which he followed in order to determine the point. It was, however, reasonable to expect, a priori, that it should constitute a constituent of sea water, otherwise it is difficult to conceive how it should make its way into the fuci which derive all their nourishment from that liquid.

1. Iodine, obtained by the process described above, is a solid substance of a blueish white colour and metallic lustre, so as to bear a considerable resemblance to plumbago. It is still more like galena; but perhaps approaches most nearly to the appearance of the crystallized black oxide of manganese. It is so soft that the particles of it may be easily squeezed together between the fingers, bearing, in this respect, a considerable resemblance to an amalgam of mercury.

It has a peculiar smell, which has been compared to that of chlorine, only it is said to be much weaker. But to us it does not convey any such resmblance. The smell seems quite peculiar, and cannot be compared to any other with which we are acquainted.

Iodine is very volatile. If it be left in the open air it speedily disappears, and perfumes the apartment with its peculiar odour. When heated to 225° it melts, and, under the common pressure of the atmosphere, it is volatilized into a violet vapour at the temperature of 350°. It may, however, be distilled over along with the vapour of water.

Iodine crystallizes into rhomboidal octahedrons as was first ascertained by Dr. Wollaston. His method was to put a little iodine into a dry phial, and to set this phial on its side near a common fire. The iodine gradually volatilized and assumed the form sometimes of rhomboidal plates, sometimes of octahedrons. The axes of these octahedrons are to each other, as nearly as can be estimated, as the numbers 2, 3, 4.

Water dissolves about one seven thousandth part of its weight of iodine and acquires an orange yellow colour. Alcohol and ether dissolve it in considerable quantities. These solutions are reddish brown. Water precipitates the iodine from the alcohol in small crystals, which appear at first reddish, but gradually assume the natural colour of iodine.

Iodine has an acrid taste notwithstanding its little solubility in water. From the experiments of M. Orfila we learn, that when taken internally it is poisonous. He swallowed six grains of it. The consequence was a most horrible taste, salivation, epigastralgia, colic, nausea, and violent vomiting. When given to dogs in the quantity of 72 grains or more it generally produces speedy vomiting, by which means it is thrown out of the system and the animal saved. But if vomiting does not take place, or if it be prevented by tying the oesophagus, death ensues in the course

of a few days, without being preceded by any particular symptoms.

Iodine stains the fingers of a deep brown; but the stain very speedily disappears. The specific gravity of this substance, according to Gay-Lussac, is 4.948 at the temperature of 62°. But Clement found it only 4, and the writer of this article found the specific gravity of iodine in pretty large crystals only 3.791. There seems reason therefore to doubt the accuracy of the number given by Gay-Lussac.

Iodine is a supporter of combustion; but a much more imperfect one than any of the other supporters previously known. Potassium, when placed in contact with it, or surrounded by its vapour, burns with a pale blue flame. Phosphorus rapidly combines with it, and much heat is evolved. According to Vauquelin light also appears. But we were not able to observe any such evolution of light in our experiments.

As all attempts to decompose iodine have failed, we must consider it as a simple substance. As it unites, like oxygen, with combustible bodies, and as during this union combustion in certain cases takes place, we must consider it as a supporter of combustion. So that the supporters of combustion at present known are three in number; namely oxygen, chlorine, and iodine. And if fluorine, which there is the strongest reason for considering as a supporter likewise, be ever obtained in a separate state, we shall in that case be acquainted with no fewer than four of these bodies. Let us now consider the compounds which iodine forms with the simple combustibles.

2. The compounds which oxygen forms with the simple combustibles have received the name of oxides, those which chlorine forms are called chlorides. Analogy leads us to give the name of iodides to the compounds which iodine forms with the same bodies. This name appears to us better than the French term iode; we shall therefore adopt it in preference. The simple combustibles have been divided into two classes; namely, simple combustibles proper, and metals. It will be convenient to take these two classes separately.

I. The simple combustibles proper at present known are phosphorus, sulphur, carbon, boron, silicon, and hydrogen, to which we must add azote, though it is not strictly speaking combustible; but in other respects it resembles the other bodies of this class, Of these seven bodies there is one with which iodine does not seem capable of uniting; namely, carbon. Its action on boron and silicon has not hitherto been tried. So that there remains four bodies, phosphorus, sulphur, hydrogen, and azote, with which its combinations have been ascertained.

(1.) Iodide of phosphorus. If iodine be put into a glass tube

shut at one end, and a bit of phosphorus be dropped in, a violent action immediately takes place. The phosphorus melts, great heat is evolved, and an iodide of phosphorus is produced. The colour of this substance is greyish black, its texture is crystalline, and it easily melts when heated. The combination takes place whatever proportion of phosphorus and iodine be employed. But there is one particular proportion in which there is no redundancy of either of the constituents. This, according to GayLussac, takes place when we mix 1 part by weight of phosphorus with 8 parts of iodine. But, according to our experiments, the true proportions are 1 phosphorus and 94 iodine. The iodide thus formed is soluble in water. The solution is colourless. When the iodide is dropped into water a kind of effervescence takes place, and a strong odour is exhaled similar to that of muriatic acid. Both the iodine and the phosphorus are converted into acids. The water is decomposed, its oxygen uniting to the phosphorus constitutes phosphorous acid, while the hydrogen uniting to the iodine constitutes the peculiar volatile acid to which the odour of the liquid is owing. This acid has received the name of hydriodic acid.

It is extremely probable that there exists likewise a biniodide of phosphorus composed of 1 phosphorus and 19 iodine. It is certain that the two substances unite in this proportion. In colour it resembles the preceding iodide. But it is fusible at the tempe rature of 84°.

