Concentrated sulphuric acid, nitric acid, and chlorine, immediately decompose hydriodic acid. They seize upon its hydrogen, and the iodine precipitates, or exhales in purple vapours. Chlorine is one of the most delicate re-agents to detect small quantities of hydriodic acid; but it must be added cautiously: for when an excess is employed, it dissolves the iodine before it has time to precipitate, or at least colour the liquid. Like hydrosulphuric acid, hydriodic acid is decomposed by solutions of peroxide of iron. When heated by the oxides which give chlorine with hydrochloric acid, iodine is evolved, together with a hydriodate or an ioduret. If for example, it be heated with the black oxide of manganese, we obtain iodine and hydriodate of manganese; but with the red oxide of lead, we obtain iodine and an ioduret. Hydriodic acid forms compounds with the different bases, which have a great deal of resemblance to the hydrosulphates and the hydrochlorates.

Let us recapitulate the principal characters of hydriodic acid. In the gaseous state it is speedily decomposed by mercury, which is converted into an ioduret of a greenish yellow colour. With chlorine it immediately produces a fine purple vapour of great intensity. In the liquid state it is speedily decomposed, and coloured when exposed to the air. Concentrated sulphuric acid, nitric acid, and chlorine, separate iodine from it. Sulphureted hydrogen does not alter it in the least. When poured into a solution of lead it forms a fine orange precipitate. In the solution of peroxide of mercury it forms a red precipitate, and with silver a white precipitate insoluble in ammonia. I thought proper to give the properties of hydriodic acid in this place, because this will render more intelligible the account which I am going to give of the combinations of iodine with other bodies.

Iodine forms with sulphur a weak compound of a greyish black colour, radiated like sulphuret of antimony. Iodine is separated from it when it is distilled with water.

Hydrogen, whether dry or moist, did not seem to me to have any action on iodine at the ordinary temperature; but if, as was done by M. Clement, in an experiment in which I was present, we expose a mixture of hydrogen and iodine to a red heat in a tube, they unite together, and hydriodic acid is produced, which gives a reddish brown colour to water. We found that 100 parts of iodine absorb 1.53 of hydrogen, in order to be converted into an acid. But this proportion is a great deal too great, as I found afterwards that hydriodic acid is composed of 100 iodine and 0-849 hydrogen.

Charcoal has no action upon iodine, either at a high or low temperature. Several metals on the contrary, as zinc, iron, tin, mercury, and potassium attack it with facility, even at a low temperature, provided they be in a divided state. Though these combinations take place readily, they produce but little heat, and but rarely any light. The compound of iodine and zinc, which I call ioduret of zinc, is white. It melts readily, and is sublimed in the state of fine acicular four-sided prisms. It is very

soluble in water, and rapidly deliquesces in the air. It dissolves in water without the evolution of any gas. The solution is slightly acid, and does not crystallize. The alkalies precipitate from it the white oxide of zinc, concentrated sulphuric acid disengages hydriodic acid and iodine, because sulphurous acid is produced. We may conceive that water dissolves the ioduret of zinc without undergoing decomposition; but as the slightest force would afterwards decompose it, and besides, as the solution has exactly the same characters as the hydriodate of zinc obtained by combining the oxide of zinc with hydriodic acid, we have the same motives for admitting that the water is decomposed during the solution of the ioduret, as for admitting that it is formed when hydriodic acid dissolves the oxide. We may, however, adopt either supposition. For the sake of greater simplicity I shall adopt the latter, in order to determine the ratio of iodine to oxygen and hydrogen.

When iodine and zinc are made to act on each other under water in vessels hermetically sealed, on the application of a slight heat the water assumes a deep reddish brown colour, because as soon as hydriodic acid is produced it dissolves iodine in abundance. But by degrees, the zinc which I suppose in excess, combines with the whole iodine, and the solution becomes colourless like water. In three experiments, which differed little from each other, and of which I have taken the mean, I found that 100 iodine combined with 26.225 zinc. But 26.225 zinc combine with 6.402 oxygen, which saturate 0849 hydrogen; consequently, the ratio of oxygen to iodine is 6·402 to 100, or 10 to 156°21, and the ratio of hydrogen to iodine is 0.849 to 100, or 1.3268 to 156-21. Thus if we represent the number for oxygen with Dr. Wollaston by 10, the number for iodine will be 156 21; the ratio which I assigned in my first experiments, as well as that of Davy, was very inaccurate.

