Heated to a low red heat in the air, the oxide combines with oxygen, forming the peroxide (BaO_{2}). If the temperature is raised still higher, or the pressure is reduced, oxygen is given off and the oxide is once more formed. The reaction BaO_{2} <--> BaO + O is reversible and has been used as a means of separating oxygen from the air. Treated with acids, barium peroxide yields hydrogen peroxide: BaO_{2} + 2HCl = BaCl_{2} + H_{2}O_{2}. ~Barium chloride~ (BaCl_{2}.2H_{2}O). Barium chloride is a white well-crystallized substance which is easily prepared from the native carbonate. It is largely used in the laboratory as a reagent to detect the presence of sulphuric acid or soluble sulphates. ~Barium sulphate~ _(barytes)_ (BaSO_{4}). Barium sulphate occurs in nature in the form of heavy white crystals. It is precipitated as a crystalline powder when a barium salt is added to a solution of a sulphate or sulphuric acid: BaCl_{2} + H_{2}SO_{4} = BaSO_{4} + 2HCl. This precipitate is used, as are also the finely ground native sulphate and carbonate, as a pigment in paints. On account of its low cost it is sometimes used as an adulterant of white lead, which is also a heavy white substance. Barium compounds color the flame green, and the nitrate (Ba(NO_{3})_{2}) is used in the manufacture of green lights. Soluble barium compounds are poisonous. RADIUM ~Historical.~ In 1896 the French scientist Becquerel observed that the mineral pitchblende possesses certain remarkable properties. It affects photographic plates even in complete darkness, and discharges a gold-leaf electroscope when brought close to it. In 1898 Madam Curie made a careful study of pitchblende to see if these properties belong to it or to some unknown substance contained in it. She succeeded in extracting from it a very small quantity of a substance containing a new element which she named radium. In 1910 Madam Curie succeeded in obtaining radium itself by the electrolysis of radium chloride. It is a silver-white metal melting at about 700 deg.. It blackens in the air, forming a nitride, and decomposes water. Its atomic weight is about 226.5. ~Properties.~ Compounds of radium affect a photographic plate or electroscope even through layers of paper or sheets of metal. They also bring about chemical changes in substances placed near them. Investigation of these strange properties has suggested that the radium atoms are unstable and undergo a decomposition. As a result of this decomposition very minute bodies, to which the name corpuscles has been given, are projected from the radium atom with exceedingly great velocity. It is to these corpuscles that the strange properties of radium are due. It seems probable that the gas helium is in some way formed during the decomposition of radium. Two or three other elements, particularly uranium and thorium, have been found to possess many of the properties of radium in smaller degree.