tions  and  decompositions,  and  it  is  simply  a  complex  problem  in
the  mechanical  theory  of  heat  to  determine  what  must  be  in
any  case  the  thermal  effect.  The  numerous  facts  with
which  we  are  acquainted  in  regard  to  the  heat  of  chemical
combination  generally  agree  with  the  mechanical  theory  ;  and,
where  the  facts  do  not  appear  to  conform  to  it,  the  discrepancy
probably  arises  from  our  ignorance  of  the  nature  of  the  chem-
ical change  in  question.  It  would  be  incompatible  with  our
design  to  discuss  these  facts  in  this  book.  It  must  be  sufficient
to  state  a  few  general  results,  which  may  be  summed  up  in  the
following  propositions :  —

First.  The  heat  absorbed  in  the  decomposition  of  a  com-
pound is  equal  to  the  heat  evolved  in  its  formation,  provided
the  initial  and  the  final  states  are  the  same.
Second.  The  heat  evolved  in  a  series  of  successive  chemical
changes  is  equal  to  the  sum  of  the  quantities  which  would  be
evolved  in  each  separately,  provided  the  bodies  are  finally
brought  into  identical  conditions.
Third.  The  difference  between  the  quantities  of  heat  evolved
in  two  series  of  changes  starting  from  two  different  states,  but
ending  in  the  same  final  state,  is  equal  to  that  which  is  evolved
or  absorbed  in  passing  from  one  initial  condition  to  the  other.
For  example,  if  a  body  m  evolves  a  certain  amount  of  heat
in  uniting  with  n  to  form  m  n,  and  if  the  body  m  n  is  decom-
posed by  a  third  body  p,  so  that  m  p  is  formed,  the  quantity
of  heat  evolved  in  this  last  reaction  is  less  than  that  which
would  be  evolved  in  the  direct  union  of  m  and  p  by  the  amount
evolved  in  the  formation  of  m  n.
