258       Dr.  C.  R.  A.  Wright  on  the  Relations  between  the 
Hence  the  formula  of  a  compound  expresses  the  nature  and 
number  of  the  constituent  atoms  present  in  a  molecule  of  the 
substance. 
27.  This  hypothesis  does  not  explain  why  a  molecule  of  free 
mercury  should  consist  of  but  1  atom,  while  the  molecule  of 
oxygen  (which  is  also  a  bivalent  element)  contains  2  atoms; 
nor  why  a  molecule  of  phosphorus  should  contain  4,  and  one  of 
sulphur  at  low  temperatures  6 ;  but  it  enables  us  to  explain  the 
allotropy  of  the  elements  by  the  assumption  that  the  molecules 
of  the  different  modifications  are  made  up  of  different  numbers 
of  constituent  atoms  (thus  the  molecule  of  gaseous  oxygen  con- 
tains 2  atoms,  of  ozone  3,  of  sulphur  at  low  temperatures  6 
atoms,  at  higher  temperatures  2  atoms),  and  therefore  allows  of 
the  inference  being  drawn  that  under  different  conditions  of 
pressure  and  temperature  phosphorus  vapour  might  contain 
only  2  atoms,  or  even  1  atom  in  the  molecule,  or  mercury  vapour 
2  or  4  atoms  to  the  molecule. 
28.  The  phenomena  of  dissociation  are  to  some  extent  ac- 
counted for  by  the  atomic  hypothesis ;  the  atoms  in  a  molecule 
are  held  together  by  certain  forces,  which  are  altered  in  character 
by  variation  of  temperature;  at  some  given  temperature  the 
molecule  is  in  a  state  of  unstable  equilibrium ;  and  at  a  higher 
temperature  the  component  atoms  alter  their  mutual  relations, 
taking  up  more  stable  positions.  The  observed  facts  that  dis- 
sociation always  causes  a  molecule  to  form  less  complex  mole- 
cules, that  atomic  weight  and  specific  heat  of  elements  are  in- 
versely proportional,  that  compounds  are  more  stable  according 
as  more  heat  is  evolved  in  their  production,  and  that  the  boil- 
ing-points and  melting-points  are  usually  higher  the  more  com- 
plex the  formula,  indicate  the  existence  of  some  general  law  con- 
necting together  the  mutual  relations  of  atoms  in  a  molecule  and 
the  effects  of  the  force  termed  heat  upon  them, — a  conclusion 
strengthened  by  the  analogous  connexions  between  electricity 
and  equivalents,  specific  refractive  energy  and  formulae,  specific 
gravity  and  formulae,  crystalline  shape  and  formulae,  which  ap- 
pear to  indicate  analogous  laws  with  respect  to  other  forms  of 
force. 
The  atomic  hypothesis  alone  does  not  account  for  these  ob- 
served facts;  it  only  colligates  them  together  and  allows  of 
their  being  expressed  in  concise  phraseology  :  thus  the  law  of 
isomorphism  may  be  stated  in  the  language  of  the  atomic  hy- 
pothesis thus  : — The  same  numbers  of  atoms  similarly  arranged 
will  give  rise  to  the  same  crystalline  form.  In  order  to  explain 
the  facts,  assumptions  other  than  the  primary  one  as  to  the 
existence  of  atoms  must  be  made  ;  thus,  to  explain  Dulong  and 
Petit's  law,  the  assumption  must  be  made  that  the  effect  of  a 
