xin THE DECOMPOSITION OF THE ALKALIS 289 



less completely, leaving behind a residue pf metallic magnesium, 

 calcium, strontium or barium. The metals oxidised rapidly 

 and were converted into unslaked or caustic earths, which were 

 thus shown to be metallic oxides : 



Magnesia = magnesium oxide MgO 



Lime = calcium oxide CaO 



Strontia = strontium oxide SrO 



Baryta = barium oxide BaO 



The slaked oxides, formed by uniting the oxides with water, are 

 called hydroxides, e.g. 



slaked lime = lime + water 



= calcium hydroxide, CaO,OH 2 or Ca(OH) 2 



Davy at first regarded the caustic alkalis as oxides of pot- 

 assium and sodium, but further experiments showed that they 

 were really hydroxides like slaked lime, since 

 potassium and chlorine gave potassium chloride only 



2K + C1 2 -> 2KC1, 



potassium oxide and chlorine gave potassium chloride and oxygen 

 K 2 O + C1 2 -> 2KC1 + O 2 , 



(i vol.) (i vol.) 



potassium peroxide and chlorine gave potassium chloride and 

 oxygen 



K 2 O 4 + C1 2 -> 2KC1 + 2O.J, 



caustic potash and chlorine gave potassium chloride, oxygen 

 and water 



2KOH + C1 2 -> 2KC1 4- K) 2 + H 2 O. 



u 



