POTASSA. 287 



and are the strongest of all bases. From their destruc- 

 tive action on animal matter, they are called caustic 

 alkalies, and are often distinguished, by this term, 

 from the carbonates of potassa and soda. 



POTASSA. 

 716. Potassa is prepared from wood 



What is the 



source of po- ashes. The ley obtained from these be- 

 ing evaporated to dryness, the mass which 

 remains is the crude potash of commerce. This, when 

 purified, becomes pearlash. 



How is potassa 717. CAUSTIC POTASSA. Commercial 

 prepared? potash and pearlash are both carbonates 

 of potash, from which the carbonic acid must be 

 removed, in order to produce potassa itself. This is 

 done by a milk of slaked lime. A solution of potash, 

 in at least ten parts of hot water, or a hot ley, made 

 directly from wood ashes, should be employed in the 

 experiment. To this, the milk of lime is added, little 

 by little, the solution boiled up after each addition, 

 and then allowed to settle. If, after settling, a por- 

 tion of the clear liquid is found no longer to effervesce 

 on the addition of an acid, it is sufficient evidence 

 that all the carbonic acid has been removed by the 

 lime, and the process is completed. This must be as- 

 certained by trial. About half as much lime as pot- 

 ash will be required in the process. Caustic soda is 

 similarly made from the carbonate of soda. 



718. The boiling in the above process 



Give a modi- 



fication of the may be omitted, if the mixture be fre- 

 above method. quent]y sha ken up, during several days. 



