874 
rochleder's  proximate  analysis. 
from  the  acetic  acid  solution,  or  the  precipitate  when  only  one  results,  must 
be  purified  by  washing  with  water  on  the  filter,  then  distributed  in  water 
to  be  decomposed  with  sulphuretted  hydrogen.  The  fluid  is  separated 
from  the  sulphuret  of  lead  by  filtration,  and  the  excess  of  sulphuretted 
hydrogen  expelled.  This  expulsion  is  most  conveniently  effected  in  a  glass 
flask,  or  in  a  tubulated  retort,  in  a  water  or  chloride  of  calcium  bath.  Car- 
bonic acid  gas  should  be  driven  through  the  fluid  to  displace  the  air  in 
the  retort  or  flask,  and  prevent  the  decomposition  of  the  sulphuretted  hy- 
drogen accompanied  by  the  separation  of  sulphur.  Heat  must  not  be 
applied  to  the  apparatus  until  all  the  air  is  displaced  by  carbonic  acid  gas. 
The  precijniate  which  is  produced  by  suhacetate  of  lead  in  the  fluid  which 
v/as  filtered  from  the  precipitate  which  sugar  of  lead  occasioned  in  the 
watery  decoction,  is  likewise  examined  with  reference  to  its  solubility  in  al- 
cohol and  in  weaker  spirit,  wherein  it  may  be  partially  or  quite  soluble, 
or  quite  insoluble.  TJie  precipitates  soluble  in  alcohol  or  spirit,  and  those 
insoluble  therein,  are  treated  in  precisely  the  same  manner  as  directed  for 
the  precipitates  derived  by  sugar  of  lead  from  the  watery  decoction.  The 
precipitate  resulting  from  sugar  of  lead  is  manifestly  always  completely 
soluble  in  acetic  acid.  In  many  cases,  indeed,  a  number  of  substances 
are  entirely  thrown  down  from  the  watery  decoction  by  sugar  of  lead,  so 
that  the  precipitate  formed  by  subacetate  of  lead  contains  none  of  the  sub- 
stances which  are  contained  in  the  precipitate  produced  by  sugar  of  lead. 
But  these  favorable  instances  of  complete  separation  are  only  exceptions 
to  the  rule.  Mostly  the  precipitate  obtained  by  subacetate  of  lead  contains 
substances  which  were  contained  in  the  first  precipitate — namely,  in  that 
part  of  the  first  precipitate  by  sugar  of  lead  which  dissolved  in  acetic  acid, 
and  was  again  thrown  down  by  subacetate  of  lead  from  this  acid  solution. 
It  is  here  particularly  necessary  to  draw  attention  to  a  circumstance, 
namely,  to  the  presence  of  bodies  in  the  precipitates  produced  by  subacetate  of 
lead,  which  are  not  contained  in  these  precipitates  in  the  form  of  lead  com- 
pounds, but  are  in  a  free  condition.  Let  it  bo  assumed  that  the  watery 
decoction  contains  an  organic  base  very  difiicultly  soluble  in  water  in  the 
form  of  a  salt  soluble  in  water,  whose  acid  produces  with  oxide  of  lead  a 
compound  insoluble  in  water,  it  will  be  evident  that  the  acid  of  this  salt 
will  be  contained  in  the  precipitate  resulting  from  sugar  of  lead,  and  the 
base  of  this  salt  will  be  found  as  an  acetate  in  the  filtered  watery  solution. 
If  this  fluid  is  neutralized  with  subacetate  of  lead,  a  weak  base  can  be  there- 
by separated  from  the  acetic  acid,  and  is  precipitated  in  water  on  account 
of  its  insolubility  in  conjunction  with  the  different  lead  compounds  formed. 
Bodies  admixed  with  precipitates  of  that  kind  which  are  produced  by  sub- 
acetate of  lead,  generally  dissolve  in  alcohol,  especially  when  heated.  They 
are  in  this  way  often  easily  obtained  pure  when  the  lead  compounds  in 
these  precipitates  are  insoluble  in  alcohol,  or  possess  a  very  slight  solu- 
bility in  that  menstruum.  What  has  been  stated  explains  the  peculiarity 
sometimes  existing  of  a  precipitate  resulting  from  subacetate  of  lead  dis- 
