this group also belong the rare elements, selenium and tellurium.

It will be noticed that the atomic weight of sulphur, 32, is just double that of oxygen.

CHLORINE.

52. Preparation of Chlorine. - Chlorine, as we have already seen (1), is prepared by heating a mixture of muriatic acid and black oxide of manganese, MnO2. The reaction is shown in the following equation:

MnO2+4HCl = MnCl2 + 2H2O + 2C1.

The hydrogen of the HCl combines with the oxygen of the MnO2, forming two molecules of water, 2H2O. Half the chlorine of the HCl unites with the manganese, to form manganic chloride (chloride of manganese), MnCl2, and the other half is set free.

53. Properties of Chlorine. - Chlorine was discovered by Scheele in 1774. It does not occur free in nature, but is found combined with metals, forming chlorides, of which rock-salt is the most common. Chlorine is, as we have seen, a greenish-yellow gas of a most disagreeable and irritating odor. It is very poisonous when inhaled in any considerable quantity. It is a very heavy gas, being nearly 2.5 times as heavy as air. Chlorine has a very strong affinity for the metals. If these in a finely divided state are brought into contact with it, they take fire spontaneously, giving rise to metallic chlorides. It has so strong an affinity for phosphorus, that this element will take fire spontaneously when put into a jar of chlorine. But the most remarkable property of chlorine is its power of combining with hydrogen to form muriatic acid. When these gases are mixed in equal volumes, they combine with an explosion

on applying a lighted taper to them, or on exposing them to the sunlight. This property of chlorine has already been illustrated in the decomposition of water and ammonia (2, 3).

Another remarkable property of chlorine is its bleaching power, which depends on its ability to decompose water. We may enclose a piece of cotton cloth colored with a vegetable substance in a bottle of dry chlorine, and no change of color takes place, even after many weeks. If, however, a few drops of water are added, the cotton is at once bleached. The chlorine combines with the hydrogen of the water, and the oxygen, at the moment of its liberation, when it is in the nascent state (32), combines with the coloring matters, forming colorless compounds. Ordinary free oxygen has not this power, at least to any great extent.

54. Chloride of Lime, or Bleaching Powder.- Since chlorine destroys all organic colors (that is all colors derived from the vegetable and animal kingdoms) it is very extensively used in bleaching cotton and linen goods and paper. Cotton fabrics may now be rendered perfectly white in a few hours; while, by the old method of laying them on the grass in the sun, it required weeks, and even months, to effect it. It was necessary, moreover, to have meadow-land suitably situated for the bleaching; hence most of the cloth manufactured in England was carried to Holland to be bleached. Besides the diminished expense, the cotton stuffs bleached with chlorine suffer less injury in the hands of skilful workmen than those bleached in the sun.

The health of the workmen is greatly endangered by the use of chlorine in the gaseous state; but it has been found that the gas is readily absorbed by slaked lime, and is as readily given up again when the lime is treated with dilute acid.

When thus combined with lime, the chlorine can be easily transported. The compound is called chloride of lime, or bleaching powder.

This powder is made on a large scale by conducting chlorine into spacious chambers, on the floor of which slaked lime is spread to the depth of two inches.

In bleaching, the goods are first dipped into a solution of the bleaching powder, and then passed through dilute acid. The chlorine is thus set free in the fibres of the cloth, where it does its work of bleaching without injury to the workmen.

Chloride of lime is also largely used as a disinfectant. The chlorine acts upon organic odors in the same way as upon organic colors; that is, it oxidizes and destroys them.

55. Hydric Chloride (Hydrochloric Acid).— Hydric chloride or hydrochloric acid is a colorless gas, somewhat heavier than air, and is very soluble in water. This solution is the ordinary muriatic acid of the shops. It may be easily prepared, by placing in a capacious retort 3 parts of fused sodic chloride (chloride of sodium, or common salt) in fragments, and adding slowly, through a bent funnel, 3 parts of oil of vitriol. If pounded salt be used, the action of the acid is apt to be too rapid. The retort is connected with a series of bottles; in the first, a small quantity of water is placed, to detain any impurities which might be carried over mechanically with the gas; the second bottle may contain 4 parts of water, and should be immersed in a vessel of cold water, as the condensation of the gas is attended with a great disengagement of heat. On applying a gentle heat to the retort, the acid comes over and is condensed; an easily soluble sodic sulphate (sulphate of soda) remains in the retort. For manufacturing purposes, the decomposition is effected in iron cylinders, like those employed in the manufacture

of nitric acid, and only one half the quantity of sulphuric acid prescribed above is used:

2NaCl +H2SO1=2HCl +Na2SO4.

Sodic sulphate, Na,SO,, remains in the cylinder, whilst the acid is condensed in a series of salt-glazed stone-ware jars.

Enormous quantities of muriatic acid are obtained as an incidental product in the manufacture of soda-ash, which will be described farther on. More than 1000 tons are made every week at the soda-ash works in South Lancashire (England) alone.

56. Hydracids. — Hydric chloride, HCl, and hydric sulphide, H,S, contain each but two elements, and no oxygen; while nitric acid, HNO, and sulphuric acid, H2SO, contain each three elements, one of which is oxygen. There are, then, two classes of acids: one containing oxygen, and called oxyacids; and the other, containing no oxygen, called hydracids.

57. Aqua Regia.-When muriatic acid is mixed with nitric acid, it forms the so-called nitro-muriatic acid. This acid was called aqua regia (royal water) by the alchemists, because it dissolves gold, the "king of metals." Both platinum and gold are insoluble in either acid separately; but when the two acids are mixed, they decompose each other. Chlorine is set free, and in its nascent state (32) acts upon the metals, and dissolves them.

58. Chlorine Oxyacids. - Chlorine has a very weak affinity for oxygen; but it forms four oxyacids: hypochlorous acid, HCIO; chlorous acid, HCIO,; chloric acid, HCIO,; and hyperchloric or perchloric acid, HCIO. Of these, the hypochlorous and the chloric form some important salts.

BROMINE.

59. Properties of Bromine.-This element, which closely resembles chlorine in its properties and compounds, was discovered by Balard, in 1826, in the salts obtained by the evaporation of sea-water. It does not occur free in nature, and is, like chlorine, found combined with sodium and magnesium as bromides in certain mineral springs.

Bromine is a dark, reddish-black, heavy liquid, being the only element, except mercury, that exists as a liquid at the ordinary temperature. Its specific gravity is 2.966, it freezes at -9°, and boils at 145°. It has a very strong, irritating smell, resembling that of chlorine, and when inhaled,, acts as a violent poison. It is quite soluble in water, and this solution bleaches, but more feebly than a solution of chlorine. Some of its safts are much used in medicine and in photography.

IODINE.

60. Properties of Iodine. -Iodine also occurs in seawater, combined with sodium and magnesium. It was discovered in 1812 by Courtois. At the ordinary temperature it is a dark gray solid, with a bright metallic lustre. It has a specific gravity of 4.95; it melts at 225`, and boils at about 350°. Its vapor is of a beautiful deep violet color, and has a faint' chlorine-like smell. Iodine does not possess such active qualities as either chlorine or bromine. Its solution does not bleach vegetable colors, and it is set free from its compounds by both the above elements. Free iodine forms with starch a remarkable compound of a rich blue color (12). Iodine acts as a violent poison; but, given in small quantities, it is much used as a medicine. Some of its salts also are used in medicine and in photography.