Solubility  in  Acid. — The  question  as  to  whether  a  min-
eral is  soluble  in  one  of  the  acids  named  is  often  of
great  importance.  To  test  the  solubility  hydrochloric  acid
is  generally  used,  except  with  metallic  sulphides  and  some
other  minerals  containing  prominently  one  of  the  heavy
metals  (lead,  copper,  silver,  etc.);  for  these  latter  nitric
acid  is  usually  better.  The  mineral  should  in  general  be
pulverized  as  finely  as  possible  in  the  agate  mortar  and  in-
troduced into  a  large  test-tube,  some  acid  poured  on,  and
the  whole  carefully  heated  over  the  Bunsen  flame,  the  tube
being  shaken  gently  during  the  process.
It  must  be  remembered  here  that  the  acid  fumes  in  the
air  are  injurious  to  breathe  and  will  act  corrosively  upon
surfaces  of  brass  in  the  neighborhood;  hence  such  tests
can  only  be  tried  with  caution  unless  the  conveniences  of
the  laboratory  are  at  hand.
Various  results  may  be  noted  during  this  trial:

A.  The  mineral  may  dissolve  quietly  with  or  without
coloring  the  solution;  this  is  true,  for  example,  of  hema-
tite, also  of  many  of  the  sulphates  and  phosphates.
B.  There  may  be  a  bubbling  off  or  effervescence  of  a  gas.
This  gas  is  usually  carbon  dioxide  or  carbonic-acid  gas


THE   USE   OF   THE   BLOWPIPE.  155

(CO,);  but  may  be  hydrogen  sulphide  or  sulphuretted  hy-
drogen (H2S).  Also  chlorine  may  be  liberated,  or  reddish
fumes  of  nitrogen.
C.  There  may  be  a  separation  of  some  insoluble  sub-
stance, as  sulphur,  silica,  etc.  These  points  will  now  be
3poken  of  more  in  detail.
