In  the  latter  all  the  heat  applied  goes  to  increase  the  mo-
lecular kinetic  energy,  while  in  the  former  the  gas  does
work  in  expanding  by  heat  under  a  constant  pressure ;  and
heat  must  be  supplied  not  only  to  increase  the  kinetic
energy  of  the  molecules  to  the  same  extent  as  when  the
volume  is  kept  constant,  but  in  addition  enough  to  do  the
external  work.  The  specific  heat  under  constant  pressure
is  therefore  greater  than  the  specific  heat  at  constant
volume.  The  ratio  of  the  one  to  the  other  for  air  is  about
1.41.  The  importance  of  this  ratio,  to  which  reference  has
already  been  made  in  Sound  (I.,  120),  will  be  discussed  in
a  later  chapter.
It  has  been  found  very  difficult  to  measure  the  specific
heat  of  gases  at  a  constant  volume,  and  till  quite  recently
the  difficulties  have  not  been  surmounted.
>    The  specific  heat  of  a  gas  under  a  constant  pressure  has


52  HEAT.