Thus it appears that the iodide of phosphorus, like the chloride of the same body, cannot be brought in contact with water without undergoing decomposition. As we are not at present acquainted with the oxide of phosphorus (unless phosphorous and phosphoric acids be distinguished by that name), we do not know whether it possesses a similar property.

(2.) Iodide of sulphur. Iodine and sulphur do not unite by simple contact; but if they be mixed in a glass tube and gently heated the combination takes place very readily. The colour of this compound is greyish black, and its texture is radiated like that of sulphuret of antimony. According to our experiments this iodide is composed of 1 part sulphur and 7.8 iodine. It does not appear to be immediately decomposed by water. But according to Gay-Lussac the iodine is separated, if this iodide be distilled with water. This iodide is of a more permanent nature than the chloride of sulphur, which is decomposed, and the sul phur deposited the instant it comes in contact with water.

(3.) Iodide of hydrogen, or hydriodic acid. When iodine is put into a vessel filled with hydrogen gas, no combination takes place. But if the vapour of iodine and hydrogen gas be passed together through a red hot tube, they unite and form hydriodic acid.

There are various ways of obtaining this acid. The first method practised was to pour water over the iodide of phosphorus, and expose the mixture to heat. The acid which is gaseous was expelled, and received in proper vessels. Gay-Lussac pointed out a still easier method of obtaining this acid in a state of purity. Put a quantity of iodine and water into a glass vessel, and cause a current of sulphureted hydrogen gas to pass through it. The iodine speedily dissolves. When that has taken place stop the process and expose the liquid to heat to drive off the excess of sulphureted hydrogen. The residual liquid is a solution of pure hydriodic acid in water. During this process the sulphureted hydrogen is decomposed. Its hydrogen unites to the iodine while its sulphur is precipitated, and is separated from the liquid by filtration.

Hydriodic acid thus prepared is a colourless liquid, having an odour very similar to that of muriatic acid, and a sharp acid taste, leaving behind it a sensation of astringency. By heat it may be driven off in the gaseous form and collected in proper vessels.

Hydriodic acid gas is colourless, and possesses the peculiar taste and smell of the liquid acid. Its specific gravity, according to the experiments of Gay-Lussac, is 4.443, that of common air being 1. So that it is by far the heaviest gaseous body at present known. When it comes in contact of mercury, it is immediately decomposed by the action of that metal. The mercury unites with the iodine, and forms an iodide, while the hydrogen gas, the other constituent, is left in the gaseous state. It occupies exactly one half of the volume of the hydriodic acid. Hence it follows, that hydriodic acid gas is composed of 1 volume of iodine in the state of vapour, and 1 volume of hydrogen gas, constituting together two volumes. The specific gravity of hydriodic acid, therefore, must be just the mean of the specific gravity of hydrogen gas and the vapour of iodine. This puts it in our power to calculate the specific gravity of the vapour of iodine, and to determine the weight of each of the constituents of hydriodic acid. The specific gravity of hydrogen gas being 0.073, we have 0.073 + x = 4.443, x being the specific gravity of the vapour of

2

iodine. Of course, by resolving the equation, we get x = 8.813. Thus the vapour of iodine is by far the densest of all the aërial bodies with which we are acquainted. Hydriodic acid is composed by weight of 73 hydrogen and 8813 iodine, or of 1 hydrogen and 120.72 iodine.

Liquid hydriodic acid, when as much concentrated as possible, is of the specific gravity 1.7. It smokes like muriatic acid, though not so perceptibly. But if a vessel containing it be placed beside another containing chlorine (supposing both open), a purple

coloured atmosphere is formed between them, showing very evidently the volatility of the hydrionic acid. This acid boils at 26210.

When hydriodic acid gas is passed through a red hot tube, it is decomposed at least partially. The decomposition is complete if the hydriodic acid gas be mixed with oxygen gas. In that case water is formed, and iodine set at liberty. From this experiment it seems to follow that iodine is incapable of decomposing water. Accordingly, if the vapours of iodine and water be passed together through a red hot tube no oxygen gas is disengaged; yet if water holding iodine in solution be exposed to the solar light, or if it be heated, its peculiar colour disappears, and the iodine is converted partly into hydriodic and partly into iodic acid. Now this last acid is a compound of iodine and oxygen; so that in this case water must be decomposed, and one of its constituents must go to the formation of hydriodic, and the other to that of iodic acid.

Liquid hydriodic acid very readily dissolves iodine, and acquires a brown colour. Even exposing the liquid acid to the atmosphere gives it this colour; because the oxygen of the atmosphere decomposes a portion of the hydriodic acid uniting with its hydrogen, and setting the iodine at liberty, which is immediately dissolved by the undecomposed portion of the acid.

Concentrated sulphuric acid, nitric acid, and chlorine, decompose hydriodic acid. They seize upon its hydrogen while the iodine precipitates or exhales in purple vapours. In consequence of this property chlorine is a delicate test of the presence of hydrionic acid, provided it be added cautiously and in small quantity. For an excess of it dissolves the iodine before it has time to precipitate, or to colour the liquid. Hydriodic acid is decomposed and the iodine evolved by peroxide of iron, black oxide of manganese, red and brown oxides of lead, and, in short, all the substances which produce chlorine when digested with muriatic acid.

Hydrionic acid combines with the different bases, and forms a class of salts called hydriodates, which we shall describe in a subsequent part of this article.

(4.) Iodide of azote. Iodine cannot be made to unite directly with azotic gas; but the compound is easily formed indirectly. It was discovered by M. Courtois, and its properties first accurately examined by M. Colin. Nothing more is necessary than to put a quantity of iodine in fine powder into liquid ammonia. A brownish black powder is speedily formed, which is the iodide of azote. In this case the ammonia is decomposed, its hydrogen uniting to iodine forms hydriodic acid, while its azote combining with another portion of the same substance constitutes the iodide