162         Prof.  M.  B.  Pell  on  the  Constitution  of  Matter. 
There  should  be  no  difficulty  then  in  making  a  similar  supposi- 
tion respecting  the  atoms  of  which  matter  is  assumed  to  consist. 
If  matter  does  not  consist  of  isolated  atoms  but  is  continuous, 
any  inquiry  into  its  real  nature  would  seem  as  hopeless  as  a 
similar  investigation  respecting  time  or  space.  We  could  not 
hope  to  give  any  explanation  of  the  facts,  for  instance,  that  gold 
is  yellow  and  soft,  and  expands  under  the  action  of  heat,  except 
that  it  consists  of  little  bits,  all  of  which  possess  those  properties. 
It  will  be  time  to  confess  that  we  are  reduced  to  such  a  method 
of  accounting  for  the  phenomena  of  nature  when  every  other  has 
been  found  to  fail. 
I  have  endeavoured  to  assume  as  little  as  possible  respecting 
the  mutual  action  of .  two  atoms,  except  that  it  must  be  such  a 
function  of  the  distance  as  to  satisfy  the  most  obvious  properties 
of  matter.  Leaving  out  of  consideration  for  the  moment  all 
theories  except  that  of  gravitation,  let  us  consider  what  are  the 
facts.  Let  us  consider  the  mutual  action  between  two  atoms  or 
molecules,  or  particles,  or  whatever  they  may  be,  of  a  substance 
such  as  mercury,  which  is  capable  under  ordinary  circumstances 
of  existing  in  the  solid,  the  liquid,  or  the  gaseous  state.  As 
mercury  has  weight,  we  can  hardly  doubt  that  at  a  sufficient 
distance  two  particles  of  that  substance  attract  one  another.  At 
some  less  distance  they  repel  one  another,  whatever  be  the  cause ; 
for  the  vapour  of  mercury,  whether  in  vacuo  or  when  mixed  with 
the  air,  tends  to  diffuse  itself.  At  a  still  less  distance,  in  the 
liquid  state,  the  particles  cohere  slightly,  or  attract  one  another, 
appearing  to  be  in  a  relative  position  of  unstable  equilibrium. 
At  a  slightly  diminished  distance,  the  mercury  becomes  solid  and 
the  attractive  force  considerable.  The  solid  mercury  resists 
further  compression,  so  that  the  action  again  becomes  repulsive. 
During  the  last  century,  Boscovich  propounded  a  theory  of 
alternate  attractions  and  repulsions ;  but  I  am  not  aware  of  the 
exact  nature  of  his  investigations  or  speculations,  never  having 
had  an  opportunity  of  consulting  his  works.  They  do  not  ap- 
pear to  have  borne  fruit,  or  to  have  been  received  with  much 
favour.  It  is  hardly  correct,  however,  to  apply  the  word  theory 
to  these  attractions  and  repulsions;  we  should  rather  say  that 
they  are  obvious  facts,  requiring  some  theory  for  their  explana- 
tion. One  theory  may  be  stated  thus :  the  action  between  two 
particles  is  some  function  of  their  distance  sc}  which  for  consi- 
derable distances  is  sensibly  equal  to  ■—-,  but  for  very  small  dis- 
OS 
tances  changes  sign  several  times,  becoming  finally  large  and 
negative,  or  repulsive.  Let  us  consider  whether  there  is  any 
other  tenable  hypothesis. 
The  dynamical  theory  of  gases,  due  chiefly  to  the  labours  ot 
