from water, at the temperature 64° F. It has a penetrating suffocating odour. It is one of the most deadly of known poisons. It readily undergoes decomposition under the influence of light.

Now, to what are we to attribute this remarkable parallelism between chlorine and cyanogen, the one a body which we assume to be simple, the other a body which we know to be compound? Is it possible to believe, that these substances, so much alike, are yet dissimilar in their essential constitution, and that the one is compound and derivative, and the other simple, and not derived from other bodies? Are we not compelled to believe, that substances so much alike in their chemical actions are of a similar nature, and that what is true of the one, with respect to its constitution, must be true of the other? Can we place in the balance against so many strong analogies, the merely negative result, that we have been unable to decompose chlorine in the laboratory? If such reasoning is allowed in Chemistry, it will be admitted that there is no other branch of physics in which it would be tolerated.

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Cyanogen is represented by C2 N, which, upon the supposition made in the table, H2 C4, and chlorine upon the same data is represented by H11 C4, so that the two bodies, under the suppositions made, present the closest analogy in composition, and both contain the elements of nitrogen. And even independently of the conclusion arrived at by this train of argument, we should have been justified, when we had determined that cyanogen contained nitrogen as a base, in assuming that chlorine likewise contained nitrogen, or the elements of nitrogen, as a base.

VII. BROMINE.

Bromine exists in salt springs and the waters of the ocean, and is found in the ashes of certain maritime plants and marine mollusca. It appears to exist in sea-water, as a bromide in combination with sodium or magnesium, and may be procured from this source. The agent employed to separate it is chlorine, which, from its superior affinity, unites with the base, and sets the bromine free. The bromine is then driven off by heat, in the form of vapour, and condensed in a tube cooled by ice; but other methods more convenient for obtaining it in quantity may be adopted.

Bromine at common temperatures is in the liquid state. It appears opaque, and of a dark reddish-brown colour, when looked at in a mass, but of a hyacinth-red, when in a drop or thin stratum, and seen by transmitted light. Its taste is acrid and strong, and its odour suffocating and offensive; from which latter character its name has been derived, Bgwuos, signifying strong or bad odour. When cooled a few degrees below zero, it congeals all at once, becomes brittle, and it may be said, metallic. It is a volatile substance, emitting, at common temperatures, red-coloured fumes, resembling those of nitrous acid, N O4. Its vapour is entirely irrespirable. A lighted taper placed in it, burns for a few instants with a flame green at the base, and red above, and then is extinguished. When certain metals are brought into contact with it, they burst into a bright flame, and burn, as in the case of chlorine, brilliantly. It is soluble, though in small quantity, in water, producing an orangecoloured solution. When in contact with water at 32°, it forms a hydrate of octahedral crystals, of a fine red tint.

When dropped into a solution of starch, it gives it an orangeyellow colour.

It acts upon vegetable colours, and destroys them. It disorganizes organic substances, as wood and muscular fibre. If it touches the skin, it gives a yellow tinge. It is a most fatal poison, if taken internally. A drop of it placed on the beak of a bird, has caused almost instant death. It is, like sulphur, chlorine, and iodine, a non-conductor of electricity. Its specific gravity in the solid state is 2.9966, and the density of its vapour is 5.54. Its combining weight has been calculated at 78.39.

In all its combinations it exhibits the closest parallelism with chlorine. Uniting with hydrogen, it forms an acid in all respects similar to hydrochloric acid. This acid, in the state of gas, has an acid taste and suffocating odour. It is rapidly absorbed by water, and, when mixed with the air of the atmosphere, it forms dense white vapours. It is wholly irrespirable.

Bromine and chlorine, then, are two substances closely connected together, and must be supposed, on every fair presumption, to be derived from some common root. By the table, they are both supposed to be derived from the secondary root C O, chlorine being 2 CO+H7, bromine 5 CO + H3.

Supposing this to be the composition, or something approaching to the composition of bromine, we can readily enough conceive that bromine may be formed in sea-water, where all the elements required to form it are present; but we can scarcely conceive it to be a physical truth, that two distinct orders of molecules have been created, the one to form chlorine, and the other to form the rare body universally associated with it in the natural state, and which may so easily be conceived to be derived from it.

VIII. IODINE.

This substance so greatly resembles chlorine in its chemical characters and relations, that it is by all chemists comprehended in the same class of bodies. It is found in the waters of the ocean, and in saline springs. It has been found in the oyster, and other marine mollusca, in the sponge and other zoophytes, and largely in sea-plants, from the ashes of which it may be obtained.

Iodine is an opaque solid, of a bluish colour and metallic lustre. It is usually in crystalline scales, but may be obtained from solutions in octahedral crystals. It has an acrid taste, and pungent odour like that of chlorine. It stains the skin of a deep brownish yellow, and, like chlorine, destroys vegetable colours, although its action is more feeble. It fuses at 225°, and enters into ebullition at 347°. But when exposed to even a gentle heat, it rises in the form of a beautiful vapour, which condenses into crystals as it cools. It is from the colour of this vapour, which is of a rich violet, that iodine has derived its name, iwons. The density of this vapour being 8.7, is greater than that of any known gas.

Iodine is very sparingly soluble in water, requiring about 7000 times its weight of the liquid for solution. It is not inflammable, but, under certain conditions, can be made to unite with oxygen. It is a virulent poison on the animal system, although used in minute quantities as a medicine. It has the property of uniting with starch, and forming a compound which is insoluble in cold water, and distinguished by its deep blue colour. It has an extensive range of chemical affinities similar to those of chlorine and bromine, but less powerful.

Hydriodic acid resembles the corresponding compound of

chlorine in its general characters. It is in the gaseous form, has an odour similar to that of hydrochloric acid, and, like it, reddens vegetable colours. It is instantly absorbed by water, and, when mixed with the air of the atmosphere, white fumes are produced.

The close relation, then, of iodine and chlorine cannot be doubted, and the question naturally arises, whether it is more reasonable to conclude, that these analogous bodies are derived from the same molecules, or that each is composed of molecules, original, distinct, and proper to itself? Surely we ought not to hesitate between the two hypotheses. The bodies themselves have all the marks of being derived from a common root; the one is an abundant product of nature, the other is found only in small quantity, and never but when the other is present, or may have been present.

In the table, iodine is represented by 8 CO+H11, making its combining weight 126.64, which agrees, within a fraction, with that assigned to it by experiment, namely, 126.57. It may be supposed to be derived from the same secondary root as chlorine and bromine, namely, CO.

If this be the composition, or nearly the composition, of iodine, we can have little difficulty in accounting for its existence when sea-water is present. It is nothing that we have been unable to form such a compound in the laboratory. It is formed in the laboratory of the animal and vegetable organs, by a chemistry which we cannot imitate; and this mutation is no more surprising than that of carbon and the elements of water into gum and sugar, nor than the innumerable processes of the animal system by which known bodies are converted into compounds which cannot be produced by art.