the  relations  of  solubility  and  temperature  in  this  salt  at  a
glance.  The  vertical  divisions  0  to  0'  register  the  range  of
temperature  from  32°  to  218° ;  the  horizontal  ones  0  Vindi-
cate the  degree  of  solubility,  which  reaches  322  parts  at  93°.
The  curve  of  solubility  then  descends  from  B  to  C,  when
210  parts  are  dissolved  at  218°.  The  cause  of  this  sud-
den diminution  of  solubility,  is  the  decomposition  of  the
hydrous  salt  in  solution  at  that  heat,  and  the  precipi-
tation of  a  portion  of  anhydrous  sulphate  of  soda.  A  solu-
tion of  Grlauber's  salts  saturated  at  boiling  heat  in  a  vessel
capable  of  being  corked  while  boiling,  and  suffered  to  cool,
will  often  crystallize  completely  on  withdrawing  the  cork,
a  change  from  the  fluid  to  the  solid  state  occasioned  by  the
concussion  of  the  air.  The  same  thing  happens  if  a  small
crystal  is  dropped   into   a    saturated  solution   of  the   salt,

Sulphate  of  soda  is  a  familiar  aperient.  In  the  arts,  its
chief  use  is  in  the  preparation  of  carbonate  of  soda,  as  will
be  presently  described.     It  is  a  result,  on  a  large  scale,  of


How  does  it  fuse  ?  How  if  exposed  'i  How  does  it  dissolve  in  water
at  different  temperatures  ?  What  is  its  curve  of  solubility  ?  Describe
the  diagram  353.  How  is  its  solutioi.  in  vacuo  crj^^tallized  ?  What  is  its
use  in  the  arts  ?


cSOl


METALLIC    ELEMENTS.
the  preparation  of  chlorohjdric  acid,  (426.)  The  other
sulphates  of  soda  require  no  mention  at  present.  The  solu-
tion of  sulphate  of  soda  (12  parts)  in  strong  chlorohydric
acid  (10  parts)  produces  cold  enough  to  freeze  a  considerable
quantity  of  water  in  summer.
