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436  ON  OXIDE  OF  ZINC. 
is  deposited,  therefore,  very  quickly,  and  is  also  very  easily 
washed  out.  To  obtain  a  good  precipitate,  it  is  necessary  to 
have  the  solution  of  soda  heated  to  boiling,  and  then  to  add  to 
it  the  solution  of  the  sulphate  of  zinc.  By  mixing  the  solutions 
cold  and  by  pouring  the  solution  of  soda  into  that  of  the  zinc,  a 
thick,  gelatinous,  bulky  precipitate  is  produced,  and  it  can  not 
be  avoided,  that  some  sub-sulphate  of  zinc,  or  some  other  basic 
combination  of  sulphuric  acid,  is  formed  with  the  oxide ;  all,  or 
any,  of  which  can  not  so  easily  be  washed  out  from  the  precipi- 
tate, on  account  of  their  difficult  solubilities. 
To  avoid,  therefore,  these  inconveniences,  Schindler  proposed 
to  add  the  solution  of  zinc  to  that  of  soda,  and  not  to  mix  until 
the  latter  is  heated  to  the  boiling  point.  The  object  of  treating 
in  this  manner  is  perfectly  attained,  and  the  formation  of  a  basic 
salt  entirely  prevented.  Although  equal  parts  of  carbonate  of 
soda  and  of  sulphate  of  zinc  will  decompose  each  other,  yet  it 
is  preferable  to  use  a  small  excess  of  the  carbonate  of  soda,  to 
prevent,  more  completely,  the  formation  of  any  basic  salt. 
The  well  washed  and  dried  precipitate  is  then  heated  in  a 
crucible,  until  diluted  sulphuric  acid  no  longer  causes  an  effer- 
vescence ;  when  a  small  quantity,  after  having  cooled,  is  tested 
with  it.  The  zinc  losing  its  carbonic  acid  at  much  less  than 
red  heat,  it  is,  consequently,  unnecessary  to  employ  so  high  a 
temperature ;  and  Mohr,  therefore,  proposes  to  burn  it  in  a 
glass  matrass  on  an  open  coal  fire,  requiring  only  one-half  the 
time  and  fuel  in  this  way.  For  this  purpose,  the  carbonate  of 
zinc,  triturated  to  fine  powder,  is  put  into  a  dry  glass  matrass 
and  placed  on  an  iron  ring,  or  in  a  wire  net,  over  a  moderate 
coal  fire.  The  carbonate  of  zinc,  when  heated,  loses  its  water 
and  carbonic  acid,  and  appears,  during  their  disengagement,  as 
a  liquid,  apparently  boiling  in  the  centre. 
The  matrass  is  heated  cautiously,  shaking  it  now  and  then 
until  all  the  carbonic  acid  has  passed  off,  and  a  small  quantity, 
taken  out  after  cooling,  no  longer  effervesces  when  brought  in 
contact  with  acids. 
The  oxide  will  now  appear  like  any  other  fine  powder,  adher- 
ing quite  closely  to  the  matrass.  Small  quantities  can,  by  this 
method,  be  very  readily  prepared  in  a  few  minutes  by  heating 
them  in  a  small  matrass  over  a  common  spirit  lamp.    The  same 
