80 POTASH. 



Potash and pearlash procured in this manner are 

 combined with a certain proportion of carbonic 

 acid, but not so much as to destroy completely its 

 alkaline properties : hence it is a sub-carbonate of 

 potash. To free the alkali from the carbonic acid, 

 another process is necessary. Twice its weight 

 of quicklime is added to the pearlash, and the 

 whole mixed with water. The carbonic acid 

 having a stronger affinity to the lime than to the 

 alkali, quits the latter, and forms carbonate of 

 lime, which, being insoluble, falls down, while the 

 purer alkali is kept in solution by the water, and 

 is afterwards separated by evaporation. Sometimes 

 it is still farther purified, if necessary, by mixing 

 the whole with alkohol, which dissolves the pure 

 alkali alone. The alkoholic solution ascends 

 to the top of the fluid, and is drawn off by decant- 

 ation. 



Potash, when thus prepared, is a solid white 

 substance, and is called caustic potash, from its 

 property of corroding the skin and flesh when it is 

 applied to it : on this account it is frequently em- 

 ployed by surgeons. 



Caustic potash, when prepared by alkohol, is 

 united to a portion of water, and is therefore a 

 hydrate of potash. It may be obtained free from 

 water by another process. 



Potash may be made to combine with a greater 

 proportion of carbonic acid than in the state of 

 sub-carbonate, by causing a stream of carbonic 

 acid gas to pass through a solution of the latter 

 salt : when this solution is then evaporated, it af- 

 fords crystals of bi-carbonate of potash. This salt 

 is milder than the subcarbonate, and its crystals 

 are not deliquescent. 



