140 SCIENTIFIC AMUSEMENTS. 



natural carbonate of soda (also known for that reason as mineral 

 alkali), and to the analogous substance carbonate of potash 

 (obtained from the ashes of vegetable matter, and thence dis- 

 tinguished as vegetable alkali); after removal of carbonic acid these 

 substances were distinguished as caustic alkali, on account of their 

 increased caustic or corrosive qualities. This operation of remov- 

 ing carbonic acid, or " causticising," is performed by treating solu- 

 tions of the mild alkalies with quicklime, boiling the whole 

 together until the action is complete. 



Expt. 144. To causticise Soda Solution. Take an ounce of 

 washing soda crystals and dissolve it in half a pint of soft water. 

 Add a tablespoonful of well-burnt quicklime, previously reduced 

 to a sort of thin paste or cream by rubbing with water by means 

 of a pestle and mortar. Boil the whole in a glass flask for a 

 quarter of an hour ; remove the flame, and allow the sediment to 

 settle ; pour off a teaspoonful of the nearly clear fluid into a 

 test-tube, and add a little dilute hydrochloric acid. If the action 

 of the quicklime is complete, little or no effervescence will be pro- 

 duced ; but if much gas is evolved, the boiling must be continued 

 until no more escape of carbonic acid gas is brought about on add- 

 ing acid to a sample of the liquid, an additional quantity of quick- 

 lime being added if the first amount used does not suffice to bring 

 about the requisite degree of causticising after an hour's boiling. 

 When the action is complete, allow the contents of the flask to 

 stand till cool, and carefully pour off the clear caustic soda solution 

 into another bottle for use ; a bottle provided with a cork that 

 has been dipped in melted paraffin-wax answers best, ordinary 

 untreated corks being easily attacked and corroded by the caustic 

 soda, whilst bottles with glass stoppers are apt to become useless 

 through the stopper becoming tightly fixed, and not easily 

 extracted without breaking the glass. 



In order to show that the quicklime has actually taken away 

 the carbonic acid from the carbonate of soda, pour into a test-tube 

 some of the thick sediment left after all the clear liquor has been 

 decanted, and place in a second test-tube about as much of the 

 original lime-cream. Add a teaspoonful of hydrochloric acid to 

 each; if the quicklime was of good quality, there will be but 

 little effervescence produced in the second test-tube, the lime dis- 

 solving comparatively quietly in the acid; whilst a copious 

 effervescence will be produced in the first test-tube, owing to the 

 escape of the carbonic acid taken up by the lime from the soda. 



If carbonate of potash * (pearl-ashes) be used instead of washing 



* Crude potashes can easily be prepared by burning almost any kind of 

 wood to ashes, boiling these with water, filtering the liquid, and evaporating 



