1. _Laboratory method._ In the laboratory chlorine is made by warming the mineral pyrolusite (manganese dioxide, MnO_{2}) with concentrated hydrochloric acid. The first reaction, which seems to be similar to the action of acids upon oxides in general, is expressed in the equation MnO_{2} + 4HCl = MnCl_{4} + 2H_{2}O. The manganese compound so formed is very unstable, however, and breaks clown according to the equation MnCl_{4} = MnCl_{2} + 2Cl. Instead of using hydrochloric acid in the preparation of chlorine it will serve just as well to use a mixture of sodium chloride and sulphuric acid, since these two react to form hydrochloric acid. The following equations will then express the changes: (1) 2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl. (2) MnO_{2} + 4 HCl = MnCl_{2} + 2Cl + 2H_{2}O. (3) MnCl_{2} + H_{2}SO_{4} = MnSO_{4} + 2HCl. Combining these equations, the following equation expressing the complete reaction is obtained: 2NaCl + MnO_{2} + 2H_{2}SO_{4} = MnSO_{4} + Na_{2}SO_{4} + 2H_{2}O + 2Cl. Since the hydrochloric acid liberated in the third equation is free to act upon manganese dioxide, it will be seen that all of the chlorine originally present in the sodium chloride is set free. The manganese dioxide and the hydrochloric acid are brought together in a flask, as represented in Fig. 53, and a gentle heat is applied. The rate of evolution of the gas is regulated by the amount of heat applied, and the gas is collected by displacement of air. As the equations show, only half of the chlorine present in the hydrochloric acid is liberated. [Illustration: Fig. 53] 2. _Electrolytic method._ Under the discussion of electrolysis (p. 102) it was shown that when a solution of sodium chloride is electrolyzed chlorine is evolved at the anode, while the sodium set free at the cathode reacts with the water to form hydrogen, which is evolved, and sodium hydroxide, which remains in solution. A great deal of the chlorine required in the chemical industries is now made in this way in connection with the manufacture of sodium hydroxide. ~Physical properties.~ Chlorine is a greenish-yellow gas, which has a peculiar suffocating odor and produces a very violent effect upon the throat and lungs. Even when inhaled in small quantities it often produces all the symptoms of a hard cold, and in larger quantities may have serious and even fatal action. It is quite heavy (density = 2.45) and can therefore be collected by displacement of air. One volume of water under ordinary conditions dissolves about three volumes of chlorine. The gas is readily liquefied, a pressure of six atmospheres serving to liquefy it at 0 deg.. It forms a yellowish liquid which solidifies at -102 deg.. ~Chemical properties.~ At ordinary temperatures chlorine is far more active chemically than any of the elements we have so far considered, with the exception of fluorine; indeed, it is one of the most active of all elements.