centim.;  another  portion  (b]  of  10  cub.  centim.  is  sub-
jected to  the  same  treatment  after  addition  of  two  drops
of  nitric  acid  ;  a  third  portion  (c)  is  treated  in  the  same
manner  after  addition  of  a  little  caustic  alkali.
1 68  MICROCHEMICAL  ANALYSIS  PART

a.  A  drop  of  the  concentrated  liquid  (a)  is  acidulated
with  nitric  acid  and  subjected  to  further  concentration  on
a  slide.  Sulphate  of  calcium,  if  present,  will  be  immediately
detected  (§21,  d).
To  the  same  drop  thallous  nitrate  is  added  to  detect
chlorides.
Two  or  three  small  drops  are  evaporated  on  the  same
spot,  with  addition  of  a  grain  of  calcium  acetate.  The
residue  is  covered  with  a  layer  of  a  tepid  solution  of
gelatine  and  the  slide  put  aside  in  a  cool  place  for  a  quarter
of  an  hour.  After  that  time  a  drop  of  hydrochloric  acid  is
spread  over  the  jelly,  which  will  speedily  penetrate  it,  evolv-
ing bubbles  of  carbonic  acid  if  soluble  carbonates  are  present
in  the  water  (§  40,  b}.
If  a  centrifuge  is  at  the  disposal  of  the  analyst,  the  test
may  be  modified  so  as  to  include  bicarbonates  and  free
carbonic  acid.  Distilled  water  is  mixed  with  acetate  of
calcium  and  ammonia,  the  mixture  whirled  to  precipitate
traces  of  calcium  carbonate.  A  sample  of  water  is  run
into  a  testing-tube  with  one  or  two  cub.  centim.  of  the
clear  mixture,  and  whirled  till  the  carbonate  of  calcium  is
condensed  into  a  thin  layer,  firmly  adhering  to  the  bottom
of  the  tube.  This  may  be  hastened  by  slightly  heating
the  tube  before  whirling.  The  water  is  run  off,  a  little
gelatine  is  spread  over  the  film  of  precipitate,  and  after
solidification  of  the  jelly  some  drops  of  hydrochloric  acid.
