the  Principle  of  the  Conservation  of  Energy,  19 
(3)  This  sum  consists  of  a  part  A,  which  may  be  ascribed 
partly  to  the  motion  of  their  centre  of  gravity,  and  partly  to 
their  rotation  about  one  another  in  space — and  of  another  part 
B,  which  the  particles  possess  relatively  to  each  other  when  con- 
sidered by  themselves.  This  latter  part,  B,  is  called  the  relative 
kinetic  energy }  or  that  belonging  to  the  system  formed  by  the  two 
particles. 
(4)  But  in  the  system  of  two  particles  there  is  a  something, 
in  addition  to  its  kinetic  energy,  which  does  not  belong  to  the 
two  particles  taken  separately,  namely  a  greater  or  less  capacity 
for  doing  work  in  virtue  of  the  mutual  action  of  the  two  particles 
upon  each  other.  The  measure  of  this  capacity  for  doing  work 
is  termed  the  potential  energy  of  the  system,  or  the  relative  poten- 
tial energy  of  the  two  particles  ;  and  that  quantity  of  work  serves 
as  the  measure  of  this  working-power  which  is  done  in  conse- 
quence of  the  mutual  action  of  the  two  particles  during  their 
transference  from  the  smaller  distance  r  =  p  to  the  greater  distance 
r=x ,  where  p  is  determined  by  the  particles  themselves  e,  e1, 
by  their  masses  e,  e',  and  by  the  constant  c. 
(5)  The  principle  of  the  conservation  of  energy,  however, 
when  specially  defined  as  above,  is  only  applicable  to  two  par- 
ticles when  their  potential  is  of  the  same  form  as  that  of  two 
electrical  particles,  namely 
r    \cc    dft        )' 
The  potential  of  two  ponderable  masses  m,  m',  on  the  con- 
trary, is 
Tr      mm1 
r 
which  (neglecting  the  sign)  can  be  included  under  the  above 
general  form  only  if  the  value  of  the  constant  c  for  ponderable 
masses  is  infinitely  great.  It  is  evident,  however,  that  it  would 
in  reality  suffice  for  the  constant  c  to  have  only  a  very  great 
value  instead  of  an  infinite  value,  in  order  that  there  might  not 
be  any  thing  perceptibly  inconsistent  with  the  results  of  ex- 
periment. And,  considering  the  extraordinarily  high  value 
which  must  be  ascribed  to  the  constant  c  in  the  case  of  elec- 
trical particles,  it  does  not  seem  at  all  necessary,  for  the  avoid- 
ance of  all  sensible  contradictions,  to  adopt  any  other  value  for 
ponderable  bodies ;  consequently  it  must  be  permissible  to  re- 
present the  potential  of  two  ponderable  particles  m,  m!  by 
v_  mm]  f         1     drl\ 
C2 
