Chlorine (sym. Cl; atom. wt. 35.4; Gr. chlōros, 'pale green') is a non-metallic element discovered by Scheele in 1774, and named by him dephlogisticated marine air. Afterwards, in 1810, Davy proved it to be an elementary body, and gave it the name which it now bears. In nature it is always found in a state of combination. United with sodium, Na, it occurs very largely as the chloride of sodium, NaCl—common salt—in the ocean; in large beds, as rock-salt; in all natural waters, including even rain-water; in clays, soils, limestone; in volcanic incrustations; and in the vegetable and animal kingdoms. The preparation of gaseous chlorine by its liberation from common salt, is described under HYDROCHLORIC ACID, SODA. See also BLEACHING POWDER, which is the form in which chlorine is prepared commercially. For experimental purposes the gas may be received in jars filled with warm water at the pneumatic trough, when the chlorine rises into the jar, and displaces the water. When thus obtained it is a yellowish-green gas with a peculiar and suffocating odour, is not combustible, and is a very feeble supporter of ordinary combustion. A lighted candle placed in it burns with a very smoky flame, owing to the hydrogen of the oil alone burning, while the carbon is liberated. Several of the metals, such as antimony, copper, and arsenic, in a fine state of division, or in the condition of thin leaves, at once become red hot, and burn when introduced into the gas. A piece of thin paper soaked in turpentine likewise bursts into flame. Chlorine is a very heavy gas, nearly times heavier than air, its specific gravity being 2.470 (air = 1.000); it is soluble in cold water to the extent of two volumes of chlorine in one of water, and yields a solution resembling the gas in colour, odour, and other properties. The principal properties of chlorine are those of a bleacher of cotton and linen (see BLEACHING) and a most powerful Disinfectant (q.v.). The gas can be condensed by pressure and cold into a transparent dark greenish-yellow limpid liquid, with a specific gravity of 1.330 (water = 1.000), which also possesses bleaching properties and a most powerful odour. On the animal system chlorine acts, in very minute quantity, by producing a sensation of warmth in the respiratory passages, and increasing the expectoration; in large quantity, by causing spasm of the glottis, violent cough, and a feeling of suffocation. The workmen in chemical manufactories, who get accustomed to the chlorine in small quantities, are generally stout—at least, lay on fat—but complain of acidity in the stomach, which they correct by taking chalk, and also suffer from the corrosion of their teeth, which are eaten away to stumps. The antidotes to the evil effects of the introduction of chlorine into the lungs are the inhalation of the vapour of water, alcohol, ether, or chloroform; but the latter two should never be resorted to except under medical supervision.
Chlorine forms with other substances a very large number of chemical compounds. Mixed with hydrogen in equal volumes, and exposed to sunlight or flame, union takes place with explosive energy, and hydrochloric acid, HCl, is produced. This acid forms an extensive class of salts called chlorides (see HYDROCHLORIC ACID), but many of these may be produced by the direct combination of chlorine with the metals, as in the experiments described above. Chlorine replaces other elements or groups of atoms in organic compounds, forming chloro-derivatives. In combination with carbon and hydrogen it forms many substances, among which Chloroform (q.v.) is perhaps chief. Chlorine forms with oxygen a series of oxygen acids—viz. Hypochlorous, HClO; Chlorous, HClO2; Chloric, HClO3; and Perchloric Acid, HClO4; the more important of which are referred to under separate heads.