BORAX,  the  sodium  borate  which  is  so  familiar  to  one
who  works  with  the  blowpipe  (see  p.  125),  also  occurs  in
nature,  and  on  so  large  a  scale  in  certain  limited  regions  as
to  be  extensively  mined.  It  is  found  in  monoclinic  crys-
tals, often  of  large  size;  they  are  clear  at  first,  but  lose
water  on  exposure  and  become  white  and  opaque.  Borax
Lake  in  Lake  County,  California,  as  also  another  lake  of
the  same  name  in  San  Bernardino  County,  affords  this  min-
eral; there  are  also  large  deposits  in  Tibet.  Borax  finds
many  uses  in  the  arts,  as  in  making  glass  and  soap,  in
soldering,  in  medicine,  etc.
Some  other  sodium  compounds  occurring  in  nature  are
the  carbonates,  including  natron  and  trona;  the  fluoride,
cr}Tolite,  already  described  on  p.  242;  the  nitrate,  called
soda-niter;  the  sulphates,  thenardite  and  mirabilite  (glauber
salt),  further,  glauberite  (containing  also  lime).  Sodium
is  also  present  in  many  silicates,  as  albite,  natrolite,  etc.
SYLVITE,  or  the  chloride  of  potassium  (KC1),  is  a  rare
mineral  occasionally  found  associated  with  halite  or  rock-
salt,  and  closely  resembling  it  in  form,  taste,  and  other
characters.  It  is  easily  distinguished  by  the  violet  color
which  it  gives  the  blowpipe  flame.
Potassium  also  forms  a  number  of  compounds,  as  sul-
phates, borates,  etc.,  but  they  are  mostly  rare  and  hardly
to  be  found  in  the  cabinet  of  most  mineralogists.  It  also
enters  into  the  composition  of  many  silicates,  as,  for
example,  orthoclase  or  potash  feldspar,  also  muscovite  or
common  potash  mica.  So,  too,  apophyllite  and  many
others.
DESCRIPTION   OF   MINERAL   SPECIES.  271

A  few  minerals,  interesting  because  they  contain  rare
elements,  may  be  mentioned  briefly  in  this  place.
