(2)  The  second  kind  of  chemical  reaction  is  decomposi-
tion or  the  opposite  of  combination.  Examples  are  fur-
nished by  the  decomposition  of  mercuric  oxide  into  mer-
cury and  oxygen  by  heat  ;  of  potassium  chlorate  into
potassium  chloride  and  oxygen  by  heat  ;  of  water  into
hydrogen  and  oxygen  by  the  electric  current  ;  and  of  cal-
cium carbonate  or  limestone,  CaCO3,  into  lime  or  calcium
oxide,  CaO,  and  carbon  dioxide,  CO2,  by  heat.  These  re-
actions are  represented  by  the  following  equations  :


CHEMICAL  EQUATIONS— THE  SCOPE  OF  CHEMISTRY.  25

HgO     =Hg    +0;
KC1O3  =  KC1  +  30  ;
H30      =  2H    +0;
CaCO3  =  CaO  +  CO2.
The  expressions  3O  and  2H  mean  respectively  three
times  the  combining  weight  of  oxygen  and  twice  the  com-
bining weight  of  hydrogen,  the  figure  being  generally
used  in  this  way  when  the  element  is  not  in  combination.
It  is,  however,  sometimes  written  the  same  as  if  the  ele-
ment were  in  combination,  as  will  be  explained  later.
