49.  Elasticity  (Greek  'ixa,fn,  striving). — This  means  the  ten-
dency of  the  molecules  of  a  body  to  go  back  to  their  original
positions  when  their  places  have  been  altered   by  force.     For
instance,  when  a  piece  of  india-rubber  is  stretched,  its  mole-
cules   submit    to    be    moved    very    far    from    their    original
places    without    getting    beyond    the    range     of    their     cohe-
sion,  although  they  resist  this   change   with    a    force   which
increases  with  the  amount  of  displacement.     But  they  do  not
rest  satisfied  with  their  new  places  ;  continued  stress  has  to  be
applied  in  order  to  keep  the  india-rubber  stretched,  and  as  soon
as  this  stress  is  taken  off,  the  molecules  come  back  to  their
precise  original  places,  and  the  piece  of  india-rubber  resumes  its
old  form.      A  similar  thing  happens  when  a  straight  rod  of
whalebone  or  steel  is  bent ;  the  molecules  on  the  outside  of  the
curve  are  pulled  farther  apart,  while  those  on  the  inside  of  the

curve  are  pushed  nearer  together
as  shown  in  fig.  2.  But  if  the
displacement  has  not  exceeded
a  certain  amount,  they  all  come
back  to  their  original  places
Fi&-  2<  when  the  pressure  is  taken  off.
