184         Prof.  M.  B.  Pell  on  the  Constitution  of  Matter. 
I  believe  that  the  constitution  of  the  luminiferous  sether  is  such 
as  to  render  it  incapable  of  propagating  waves  of  less  than  a  cer- 
tain length. 
I  see  some  hope  also  of  an  explanation  of  what  has  always 
appeared  to  me  one  of  the  greatest  difficulties  in  connexion  with 
molecular  physics — that  the  wave-length  should  be  so  nearly  the 
same  for  all  kinds  of  heat.  It  is  not  difficult  to  conceive  that  the 
molecules  in  the  sun  and  elsewhere,  whose  vibrations  are  the 
chief  sources  of  heat,  should  have  been  so  constituted  as  to  vi- 
brate nearly  in  the  same  time.  The  difficulty  is  to  understand 
why  the  molecules  of  bodies  of  all  kinds  and  constitutions,  being- 
heated  and  then  left  to  vibrate  in  their  own  way,  should  all  vi- 
brate so  nearly  in  unison.  But  if  we  hold  that  the  arrangement 
of  atoms  into  molecules  is  caused  by  the  prevalent  heat  and  de- 
pends upon  its  wave-length,  the  difficulty  disappears.  Let  us 
suppose  for  a  moment  that  the  sun  should  radiate  heat  of  one 
uniform  wave-length  only,  and  that  the  values  of  m  for  all  sub- 
ir 
stances  and  combinations  were  such  that  r—-  were  in  every  case 
2^  J 
a  whole  number  exactly.  All  the  atoms  under  the  influence  of 
the  sun's  heat  would  be  arranged  into  molecules,  all  having  the 
same  fundamental  note,  and  collections  of  such  molecules,  after 
being  heated,  would  give  back  that  note  alone.  No  substance 
having  its  atoms  otherwise  arranged  could  continue  to  exist;  for 
every  ray  of  heat  which  it  encountered  would  assist  in  decompo- 
sing and  rearranging  its  atoms  according  to  the  prevailing  code. 
There  would  be  one  uniform  stability  of  molecular  constitution 
and  one  uniform  colour,  fi  appears  to  be  as  nearly  constant  as 
the  necessity  that  p  should  be  a  whole  number  allows.  If  p 
wrere  not  nearly  constant  for  heat  of  considerable  intensity,  there 
would  be  no  stability  in  the  constitution  of  matter ;  for  an  ar- 
rangement made  under  one  wave-length  would  be  liable  to  be 
decomposed  under  another.  Suppose  that  a  mass  of  any  sub- 
stance, such  as  iron,  were  brought  from  some  other  system,  if 
there  is  any  such,  where  a  much  longer  wave-length  prevails ; 
we  should  probably  not  recognize  it  as  iron  at  all.  If  melted, 
its  atoms  would  be  immediately  arranged  according  to  the 
fashion  of  our  system.  It  might  perhaps  be  preserved  in  its 
original  state  if  carefully  kept  in  a  cool  place.  If  exposed  to 
the  heat  of  the  day,  it  would  probably  be  gradually  trans- 
formed, suffering  disintegration  in  the  process ;  it  would  decay, 
in  fact,  much  as  a  piece  of  wood  does,  and  with  more  or  less 
rapidity,  according  to  the  degree  in  which  its  constitution  dif- 
fered from  our  standard.  Is  it  possible  that  organic  compounds, 
which  can  be  produced  and  exist  under  exceptional  circumstances 
only,,  which  are  so  liable  to  decay,  so  sensitive  to  the  action  of 
