Atomic  Hypothesis  and  Dissected  (Structural)  Formula.     245 
because  two  volumes  of  steam  contain  two  volumes  of  hydrogen 
and  one  volume  of  oxygen.  Necessarily,  therefore,  a  compound 
symbol  expresses  the  quantitative  composition  by  weight  as  well 
as  by  volume;  the  expression  H-O1  indicates  that  in  water  the 
hydrogen  and  oxygen  are  in  the  proportion  of  2  x  1  parts  of  the 
first  to  1x16  parts  of  the  second. 
These  collocations  of  symbols  and  suffixes  are  termed  Formula  ; 
usually  the  suffix  1  is  omitted  for  brevity.  In  the  majority  of 
known  formulae  the  suffix-values  are  not  high,  comparatively  few 
being  known  where  the  values  exceed  4,  except  in  the  case  of 
carbon  compounds. 
8.  Since  two  volumes  of  hydrogen  weigh  2  grammes,  the 
formula  of  hydrogen  is  written  H2.  Similarly  two  volumes  of 
oxygen  weigh  2x16  grammes,  and  therefore  the  formula  of 
oxygen  is  O2;  so  of  chlorine,  CI"2;  of  nitrogen,  N2;  of  sulphur 
at  high  temperatures,  S2, . . .  This  is  tantamount  to  an  expres- 
pression  of  the  fact,  that  in  the  case  of  such  elements  as  these 
the  specific  gravity  with  respect  to  hydrogen  is  expressed  by  the 
combining  number. 
This  rule,  however,  does  not  hold  good  for  all  gasefiable  ele- 
ments, nor  for  allotropic  modifications.  Thus  two  volumes  of  the 
homogeneous  vapour  of  mercury-compounds  contain  200  grins. 
of  mercury  as  the  minimum  number ;  200  is  therefore  the  com- 
bining number  :  two  volumes  of  mercury- vapour,  however,  only 
weigh  200  grammes,  and  hence  the  formula  of  mercury  is  Hg 
and  not  Hg2 ;  i.  e.  the  specific  gravity  of  its  vapour  with  respect 
to  hydrogen  is  only  half  its  combining  number.  On  the  other 
hand,  phosphorus  and  arsenic  are  found  to  have  the  formulae  P4 
and  As4;  i.  e.  the  vapour-densities  of  these  elements  are  double 
their  combining  numbers  respectively.  So  sulphur  at  low  tem- 
peratures has  the  formula  S6  and  ozone  O3.  Of  the  different 
modifications  of  an  element,  that  which  thus  gives  the  lowest 
formula  is  regarded  as  beins:  the  element  itself,  and  those  of 
higher  formulae  as  modifications  of  it ;  and  hence  it  is  possible  that 
ordinary  arsenic  and  phosphorus  are  not  the  true  elements,  but 
only  allotropic  modifications. 
9,  In  some  instances  homogeneous  compounds  yield,  on  heat- 
ing, non-homogeneous  vapours,  which,  however,  reproduce  the 
original  compounds  on  cooling :  the  non-homogeneity  of  the 
vapour  is  found  by  physical  or  other  tests  (e.  g.  diffusion  in  the 
case  of  sulphuric  acid  or  ammonium  chloride,  amalgamation  of 
gold  leaf  in  the  case  of  calomel- vapour).  In  these  cases  Disso- 
ciation is  said  to  take  place.  Such  vapours  always  lead  to  formulae 
different  from  those  ascribable  to  the  compounds  from  analogy 
and  other  considerations,  and  frequently  indicate  the  combining 
number  of  an  element  as  less  than  the  mmimum  number  derived 
