Salts  are  crystallised,  either  by  allowing
their  hot  and  saturated  solutions  to  cool  slowly,
or  by  simply  evaporating  the  menstrua  as  long
as  crystals  form.  In  the  first  case  the  liquid
is  commonly  evaporated  until  a  pellicle  appears
on  the  surface,  when  the  vessel  is  set  aside  in
some  sheltered  situation  until  cold,  at  which
time  the  crystals  are  collected,  and  the  process
repeated  for  fresh  crystals.  In  the  second
case  the  crystals  are  usually  removed  from  the
liquid  as  soon  as  they  are  deposited.  The  first
method  is  adopted  for  those  salts  that  are  con-


CEYSTALLOLD— CUPPING


623


■iderably  more  lolnlile  in  hot  than  in  cold
water,  as  cnrbonnte  of  toda,  Epsom  salts,  &c. ;
the  last  method,  for  those  that  possess  nearly
equal  solnbility  in  both  cases,  and  also  for
many  salts  which  are  not  required  in  handsome
crystals;  thus  common  salt  and  chromate  of
potash  are  crystallised  in  this  way.  Many  of
the  alkaloids,  and  their  salts,  are  obtained  in
cr\stalsi,  by  allowing  their  solutions  (generally
alcoholic  or  ethereal)  to  evaporate  sponta-
neous! v.  Uy  repeating  the  processes  of  solu-
tion unil  crvstallisation  two  or  three  times  with
the  same  body,  the  crystals  obtained  by  the  last
operation  will  usually  be  found  to  be  quite  pure.
