Animal charcoal is obtained by heating animal substances, such as bones, dried blood, horns, etc., to redness, in close vessels, until all volatile empyreumatic matters have been driven off, and a residue of carbon remains. When prepared from bones it contains a large quantity of inorganic matter in the shape of carbonate and phosphate of lime, the former of which produces _alkalinity_ in reacting upon nitrate of silver. Animal charcoal is freed from these earthy salts by repeated digestion in hydrochloric acid; but unless very carefully washed it is apt to retain an acid reaction, and so to liberate free nitric acid when added to solution of nitrate of silver. _Properties._--Animal charcoal, when pure, consists solely of carbon, and burns away in the air without leaving any residue: it is remarkable for its property of decolorizing solutions; the organic coloring substance being separated, but not actually _destroyed_, as it is by _chlorine_ employed as a bleaching agent. This power of absorbing coloring matter is not possessed in an equal degree by all varieties of charcoal, but is in great measure peculiar to those derived from the animal kingdom. China Clay or Kaolin. This is prepared, by careful levigation, from mouldering granite and other disintegrated felspathic rocks. It consists of the _silicate of alumina_,--that is, of silicic acid or _flint_, which is an oxide of silicon, united with the base alumina (oxide of aluminum). Kaolin is perfectly insoluble in water and acids, and produces no decomposition in solution of nitrate of silver. It is employed by photographers to decolorize solutions of nitrate of silver which have become brown from the action of albumen or other organic matters. Chlorine. Symbol, Cl. Atomic weight, 36. Chlorine is a chemical element found abundantly in nature, combined with metallic sodium in the form of chloride of sodium, or sea-salt. _Preparation._--By distilling common salt with sulphuric acid, sulphate of soda and hydrochloric acid are formed. Hydrochloric acid contains chlorine combined with hydrogen; by the action of _nascent_ oxygen (see oxygen), the hydrogen may be removed in the form of water, and the chlorine left alone. _Properties._--Chlorine is a greenish-yellow gas, of a pungent and suffocating odor; soluble to a considerable extent in water, the solution possessing the odor and color of the gas. It is nearly 2-1/2 times as heavy as a corresponding bulk of atmospheric air.