The Meta-Encyclopedia

Chlorine

2 of the 6 encyclopedias on this shelf carry an entry for Chlorine. Both are reproduced below, so you can see where they agree and where they differ.

Collier's New Encyclopedia (1921)

a monatomic element; symbol Cl; at. wt. 35.5. Discovered by Scheele in 1774. It was thought by Berthollet to contain oxygen, and was called by him oxymuriatic acid. It was found to be an element by Davy in 1810. Chlorine is a yellow-green incombustible gas. It has a powerful irritating smell, and attacks violently the mucous membranes and the lungs. It is very soluble in water, acts strongly on metals, and is best collected by displacement. Sp. gr. 2.47. At the pressure of five atmospheres it is condensed into a heavy yellow liquid. It is obtained b by heating common salt, sodium chloride, with sulphuric acid and black oxide of manganese. It combines with hydrogen to form hydrochloric acid, with an explosion in direct sunlight or when fire is applied to a mixture of the two gases, but slowly in diffused daylight. A solution of it in water is gradually converted in the sunlight into HCl with liberation of oxygen. A lighted candle burns in Cl with a smoky flame. Phosphorus, antimony, arsenic, and turpentine take fire in chlorine. Chlorine destroys animal and vegetable matter; and forms addition and substitution compounds with organic compounds; an aqueous aqueous solution of it has powerful bleaching properties. It is also in naa powerful disinfectant. It occurs ture in the form of metallic chlorides. Three oxides of Chlorine are known, Cl₂O; Cl2O3; Cl2O4. Chlorine was extensively used during the World War in connection with the manufacture of ASPHYXIATING GAS (q. v.).

Aiton's Encyclopedia (1910)

klo'rin, an elementary substance. It never occurs free, but always in combination with other substances, very often with sodium. It is a poisonous, greenish gas about thirty-five and one-half times as heavy as hydrogen and two and one-half times as heavy as air. It has an offensive odor and irritates the nostrils when breathed. It may be liquefied by cold and pressure. Chlorine has a strong affinity for hydrogen, and, for that reason, is much employed in bleaching cotton and linen. It decomposes moisture by uniting with the hydrogen, leaving the oxygen free to neutralize the coloring material. This gas is destructive to animal and vegetable life because it absorbs the hydrogen of the water living things contain. Chlorine unites with metals and other substances to form a large class of compounds known as chlorides. Chloride of lime makes an excellent deodorizer or disinfectant, as it not only neutralizes offensive smells but kills bacterial germs. Chlorine enters into the composition of common salt from which it may be obtained by chemical action. See Bleaching

What Has Changed in the Last Century

Both accounts give the atomic weight as about 35.5 without explaining the odd fraction; F. W. Aston's mass spectrograph work from 1919 showed that natural chlorine is a mixture of two isotopes, chlorine-35 and chlorine-37, in roughly a 3:1 ratio. The laboratory route Collier's gives in 1921 -- salt heated with sulphuric acid and manganese dioxide -- has long since been displaced industrially by electrolysis of brine, the chlor-alkali process that also yields sodium hydroxide and hydrogen and now makes tens of millions of tonnes a year, much of it for PVC plastic. Aiton's praise of chloride of lime as a germ-killer anticipated the routine chlorination of drinking water, one of the largest public-health gains of the twentieth century. Collier's cross-reference to asphyxiating gas points to chlorine's use at Ypres in 1915; such use was outlawed by the 1925 Geneva Protocol and the 1993 Chemical Weapons Convention, though chlorine has been used as a weapon in Syria since 2013. One later reversal neither account could foresee: chlorine released from CFCs was found to destroy stratospheric ozone, leading to the 1987 Montreal Protocol.

Written for this edition, 2026.