Iron is acted upon by iodine in the same way as zinc. The ioduret of iron is brown, and fusible at a red heat. It dissolves in water, and the colour of the solution is a light green, like that of the chloruret of iron.

Iodine and potassium combine with a great deal of heat, and with the disengagement of a light which appears violet through the vapour of iodine. The compound melts, and sublimes at a red heat. On cooling it assumes a pearly and crystalline appearance. Its solution in water is perfectly neutral. It is easy to determine the proportion of these iodurets from those of ioduret of zinc. If we attend to this circumstance, that the quantities of iodine which combine with each metal, are proportional to the quantities of oxygen with which it combines; of course 100 potassium, which requires 20-425 oxygen to convert it into potash, combine with 319.06 of iodine.

The ioduret of tin is very fusible. When in powder, its colour is a dirty orange yellow, not unlike that of glass of antimony. When put into a considerable quantity of water it is completely

decomposed. Hydriodic acid is formed, which remains in solution in water, and the oxide of tin precipitates in white flocks. If the quantity of water be small, the acid being more concentrated, retains a portion of oxide of tin, and forms a silky orange-coloured salt, which may be almost entirely decomposed by water. Iodine and tin act very well on each other in water of the temperature of 2120. We may, by employing an excess of tin, obtain pure hydriodic acid, or at least an acid containing only traces of the metal. The tin must be in considerable quantity, because the oxide which precipitates on its surface diminishes much its action on iodine.

Antimony presents with iodine the same phenomena as tin; so that we might employ either for the preparation of hydriodic acid, if we were not acquainted with preferable methods.

The iodurets of lead, copper, bismuth, silver, and mercury, are insoluble in water, while the iodurets of the very oxidable metals are soluble in that liquid. If we mix a hydriodate with the metallic solutions, all the metals which do not decompose water will give precipitates, while those which decompose that liquid will give none. This at least is the case with the metals of which I have spoken and if this fact, which I consider as general, be not a sufficient proof of the existence of hydriodates, it at least renders their existence probable.

There are two iodurets of mercury: the one yellow, the other red; both are fusible and volatile. The yellow, which corresponds to the protoxide of mercury, contains one-half less iodine than the red, which corresponds to the protoxide. In general there ought to be for each metal as many iodurets as there are degrees of oxidation.

All the iodurets are decomposed by concentrated sulphuric and nitric acids. The metal is converted into an oxide, and iodine is disengaged. They are likewise decomposed by oxygen at a red heat, if we except the iodurets of potassium, sodium, lead, and bismuth. Chlorine likewise separates iodine from all the iodurets; but iodine, on the other hand, decomposes most of the sulphurets and phosphurets.

(To be continued.)

ARTICLE IV.

Some Observations on the Sap of the Vine. By Dr. Prout.

ABOUT the middle of April last I was favoured by Mr. Astley Cooper with some sap which he had collected from a common white vine.* The following are a few of its properties :

It was slightly opake, or rather had the whitish appearance of common river water. Taste sweetish, but not rough. No smell.

Mr. C. informs me that the vine, although it bled very profusely, seemed to produce a greater number of leaves than usual, but no grapes.

It did not affect litmus or turmeric papers in their natural states, nor the former when it was faintly reddened by acetic acid. Specific gravity not sensibly different from that of water.

1. Potash.-A solution of pure potash changed it to a beautiful reddish copper colour, and caused after some little time a flaky precipitate of the same colour, leaving the fluid nearly colourless and transparent. This precipitate was not redissolved by excess of potash even when heat was applied, but the application of heat changed it to a deepish brown colour. Acetic acid added in slight excess readily redissolved this precipitate.

