62 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



In each case, if dilute acid is used, solid gypsum or selenite, 

 CaSO 4 ,2H 2 O, is produced. 



(CaCO 3 + 2HC1 -> CaCl 2 + H 2 O-f CO 2 

 2. Muriatic aa'd\Ca(OH) z + 2HC\ -> CaCl 2 + 2H 2 O 

 ICaO +2HC1 -> CaCl 2 +H 2 



In each case a solution of muriate of lime (calcium chloride, 

 CaCl 2 ) is produced. 



B. THE ALKALIS 



Boyle recognised an alkali by the facts that " it had a fiery 

 taste upon the tongue," that it would " make an ebullition with 

 acid spirits and precipitate diverse spirits made with them " and 

 would " turn syrup of violets green " (Producibleness of Chymical 

 Principles, 1680, pp. 35 and 37 ; Works, 1725, III. 372-373). 



Black's observations on the alkalis are set out with modern 

 equations in the following paragraphs : 



1. Mild alkalis contain fixed air since they are able to 

 re-convert lime into chalk 



K 2 CO 3 + Ca(OH) 2 - 2KOH + CaCO 3 

 potash + slaked lime -> caustic potash + chalk 



(Potassium (Calcium (Potassium (Calcium 



carbonate.) hydroxide.) hydroxide.) carbonate.) 



2. The caustic alkali which is produced in this action con- 

 tains neither lime nor fixed air. It is very corrosive, but can be 

 separated by evaporating in a silver dish. 



3. The caustic alkali absorbs fixed air from the atmosphere, 

 and is rendered mild thereby 



2KOH + CO 2 -> K 2 CO 2 -r-H 2 O. 



4. The caustic alkalis, unlike the mild alkalis, do not effer- 

 vesce with acids, e.g. 



KOH + HC1 -> KC1 +H 2 O 



caustic potash + muriatic acid -> sal sylvii + water 

 K 2 CO 3 + 2HC1 -> CO 2 + 2KC1 + H 2 O 

 potash + muriatic acid ->fixed air + sal sylvii-h water 



5. Sal volatile is a mild alkali, which can be rendered caustic 

 by means of lime 



(NH 3 ) 2 CO 2 + CaO-> 2NH 3 +CaCO 3 

 sal volatile + lime -> ammonia + chalk 



(Ammonium carbamate.) 



