The  word  atom  is  derived  from  a,  privative,  and  rf/zi/w  (I
cut),  and  recalls  a  famous  controversy  in  regard  to  the  infinite
divisibility  of  matter,  which  for  many  centuries  divided  the
philosophers  of  the  world.  But  chemistry  does  not  deal  with
this  metaphysical  question.  It  asserts  nothing  in  regard  to  the
possible  divisibility  of  matter ;  but  its  modern  theories  claim
that,  practically,  this  division  cannot  be  carried  beyond  a  certain
extent,  and  that  we  then  reach  particles  which  cannot  be  fur-
ther divided  by  any  chemical  process  now  known.  These  are
the  chemical  atoms,  and  the  atom  is  simply  the  unit  of  the
chemist,  just  as  the  molecule  is  the  unit  of  the  physicist,  or  the
stars  the  units  of  the  astronomer.  The  molecule  is  a  group  of
atoms,  and  is  a  unit  in  the  microcosm,  of  which  it  is  a  part,  in
the  same  sense  that  the  solar  system  is  a  unit  in  the  great  stel-
lar universe.  The  molecule  has  been  defined  as  the  smallest
particle  of  any  substance  which  can  exist  by  itself,  and  the
atom  may  be  now  defined  as  the  smallest  mass  of  an  element
that  exists  in  any  molecule.
