rochleder's  proximate  analysis. 
375 
solving  partially  in  alcohol  or  spirit,  without  more  than  a  trace  of  lead 
being  detectable  in  the  solution. 
If  acetate  and  subacetate  of  lead  have  produced  two  precipitates,  if 
these  have  been  divided  into  two  parts  by  strong  or  weaker  alcohol,  if  that 
portion  of  the  sugar  of  lead  precipitate,  wl)ich  is  insoluble  in  alcohol,  has 
again  been  divided  into  a  portion  soluble  and  another  one  insoluble  in 
water  containing  acetic  acid,  and  if  all  these  portions  have  been  freed 
from  lead  by  sulphuretted  hydrogen ;  then  these  substances  thus 
freed  from  lead,  as  well  as  the  different  portions  of  sulphuret  of  lead,  must 
bc'further  examined. 
For  this  purpose,  the  various portio7is  of  the  sulphuret  of  lead  are  treated 
immediately,  after  being  washed,  first  with  boiling  water  and  then  with 
boiling  spirit,  to  ascertain  whether  these  fluids  extract  anything  from  the 
sulphuret  of  lead.  After  the  sulphuret  of  lead  has  been  exhausted  with 
water  and  spirit,  it  is  exhausted  with  dilute  aqueous  ammonia,  and  the  fil- 
tered ammoniacal  solution  is  completely  neutralized  with  acetic  acid. 
The  oxidation  products  of  tannin  and  analogous  colored  substances  are 
often  in  this  way  extracted  by  the  water  containing  ammonia  from  the 
sulphuret  of  lead,  and  precipitated  from  their  solutions  by  acetic  acid. 
It  is  self-evident  that  the  various  (^portions  of)  substances  free  from  lead, 
which  are  obtained  from  those  portions  of  the  precipitates  by  acetate  and 
subacetate  of  lead  insoluble  and  soluble  in  strong  or  weaker  alcohol,  and 
from  those  portions  of  the  sugar  of  lead  precipitate  insoluble  in  alcohol, 
which  are  soluble  and  insoluble  in  acetic  acid,  may  still  be  a  mixture  of  sev- 
eral bodies.  To  ascertain  this,  the  individual  parts,  separated  according  to 
the  directions  above  given,  are  further  investigated  h-^  fractional  precipi- 
tation with  regard  to  their  homogeneousness.  It  is  scarcely  necessary  to 
mention  that  bodies  which  have  been  separated  in  a  crystalline  form  by 
the  standing  of  the  fluids,  properly  evaporated  in  the  air  or  in  exsiccator, 
must  be  previously  removed  from  the  mother  liquors,  and  that  in  an  such 
cases  the  diluted  mother  liquors  must  be  submitted  to  examination  by 
fractional  precipitation. 
For  fractional  'precipitation,  the  two  lead  salts,  sugar  of  lead  and  5tt6- 
ace^ai^e  ofZeac?  are  employed,  which  served  for  separating  up  to  a  certain 
point,  as  previously  described.  The  fluids  which  afford  no  precipitate 
with  sugar  of  lead  are  treated  with  subacetate  of  lead.  Fractional  precipi- 
tation is  conducted  in  the  following  way  : — The  whole  quantity  of  the  fluid 
to  be  examined  is  measured  in  a  graduated  glass  to  estimate  its  volume, 
and  then  it  is  ascertained  how  much  of  a  certain  solution  of  sugar  of  lead 
or  subacetate  of  lead  is  required  by  volume  to  completely  precipitate  the 
fluid.  With  this  object,  a  small  portion  of  the  fluid  to  be  examined  is  taken 
before  it  is  measured  off,  and  exactly  neutralized  with  ammonia.  If  the 
quantity  of  solution  of  sugar  of  lead  or  of  subacetate  of  lead  is  known  which 
is  required  to  precipitate  the  whole  quantity  of  the  fluid,  the  whole  quantity, 
