~Compounds of chlorine with oxygen and hydrogen.~ Chlorine combines with oxygen and hydrogen to form four different acids. They are all quite unstable, and most of them cannot be prepared in pure form; their salts can easily be made, however, and some of them will be met with in the study of the metals. The formulas and names of these acids are as follows: HClO hypochlorous acid. HClO_{2} chlorous acid. HClO_{3} chloric acid. HClO_{4} perchloric acid. ~Oxides of chlorine.~ Two oxides are known, having the formulas Cl_{2}O and ClO_{2}. They decompose very easily and are good oxidizing agents. BROMINE ~Historical.~ Bromine was discovered in 1826 by the French chemist Ballard, who isolated it from sea salt. He named it bromine (stench) because of its unbearable fumes. ~Occurrence.~ Bromine occurs almost entirely in the form of bromides, especially as sodium bromide and magnesium bromide, which are found in many salt springs and salt deposits. The Stassfurt deposits in Germany and the salt waters of Ohio and Michigan are especially rich in bromides. ~Preparation of bromine.~ The laboratory method of preparing bromine is essentially different from the commercial method. [Illustration Fig. 55] 1. _Laboratory method._ As in the case of chlorine, bromine can be prepared by the action of hydrobromic acid (HBr) on manganese dioxide. Since hydrobromic acid is not an article of commerce, a mixture of sulphuric acid and a bromide is commonly substituted for it. The materials are placed in a retort arranged as shown in Fig. 55. The end of the retort just touches the surface of the water in the test tube. On heating, the bromine distills over and is collected in the cold receiver. The equation is 2NaBr + 2H_{2}SO_{4} + MnO_{2} = Na_{2}SO_{4} + MnSO_{4} + 2H_{2}O + 2Br. 2. _Commercial method._ Bromine is prepared commercially from the waters of salt wells which are especially rich in bromides. On passing a current of electricity through such waters the bromine is first liberated. Any chlorine liberated, however, will assist in the reaction, since free chlorine decomposes bromides, as shown in the equation NaBr + Cl = NaCl + Br. When the water containing the bromine is heated, the liberated bromine distills over into the receiver. ~Physical properties.~ Bromine is a dark red liquid about three times as heavy as water. Its vapor has a very offensive odor and is most irritating to the eyes and throat. The liquid boils at 59 deg. and solidifies at -7 deg.; but even at ordinary temperatures it evaporates rapidly, forming a reddish-brown gas very similar to nitrogen peroxide in appearance. Bromine is somewhat soluble in water, 100 volumes of water under ordinary conditions dissolving 1 volume of the liquid. It is readily soluble in carbon disulphide, forming a yellow solution.