Thus,  two  hydrogen  atoms  in  a  molecule  give  a  gas  almost  insoluble
in  water;  two  chlorine  atoms,  a  gas  which  is  moderately  soluble;  but
one  of  each  gives  hydrogen  chloride,  which  dissolves  in  water  in
extraordinary  quantities.  So  also,  colorless  substances  give,  by
chemical  union,  strongly  colored  ones,  and  odorless  substances,  by
chemical  union,  strongly  odorous  ones.  Putting  pieces  of  iron  and
sulphur  side  by  side  causes  absolutely  no  change  in  the  properties  of
either.  And  yet  this  hypothesis  compels  us  to  assume  that  if  the
particles  are  made  fine  enough,  and  placed  close  enough  to  one
another,  the  individual  properties -of  the  constituents  will  entirely
disappear.  Hitherto  we  have  failed  to  think  of  any  qualities  which
might  be  attributed  to  the  atoms  in  order  to  account  for  facts  of
this  class.  Why  should  oxygen  (O2)  and  ozone  (03)  be  so  different
in  behavior,  although  the  atomic  theory  hints  at  nothing  but  a  sub-
stitution of  three  atoms  for  two?  What  atomic  properties  shall
account  for  the  difference  between  red  and  yellow  phosphorus?
Usually,  in  explaining  these  matters,  we  forsake  atoms  and  employ


156  COLLEGE   CHEMISTRY

the  conception  of  energy.  We  say  that  ozone  has  more  energy  than
oxygen,  and  yellow  phosphorus  more  than  red  phosphorus.
There  are  certain  facts,  however,  such  as  the  chemical  actions
known  as  internal  rearrangements  (p.  10),  which  are  successfully
explained  by  supposing  that  the  atoms  composing  a  given  kind  of
molecule  are  disposed  towards  one  another  in  a  definite  geometrical
arrangement.  We  speak  then  of  the  structure  of  the  molecule  and
the  constitution  of  the  substance,  and,  to  represent  our  conclusions,
we  use  graphic  formulae  (q.v.).
