PROPERTIES.-Bromine, at ordinary temperatures, and pressures, is a deep brown liquid, of very peculiar odour. Its specific gravity is about 3. It may be solidified by exposure to a temperature somewhat below 0. It does not conduct electricity. Is sparingly soluble in water, with which it forms a hydrate at low temperatures, like chlorine. In alcohol bromine is more soluble, but chiefly so in ether. It is a bleaching agent,—and a supporter of combustion, but not a combustible.

IODINE. ETYMOLOGY, HISTORY, NATURAL HISTORY, PREPARATION, AND PROPERTIES.

ETYMOLOGY.-iwdns, violaceus.

HISTORY.-Iodine was discovered in 1812, by M. Courtois, a chemical manufacturer at Paris*, but for a knowledge of its chief chemical relations we are indebted to Sir H. Davy.

NATURAL HISTORY.- Iodine, like bromine, is widely, though sparingly distributed. In the inorganic kingdom, it is a constituent of all sea water, and many springs, also in certain mineral species;-thus, in certain Mexican silver ores, according to Vauquelin ;† and Bustamenta detected it in a white-lead ore from Mexico;‡

* Vauquelin, Ann. de Chim. xc.; Gay Lussac, ibid, lxxxviii. xc. and xci.; Davy, Phil. Trans. 1814.

† Ann. C. et P. xxix.

Ann. Ch. et de Ph lxxii.

of numerous fuci, certain sea mollusks, and other animals still higher in the scale of creation. Iodine is now chiefly obtained from kelp, or the fused ash of certain sea-weeds more especially the fucus palmatus and vesiculosus.

PREPARATION.-Iodine is prepared from kelp in the following manner :-An aqueous solution of kelp, is evaporated, until all readily crystallisable bodies separate. The mother water is then evaporated to dryness, the residue mixed with black oxide of manganese and sulphuric acid, and distilled.

Theory of the Process.-The residue of the evaporation of kelp solution being a mixture of iodide of sodium with iodide of potassium and magnesium, these salts, especially the former, may be regarded as the proximate source of the resulting iodine; and by transposing chloride of sodium for iodide of sodium, &c., the general chart of decomposition given in the diagram relating to the evolution of chlorine, it will apply in this case.

PROPERTIES.-A dark resplendent looking crystalline solid at ordinary temperatures and pressures; crystallises in the rhombohedral system of Weiss and Mohs;* colour, bluish black. When heated, iodine

*The primitive form of its crystal being a rhombic octohedron. (Buchner's Repert für die Pharmacie, 2te Reiter, Band xx. S. 43.)

evolves violet coloured fumes, whence the term iodine, from ions violaceus, is derived. Fuses at 228° F., and at 350° F. boils. 100 cubic inches of its vapour weigh 270 grains. Hence its specific gravity, air being unity, is 8·7, and it is 126 times heavier than hydrogen, which also is the ratio existing between their atomic weights. Like chlorine and oxygen, it is evolved at the positive electrode or anode, of a voltaic combination,—and hence it is an anion. Like chlorine and bromine also, it is sparingly soluble in water—namely, about one part in 7000. In alcohol and ether, it is far more soluble. When added to a solution of potash or soda, two salts are formed in either case, iodide of potassium and of sodium respectively. The distinctive test for iodine is solution of starch, with which it strikes a deep blue colour. The solution, however, must be cold, and no alkali must be present.

LECTURE II.

CHLORINE. BROMINE. IODINE.

MATERIALS AND APPARATUS REQUIRED FOR ILLUSTRATING
THE LECTURE.

Mixture of hydrochloric acid and peroxide of manganese in a retort, for the purpose of generating chlorine.

Pneumatic water-trough, jars, and bottles, for the purpose of collecting chlorine.

Some chlorine collected.
An aqueous solution of chlorine.
Table of the weight of chlorine.
Phosphorus.

Sulphur.
Oil of turpentine.
Deflagrating ladles.
Glass plates.

Piece of charcoal on copper wire. Blow-pipe and spirit-lamp, for the purpose of making the charcoal red-hot.

Powdered antimony.

Copper leaf loosely thrust into a glass jar, to match with another glass jar containing chlorine.

A piece of blue litmus paper and

THE non-metallic simple bodies which I design to be the subjects of discourse on this occasion are Chlorine, Bromine, and Iodine, bodies which constitute a wellmarked triad, as possessing analogous properties amongst

themselves, and which, although they are not so largely distributed as oxygen, play nevertheless a very important part in the world's economy.

First, then, directing our attention to Chlorine, let us observe the more obvious physical properties of the element, of which we have specimens before us already prepared. Like oxygen, this element, when liberated from combination and obtained in its free or uncombined state, is a gas; but the physical qualities of chlorine differ from those of oxygen in several respects. In the first place, you will observe it to be endowed with a peculiar yellowish-green colour, whence its name is derived. In the second place, it possesses a highly irritating, remarkable odour, only too evident despite all our precautions, and which is altogether distinctive. Although gaseous, then, it appears that chlorine differs from oxygen in its most obvious physical properties. Again, it may be demonstrated by an experiment which could not be conveniently performed here, that it possesses a marked difference as to weight. 100 cubic inches of oxygen gas weigh, as we have seen, 34.6 grains; whereas 100 cubic inches of this gas weigh 76.59 grains; hence it is more than twice the specific gravity of atmospheric air; but these relations of weight are most conveniently manifested by a diagram.

ABSOLUTE AND RELATIVE WEIGHT OF CHLORINE GAS.