~Technical preparation.~ The sodium hydroxide is melted in a cylindrical iron vessel (Fig. 76) through the bottom of which rises the cathode K. The anodes A, several in number, are suspended around the cathode from above. A cylindrical vessel C floats in the fused alkali directly over the cathode, and under this cap the sodium and hydrogen liberated at the cathode collect. The hydrogen escapes by lifting the cover, and the sodium, protected from the air by the hydrogen, is skimmed or drained off from time to time. Oxygen is set free upon the anode and escapes into the air through the openings O without coming into contact with the sodium or hydrogen. This process is carried on extensively at Niagara Falls. [Illustration: Fig. 76] ~Properties.~ Sodium is a silver-white metal about as heavy as water, and so soft that it can be molded easily by the fingers or pressed into wire. It is very active chemically, combining with most of the non-metallic elements, such as oxygen and chlorine, with great energy. It will often withdraw these elements from combination with other elements, and is thus able to decompose water and the oxides and chlorides of many metals. ~Sodium peroxide~ (NaO). Since sodium is a univalent element we should expect it to form an oxide of the formula Na_{2}O. While such an oxide can be prepared, the peroxide (NaO) is much better known. It is a yellowish-white powder made by burning sodium in air. Its chief use is as an oxidizing agent. When heated with oxidizable substances it gives up a part of its oxygen, as shown in the equation 2NaO = Na_{2}O + O. Water decomposes it in accordance with the equation 2NaO + 2H_{2}O = 2NaOH + H_{2}O_{2}. Acids act readily upon it, forming a sodium salt and hydrogen peroxide: 2NaO + 2HCl = 2NaCl + H_{2}O_{2}. In these last two reactions the hydrogen dioxide formed may decompose into water and oxygen if the temperature is allowed to rise: H_{2}O_{2} = H_{2}O + O. ~Peroxides.~ It will be remembered that barium dioxide (BaO_{2}) yields hydrogen dioxide when treated with acids, and that manganese dioxide gives up oxygen when heated with sulphuric acid. Oxides which yield either hydrogen dioxide or oxygen when treated with water or an acid are called peroxides. ~Sodium hydroxide~ (_caustic soda_) (NaOH). 1. _Preparation._ Sodium hydroxide is prepared commercially by several processes. (a) In the older process, still in extensive use, sodium carbonate is treated with calcium hydroxide suspended in water. Calcium carbonate is precipitated according to the equation Na_{2}CO_{3} + Ca(OH)_{2} = CaCO_{3} + 2NaOH. The dilute solution of sodium hydroxide, filtered from the calcium carbonate, is evaporated to a paste and is then poured into molds to solidify. It is sold in the form of slender sticks.