so  that  it  will  readily  wet-out,  but  also  to  completely  remove
all  resinous  substances,  otherwise  a  good  clear  white  will  not  be
obtained  in  the  bleaching.  For  this  purpose,  it  is  generally
necessary  to  boil  the  cotton  with  a  solution  of  caustic  soda  (or
other  alkalies)  for  a  number  of  hours  and  usually  under  more  or
less  pressure.  This  operation  is  termed  "  boiling-out "  to  dis-
tinguish from  mere  "  wetting-out.'*  In  the  scouring  of  cotton
it  is  probable  that  the  waxy  and  resinous  substances  in  the
fibre  are  emulsified  or  dissolved.  Caustic  soda  is  probably  the
most  generally  employed  chemical  for  the  scouring  of  cotton,
though  soda  ash  and  sodium  silicate  are  also  extensively  used.