2. Ammonia.-The same phenomena followed the use of this alkali as of potash above-mentioned. Acetic acid also as readily redissolved the precipitate occasioned by it as by potash. 3. Muriatic Acid produced no apparent change. After the addition of this acid, ammonia was added in excess, when precisely the same sort of precipitate as that above-mentioned was observed." 4. Oxymuriatic Acid produced no apparent change.

5. Muriate of Barytes.--No apparent change.

6. Oxalate of Ammonia produced a very sensible white precipitate.

7. Prussiate of Potash caused a very slight whitish precipitate. 8. Hydro-sulphuret of Potash.-A slight dark brown flaky precicipitate.

9. Nitrate of Silver.-A slight flaky precipitate, which soon became of a purple hue.

10. Oxymuriate of Mercury.-No apparent change.

11. Subacetate of Lead.-A copious yellowish-white precipitate. 12. Infusion of Galls.-No apparent change.

13. Gelatine-No apparent change.

Four hundred and sixty grains of the sap were evaporated in a glass capsule on a sand-bath. During the evaporation air bubbles collected on the sides and bottom of the vessel. The fluid became slightly opake, and towards the end of the operation brown flocculi precipitated. There was left only gr. of solid matter (= ·044 per cent.), about half of which was carbonate of lime, the rest a peculiar vegetable matter. This peculiar vegetable matter was not soluble in alcohol, and therefore did not agree in this respect with the ill-defined class of substances called extracts or extractives. Both it, however, as well as the lime, were evidently held in solution by some volatile acid or acids. One of these acids was doubtless the carbonic. There were also traces of the acetic acid, and likewise of an alkali (potash?), since the glass capsule on exposure to the air became sensibly moist. The quantities, however, of the last two were extremely minute.

Every thing connected with vegetable physiology is exceedingly The opinion, however, appears to be correct, which

obsure.

Darwin's Phytologia, p. 28, &c. Mr. Knight, in Philosophical Transac tions, 1805, p. 70, &c.

supposes that one use of the large quantity of watery sap which flows in plants in the spring is to dissolve the thick and otherwise inactive juices which had been deposited in nearly a solid state in the vessels during the winter, and thus adapt them to the further uses required by the economy of the plant. It appears also that this solution is not effected solely by the agency of the water, which alone would perhaps be insufficient, but by the assistance of some acid or saline agent which probably pre-existed in the plant itself, and only required the presence of water to render it effective. And certainly we cannot conceive menstrua better adapted for this purpose than the above acids, not only from their considerable solvent powers, but from the ease with which they may be got rid of, either by decomposition, exhalation, &c. when they have performed their office.

I know of no use of the large proportion of lime found in all saps; but I have some reason for believing that there is a much greater relation between this earth and the saccharine principle than has been commonly imagined.

The saps of different vegetables have been examined by Vauquelin, Chaptal, and others. There is some resemblance between the sap of the common elm as observed by Vauquelin, and the above. See Ann. de Chim. xxxi. 20; Memoires de l'Institut National, i. 288; also Dr. Thomson's Chemistry, vol. v.

ARTICLE V.

On the Use of the Cerebellum, on the Spinal Marrow, and on Respiration. By Dr. John Cross.

SIR,

(To Dr. Thomson )'

Glasgow, Nov. 18, 1814.

PLEASE to announce in your Annals of Philosophy the following discovery of the function of the cerebellum, and new theory of respiration.

From considering that the cerebrum, cerebellum, and face have in the gradual progress of animality originated and evolved together, and that organs in general have their sphere of action in their own vicinity, I was led about a year ago to conclude that as the cerebrum is the fountain of sensation and intellect, the cerebellum must be the organ which supplies with nervous energy the face and other parts of the head extrinsical of the brain, perhaps also the cerebrum itself. This opinion pressed more and more strongly on my mind until I was at length tempted to use the trephine upon living animals. Having cut out a circular piece of the occipital bone of a sheep, and laid bare the cerebellum, I applied pressure upon it with the handle of a scalpel; immediately the ears, eyes, mouth, tongue, in short the whole muscles about the head and face, became convulsed. On thrusting the handle of the scalpel