to give their generic characters and their principal properties.

Sulphurous, hydro-chloric, and hydro-sulphuric acids, produce no change on the hydriodates at the usual temperature of the atmosphere.

Chlorine, nitric acid, and concentrated sulphuric acid, instantly decompose them, and separate the iodine.

With solution of silver they give a white precipitate insoluble in ammonia; with the pernitrate of mercury, a greenish-yellow precipitate; with corrosive sublimate, a precipitate of a fine orangered, very soluble in an excess of hydriodate; and with nitrate of lead, a precipitate of an orange-yellow colour.

They dissolve iodine, and acquire a deep reddish-brown colour.

Hydriodate of Potash,

When a solution of hydriodate of potash is made to crystallize, the oxygen combined with the metal, and the hydrogen with the iodine, unite together, and form water, and we obtain crystals of ioduret of potassium similar to those of chloruret of sodium. This salt easily melts, and sublimes at a red heat. When heated in contact of air, it undergoes no alteration. It is more deliquescent than the hydro-chlorate of soda. 100 parts of water, at the temperature of 64°, dissolve 143 of the salt. We may consider it as a hydriodate while it is in solution in water; but when melted, or even dried, is obviously an ioduret of potassium. I find that when ioduret of potassium is dissolved in water, and afterwards dried, its weight is not increased.

Ioduret of potassium is composed of

Hydriodate of Soda.

I obtained it in pretty large flat rhomboidal prisms. These prisms uniting together form larger ones, terminated in echelon, and striated longways, like those of sulphate of soda. They contain a great deal of water of crystallization, and yet are very deliquescent. Heat drives off this water, melts the salt, and then renders it somewhat alkaline. It does not sublime so easily as hydriodate of potash. 100 parts of water, at the temperature of about 57°, dissolve 173 of the salt. When dried, it must be considered as an ioduret of sodium. I found that 100 parts of iodate of soda give, when decomposed by heat, 24-45 of oxygen. From the data given by that analysis, we may conclude the composition of the ioduret of sodium and hydriodate of soda to be as follows:-

Ioduret of sodium... ƒ Iodine

.....

Sodium

...

100
18.536

Hydriodate of soda.....

f Acid..
Soda

100

24.728

The hydriodates of potash and soda converted into iodurets by desiccation, are the only ones not altered when heated to redness in contact with the air. The reason is, that iodine decomposes the oxides of potassium and sodium.*

Hydriodate of Barytes.

This salt crystallizes in very fine prisms, very similar in appearance to the hydro-chlorate of strontian. After about a month's exposure to the air, I found it partly decomposed. Water dissolved the hydriodate coloured by iodine, and there remained undissolved subcarbonate of barytes. Hence the hydriodic acid is gradually destroyed by exposure to the air. Its hydrogen has formed water, and its iodine has been dissipated in the atmosphere, or has remained dissolved in the undecomposed hydriodate. The hydriodate of barytes, though very soluble in water, is but faintly deliquescent. When evaporated in a close vessel, and heated to redness, it does not melt, nor is its state of neutralization altered. If air, or, still better, if oxygen, be made to play on its surface when thus heated, vapours of iodine show themselves in abundance, and the salt becomes alkaline. I did not continue the experiment till the iodine ceased to be disengaged; but I presume that the hydriodate would be changed into a subioduret, as we have seen before that this was the compound obtained when iodine in vapour was passed over barytes at a red heat. I have said that iodine does not disengage oxygen from barytes; yet I have no doubt that a red heat changes hydriodate of barytes into ioduret of barium. I have passed

* As iodine disengages oxygen from the oxides of lead and bismuth, it is evident that the iodurets of these metals will not be decomposed by the air at a red heat,

hydriodic gas cooled to - 4° over barytes obtained from the recent calcination of the subnitrate; the barytes instantly became incandescent, and water made its appearance in the vessel; yet this barytes gave no oxygen gas when dissolved in water; nor did it ndergo any alteration when I passed over its surface a current of dry hydrogen gas. I ascertained, likewise, that sulphur disengaged nothing; but that hydro-sulphuric gas produced much water when it combined with it. We cannot, therefore, doubt that at a red heat, and even at a lower temperature, hydriodate of barytes is converted into ioduret of barium.

The hydriodates of lime and strontian are very soluble, and the first is exceedingly deliquescent. I have neither determined the shape of their crystals, nor the quantity of water necessary to dissolve them. The hydriodate of strontian melts below a red heat, while the hydriodate of lime requires a higher temperature for its fusion. If they are heated in close vessels they become only slightly alkaline; but if air or oxygen have access to them while hot, thick vapours of iodine are immediately exhaled. If we consider these compounds as iodurets, the calcium and strontium are oxydized, and abandon a portion of the iodine. If we consider them as hydriodates, the hydrogen of the acid must combine with oxygen, and water be formed. I endeavoured to ascertain whether this was the case by passing dry oxygen gas over hydriodate of lime

The action of the hydrosulphuric gas was accompanied by a strong heat. The compound which was partly fused, being treated with hydrochloric acid, hydrosulphuric gas was disengaged, and little sulphur precipitated. From this it is probable that a sulphuret with excess of sulphur is formed, and hydrogen disengaged. But as I employed a sulphuret which yielded a gas not totally absorbed by the alkalies, I could not ascertain the fact. Yet the abundant production of water which accompanies the combination of hydrosulphuric gas with barytes, and even with strontian, cannot be explained, except by admitting that these alkalies are reduced by the hydrogen in consequence of the united affinities of the oxygen for hydrogen, and of the metals for sulphur. But if this be the case, it is very probable that many metallic precipitates, which have been taken for hydrosulphates (hydrosulphurets) are only sulphurets. At a red heat, all the oxides which combine with sulphur give out water, and are changed into sulphurets when hydrosulphuric gas is brought in contact with them. This fact proves nothing against the existence of hydrosulphates at a low temperature. But hitherto there is not a single decisive experiment in proof of their existence; while the insolubility of them all seems to be a fact of an opposite nature. To confirm these conjectures, I dissolved a determinate weight of zinc in hydrochloric acid. I supersaturated the solution by ammonia, and precipitated by hydrosulphuric acid. The precipitate dried in the temperature of between 140 and 176° assumed the appearance of horn. Its weight was too great for a sulphuret, and too small for a hydrosulphuret. When heated to 212° it gave out water, and a new quantity was disengaged at a higher temperature. This experiment is not entirely decisive; but from the appearance of the precipitate I think it was a hydrate. At all events, this experiment is rather favourable than otherwise to my conjecture.