528 
Basis  of  Thermo- Chemistry. 
f  Am.  Jour.  Pharic. 
\       Oct.,  1883. 
assumed  that  the  atoms  are  particles  at  rest,  more  especially  in  the  more  or 
less  established  generalization  that  in  absence  of  disturbing  circumstances 
the  final  result  of  a  chemical  change  is  tbe  combination  of  those  affinities 
which  evolve  the  greatest  heat  in  their  satisfaction.  If,  on  this  assumption, 
2  atoms,  A  and  B,  are  attracted,  and,  by  their  mutual  impact  combine  with 
one  another,  then  their  potential  energy  is  converted  into  kinetic,  which 
either  wholly  or  partially  assumes  the  form  of  heat.  Supposing,  then,  a 
third  atom,  C,  whose  attraction  for  A  is  greater  than  that  of  B,  to  be 
brought  into  such  a  condition  that  it  can  oust  B,  then  the  kinetic  energy 
evolved  as  heat  is  equivalent  to  the  potential  energy  between  A  and  C 
diminished  by  the  work  done  in  tbe  separation  of  A  and  B.  A  decomposi- 
tion of  the  compound  AC  by  B  is  practically  excluded,  and  also  every 
chemical  change  which  is  associated  with  a  consumption  of  heat. 
But  atoms  and  molecules  are  not  at  rest :  for,  not  only  is  every  molecule 
in  motion  as  a  whole,  but  each  of  its  constituent  atoms  is  likewise  moving^ 
the  movement  of  the  latter  being  so  far  limited  that  no  individual  atom  can 
be  separated  from  the  others  which  form  the  molecule. 
The  form  and  force  of  the  movement  is  dependent  on  the  nature  of  the 
substance,  temperature,  and  space  occupied  by  a  given  quantity  of  it. 
Although  no  ultimate  conclusion  can  be  arrived  at  as  to  the  actual  form  of 
the  motion,  yet  it  can  d  priori  be  suspected  that  it  is  either  of  such  a  kind 
that  the  vis  viva  remains  the  same,  or  alternately  increases  and  decreases 
at  the  expence  of  the  potential  energy.  The  former  case  Avould  obtain 
when  the  severa^atoms  describe  a  circle  around  some  centre  of  gravity,  the 
latter  when  the  path  described  is  an  ellipse:  if  the  latter  represent  the  form 
of  motion,  then  the  readiness  of  decomposition  of  the  molecule  is  dependent 
on  the  relative  position  of  the  atoms  in  a  given  unit  of  time ;  and  it  is  thus 
possible  that  one  and  the  same  compound  by  the  action  of  one  and  the 
same  substance  is  in  certain  cases  decomposed,  and  in  others  unaltered 
without  any  perceptible  difference  in  the  external  conditions,  especially 
temperature.  Then  for  the  decomposition  of  a  compound  AB  by  C,  it  is 
not  even  necessary  that  in  any  given  position  the  attraction  between  A  and  B 
should  be  less  than  that  between  B  and  C,  for  it  is  a  possible  case  that  while 
A  and  B  are  indefinitely  apart  from  one  another,  C  strikes  with  such 
velocity  on  A  that  it  ousts  B  from  the  sphere  of  action.  Whether,  after 
such  a  decomposition,  the  kinetic  energy  of  the  system  is  greater  or  less 
than  before  is  dependent  upon  whether  C  or  B  possesses  the  greater  affinity 
for  A,  but /or  effecting  the  decomposition  it  is  not  necessary  that  B  should 
possess  the  greater,  for  C  can  compensate  a  less  degree  of  affinity  by  a 
greater  vis  viva.  In  accordance  with  this  view  not  only  the  decomposition 
of  AB  by  C,  but  the  re-formation  of  AB  from  AC  and  B  is  possible.  Finally, 
the  possibility  and  ease  of  a  chemical  combination  is  dependent,  not  upon 
the  force  of  the  affinities,  but  upon  other  properties  of  atoms  and  molecules 
and  upon  external  conditions. 
The  author  also  remarks  that  there  is  no  fundamental  distinction 
between  positive  and  negative  heat-change  :  for  the  combination  of  two 
atoms  could  scarcely  be  dissolved  without  their  kinetic  energy  taking  some 
part  in  work  done  in  overcoming  the  ^nities,  but  the  loss  of  energy  will 
be  marked  by  the  heat  developed  in  the  newly  formed  affinities.  The 
