164         Prof.  M.  B.  Pell  on  the  Constitution  of  Matter. 
cannot  be  the  law  of  nature  that  two  atoms  should  attract  one 
another  for  all  distances,  however  small.  It  seems  natural  to 
suppose  that  the  total  quantity  of  heat  or  energy  in  any  solid 
body  or  collection  of  atoms  is  finite,  and  capable  of  being  ex- 
pressed as  some  function  of  the  masses,  velocities,  mutual  actions, 
and  relative  coordinates  of  the  atoms.  Now,  if  the  atoms  attract 
one  another  for  all  distances  however  small,  up  to  actual  contact, 
then  the  potential  energy,  or  that  due  to  any  relative  position  of 
the  atoms,  would  depend  partly  upon  the  magnitude  and  internal 
constitution  of  the  atoms  themselves,  and  would  be,  humanly 
speaking,  incapable  of  definite  expression.  If  the  atoms  be  sup- 
posed to  be  mere  poiuts  or  centres  of  force  having  inertia  but  no 
magnitude,  then  the  potential  energy  due  to  the  relative  position 
of  the  atoms  of  any  solid  body  is  infinite.  These  difficulties 
disappear  if  we  accept  the  fact  which  stares  us  in  the  face,  that 
the  final  action  between  two  atoms  when  the  distance  is  dimi- 
nished is  repulsive,  and  that  for  any  two  atoms  of  a  solid  body 
there  is  a  relative  position  of  stable  equilibrium  short  of  actual 
contact  or  coincidence. 
Certain  considerations  in  connexion  with  the  dynamical  theory 
of  gases  require  us  to  suppose  that  the  particles  of  a  gas  are  not 
atoms,  but  molecules  or  collections  of  atoms.  The  facts  revealed 
by  the  spectroscope  indicate  also  that  the  particles  of  vapours, 
even  in  their  most  attenuated  condition,  have  a  complicated 
constitution,  and  that  they  probably  consist  of  a  considerable 
number  of  atoms.  Presuming  that  the  mutual  action  of  atoms 
is  the  cause  of,  and  of  the  same  kind  as  that  which  takes  place  be- 
tween molecules,  that  action  may  be  rather  rudely  represented 
by  the  annexed  diagram.     The  abscissa  represents  the  distance 
between  two  atoms,  and  the  ordinates  the  force,  the  positive  or- 
