By  subtracting,  we  get  the  quantity  of  oxygen.  The  proportion  of
the  constituents  and  the  steps  in  the  calculation  (p.  41)  are  as
follows:


PERCENTAGE      AT.  WT.         FACTOR
Sulphur,          50.05     =     32.06     X     1.561      =
Oxygen,          49.95     =     16.00      X     3.122     =


•M.561

S  X  1.561        S  X  1
O  X  3.122       O  X  2


The  formula  of  the  product  is  therefore  S02  and  the  equation  S  +  2O
->S02.
Similarly,  phosphoric  anhydride  may  be  shown  to  have  the  for-
mula P2O5,  carbon  dioxide  CO2,  and  magnetic  oxide  of  iron  Fe3O4.
To  make  the  equations  representing  their  formation,  the  rules  given
on  pp.  41-42  should  be  applied.
To  make  the  equation  for  the  formation  of  oxygen  and  hausman-
nite  by  heating  manganese  dioxide  (p.  46)  the  same  rules  are  used.
The  skeleton  equation  is:  Mn02  — >Mn304  +  0.  These  being  the
formulae  of  the  substances  actually  concerned,  in  balancing  the
equation  it  is  not  permissible  to  alter  them  except  by  placing
coefficients  in  front.  The  most  complex  formula  is  Mn304  and,  to
get  a  sufficient  number  of  atomic  weights  to  produce  it,  at  least


OXYGEN  51

3MnO2  is  required.  This  furnishes  3Mn  and  6O,  so  that,  after  using
40,  there  remains  20 :

3MnO2  ->  Mn304  +  20.
