Atomic  Hypothesis  and  Dissected  (Structural)  Formula.      243 
Mercuric  oxide  when  heated,  however,  yields  not  only  oxygen 
identical  with  that  from  ozone,  but  also  metallic  mercury; 
hence  mercuric  oxide  is  said  to  be  a  compound  of  mercury  and 
oxygen. 
The  questions  as  to  whether  oxygen  and  mercury  are  present 
as  such  in  mercuric  oxide,  whether  both  are  essentially  altered 
in  the  combined  state,  whether  bodies  generally  are  simply  force 
determined  in  some  or  other  different  ways,  &c,  are  speculations 
with  which  the  chemist  who  wishes  to  deal  solely  with  facts  has 
nothing  to  do.  Mercuric  oxide  is  a  name  given  to  a  body  which, 
on  heating,  furnishes  the  two  bodies  termed  oxygen  and  mercury 
respectively ;  and  this  fact  is  referred  to  when  it  is  said  that  mer- 
curic oxide  is  a  compound  of  mercury  and  oxygen ;  this,  and  the 
converse  proposition  that  mercury  and  oxygen  can  under  certain 
conditions  reproduce  the  original  mercuric  oxide,  are  referred  to 
when  it  is  said  that  mercury  and  oxygen  unite  together  to  form 
mercuric  oxide,  or  that  mercury  and  oxygen  are  components  of 
mercuric  oxide. 
4.  Bodies  that  are  not  decomposed  by  a  rise  in  temperature 
are  either  aeriform  at  common  temperatures,  gasefiable  at  higher 
temperatures,  or  non-volatile  at  measurable  temperatures.  Many 
elements  which  are  themselves  non-volatile  yield  gasefiable  com- 
pounds with  elements  belonging  to  the  other  two  classes ;  the 
composition  by  weight  of  these  compounds  is  known  by  analysis 
or  synthesis.  From  this  composition  and  the  vapour-densities 
of  the  compounds  the  volumetric  ratio  between  the  compounds 
and  the  gasefiable  components  is  known.  In  many  instances 
this  ratio  admits  of  being  also  directly  determined  by  experi- 
menting with  the  components  in  the  gaseous  state. 
Thus  water  is  found  to  consist  of  88*89  per  cent,  oxygen  and 
ll'll  per  cent,  of  hydrogen:  the  vapour-densities  of  steam, 
oxygen,  and  hydrogen  are  respectively  9,  16,  and  1.     Hence 
1  volume  of  water-vapour  consists  of  X  y  volumes  of  hy- 
drogen and  -  x  r-^  volumes  of  oxygen,  i.  e.  of  1  volume  of 
hydrogen  and  J  volume  of  oxygen  :  volumetric  analysis  or  syn- 
thesis of  steam  verifies  this  calculation. 
Experiment  shows  that  a  volume  of  the  vapour  of  almost  all 
gasefiable  compounds  contains  the  gasefiable  elements  in  the 
proportion  of  ->  ->  -t  ->  -, . . .  volumes,  i.  e.  that  two  volumes 
of  the  compound  vapour  contain  l}  2,  3,  4,  5, . . .  volumes  of 
the  gasefiable  components.  Two  volumes  is  therefore  chosen  as 
the  relative  unit  of  volume  for  compound  vapours. 
R2 
