_Chemical Properties._--Chlorine belongs to a small natural group of elements which contains also bromine, iodine, and fluorine. They are characterized by having a strong affinity for hydrogen, and also for the metals, but are comparatively indifferent to oxygen. Many metallic substances actually undergo _combustion_ when projected into an atmosphere of chlorine, the union between the two taking place with extreme violence. The characteristic bleaching properties of chlorine gas are explained in the same manner:--Hydrogen is removed from the organic substance, and in that way the structure is broken up and the color destroyed. Chlorine is more powerful in its affinities than either bromine or iodine. The salts formed by these three elements are closely analogous in composition and often in properties. Those of the alkalies, alkaline earths, and many of the metals are soluble in water, but the silver salts are insoluble; the lead salts sparingly so. The combinations of chlorine, bromine, iodine, and fluorine, with hydrogen, are acids, and neutralize alkalies in the usual manner, with formation of alkaline chloride and water. The test by which the presence of chlorine is detected, either free or in combination with bases, is _nitrate of silver_; it gives a white curdy precipitate of chloride of silver, insoluble in nitric acid, but soluble in ammonia. The solution of nitrate of silver employed as the test must not contain iodide of silver, as this compound is precipitated by dilution. Chloride of Ammonium. Symbol, NH{4}Cl. Atomic weight, 54. This salt, also known as muriate or hydrochlorate of ammonia, occurs in commerce in the form of colorless and translucent masses, which are procured by _sublimation_, the dry salt being volatile when strongly heated. It dissolves in an equal weight of boiling, or in three parts of cold water. It contains more _chlorine_ in proportion to the weight used than chloride of sodium, the atomic weights of the two being as 54 to 60. Chloride of Barium. Symbol, BaCl+2HO. Atomic weight, 123. Barium is a metallic element, very closely allied to calcium, the elementary basis of _lime_. The chloride of barium is commonly employed as a test for sulphuric acid, with which it forms an insoluble precipitate of sulphate of baryta. It is also said to affect the color of the photographic image when used in preparing positive paper; which may possibly be due to a chemical combination of baryta with albumen: but it must be remembered that this chloride, from its high atomic weight, contains _less_ chlorine than the alkaline chlorides. _Properties of Chloride of Barium._--Chloride of barium occurs in the form of white crystals, soluble in about two parts of water, at common temperature. These crystals contain two atoms of water of crystallization, which are expelled at 212°, leaving the anhydrous chloride. Chloride of Gold. (_See_ Gold, Chloride of.) Chloride of Sodium. Symbol, NaCl. Atomic weight, 60.