PREPARATION. 357 



oxygen, and the metal remains in the state of protochloride. 

 Indeed only one half of the chlorine of the decomposed hydro- 

 chloric gas, is obtained as gas, the other half being retained by 

 the manganese, as will appear by the following diagram : 



Process for chlorine from hydrochloric acid and peroxide of 

 manganese. 



Before decomposition. After decomposition. 



Hydrochloric f Chlorine. . . Chlorine. 



acid I Hydrogen. .--^.Water. 



{ Manfanese'. Chloride of manganese 



[ Oxygen. . ^^s^^ 

 Hydrochloric J Chlorine. . ^ >< \ N ^ 



acid \ Hydrogen. . _^ Water. 



Or in symbols ; MnO 2 + 2HCl == MnCl and 2HO and Cl. 



The most convenient method of preparing chlorine gas is by 

 mixing in a flask 2 ounces of peroxide of manganese, with 6 

 ounce measures of hydrochloric acid, diluted with an ounce or 

 two of water to prevent it fuming. Effervescence, from escape 

 of gas, takes place in the cold, but is greatly promoted by the 

 application of a gentle heat. This gas is collected over water, of 

 which the temperature should not be less than 90, otherwise a 

 great waste of the gas occurs from its solution in the water, and 

 also a consequent annoyance to the operator from the escape of 

 the chlorine into the atmosphere, by evaporation from the sur- 

 face of the water trough. If the gas is not to be used imme- 

 diately, but preserved, it should be collected in bottles, into 

 which when filled with the gas, their stoppers greased, should 

 be inserted before they are removed from the trough. Before 

 the gas obtained by this process can be considered as pure, it 

 should always be transmitted through water, to remove hydro- 

 chloric acid ; an intermediate Wolfe's bottle containing water 

 may be employed to wash the gas, as was done with sulphurous 

 acid. If the gas is to be dried, it must be sent through a tube 

 containing chloride of calcium, of two or three feet in length, 

 great difficulty being experienced in drying this gas in a perfect 

 manner, owing to its low diffusive power : it is three times more 

 difficult to dry than carbonic acid. Chlorine cannot be collected 

 over mercury, as it combines at once with that metal, 



