So 
Colloids  and  Crystals. 
{Am.  Jour.  Pharm. 
February,  1914. 
by  the  particles,  are  totally  reflected  from  the  cover-glass  over  the  sol. 
This  instrument  is  simple,  easily  adjusted  and  cheap.  It  is  made 
commercially  by  the  firm  of  Zeiss.  It  would  seem  to  be  admirably 
adapted  to  school  purposes.  In  fact,  after  a  look  into  the  ultra- 
microscope,  the  study  of  the  molecular  topics  ceases  to  be  drudgery 
and  becomes  a  positive  intellectual  need. 
VIII. 
Even  a  brief  glance  at  the  subject  of  colloid  systems  must  at  least 
mention  the  classic  work  of  Perrin  10  on  the  distribution  of  the  par- 
ticles in  suspensions  of  gamboge  and  mastic.  He  succeeded,  by  an 
ingenious  and  simple  method,  in  preparing  emulsions  of  gamboge 
in  water  in  which  the  spherical  yellow  granules  were  all  of  the  same 
diameter.  If  we  consider  a  mass  of  such  a  liquid  in  a  tube,  it  is  clear 
that  the  granules,  if  at  rest,  would,  since  they  are  denser  than  water, 
all  fall  to  the  bottom.  The  fact  that  they  remain  suspended  is  due  to 
their  movement.  In  other  words,  the  state  of  things  is  the  same  as 
in  the  earth's  atmosphere,  and  just  as  the  molecules  are  more  crowded 
near  the  earth's  surface,  so  the  granules  of  gamboge  must  be  more 
numerous  near  the  bottom  of  the  liquid  than  in  the  upper  layers. 
Perrin  verified  this  prediction  by  direct  counting  of  the  granules 
under  the  microscope.  The  barometric  formula  which  describes  the 
progressive  rarefaction  of  air  with  increasing  height  also  describes 
the  distribution  of  the  granules  in  Perrin's  uniform  emulsions.  The 
only  difference  is  that,  while  the  aviator  must  ascend  six  kilometres 
in  order  to  reach  air  half  as  dense  as  at  sea  level,  the  same  effect  is 
produced,  in  Perrin's  emulsion,  by  an  ascent  of  o.i  millimetre. 
That  the  mean  energy  of  rotation  of  a  molecule  must  be  equal  to 
its  mean  energy  of  translation  is  one  of  the  chief  propositions  of  the 
kinetic  theory.  Perrin  has  proved  this  by  direct  measurement  of  the 
rotation  of  granules  under  the  microscope.  For  this  purpose,  large 
granules  (15  /x)  of  mastic  were  employed.  These  are  far  too  heavy 
to  remain  suspended  in  water,  so  a  solution  of  urea  was  used.  For- 
tunately, the  granules  contain  little  inclusions  which  make  it  possible 
to  measure  their  rotation. 
10  Annates  de  Chimie  et  de  Physique,  3d  series,  vol.  18,  p.  5  (1909).  There 
is  a  German  translation  by  Donau  in  Kolloidchemische  Beihefte,  vol.  1,  p.  1 
(1910).  An  English  translation  by  Soddy  has  appeared  in  book  form  under 
the  title  "  The  Brownian  Movement  and  Molecular  Reality." 
