17.  The  cohesion  in  liquids  is  much  greater  than  is  com*
monly  imagined.      A  disc  of  glass   suspended   from    the
beam  of  a  balance  over  a  surface  of  water  will  adhere  with
a  measurable  force  to  the  water  when  brought
in  contact  with  it.     The  force  required  to  with-
draw it  is  that  which  will  rupture  the  cohe-
sion of  the  outer  row  of  particles  at  the  edge  of
the  disc,  then  the  next  row,  and  so  on  to  the
centre  a,  as  shown  in  the  circles  on  fig.  7.     In
Fig.  7.        the  soap-bubble  we  see  a  thin  film  of  water,

Illustrate  this  by  fig.  5.  Explain  the  action  of  light  bodies  on  water.
What  is  wettmg?  17.  What  of  cohesion  in  liquids?  Explain  the  adhe-
sive disc  and  the  soap-bubble.
THE   ATMOSPHERE.
23


giving  us  a  beautiful  example  of  the  cohesive  power  of  water.
It  is  a  great  hollow  drop  of  water.  The  cohesive  power
in  the  film  of  the  bubble  is  so  great  that  if  the  pipe  be
taken  from  the  mouth  before  the  bubble  leaves  it,  a  stream
of  air  will  be  forcibly  driven  from  the  bore  by  the  contrac-
tion of  the  film,  which  will  deflect  the  flame  of  a  candle.  To
the  same  cause  is  ascribed  the  spherical  form  of  the  dew-
drop,  the  cohesion  in  the  outer  row  of  particles.
