rociileder's  proximate  analysis. 
373 
corresponding  precipitates  can  be  united  with  one  another,  and,  as  a  whole, 
be  employed  for  examination,  which  must  not  be  done  when  the  animal 
charcoal  has  separated  any  substances.  In  the  latter  instance  the  preci- 
pitates are  separately  examined  with  regard  to  their  composition,  and  in 
the  same  way. 
The  precipitate  produced  hy  sugar  of  lead  is  extracted  with  alcohol  when 
a  small  portion  has  shown  that  it  is  partially  soluble  in  alcohol.  If  the 
alcohol  dissolves  nothing,  a  small  portion  is  tried  with  spirit  of  specific  gravi- 
ty '825,  to  ascertain  whether  that  effects  a  partial  solution.  If  the  alco- 
Jiol  or  spirit  has  partially  dissolved  the  precipitate,  the  insoluble  is  sepa- 
rated from  the  dissolved jjart  by  a  filter.  The  lead  compounds  of  sulphuric 
and  phosphoric  acids,  as  well  as  those  of  oxalic  acid  and  many  other  or- 
ganic acids  frequently  present,  do  not  dissolve  in  alcohol  or  spirit.  On  the 
contrary,  there  is  a  great  number  of  substances  which  form  compounds  with 
lead  insoluble  in  water,  but  soluble  in  alcohol  and  weaker  spirit.  The  alco- 
holic solution  is  decomposed  by  a  stream  of  sulphuretted  hydrogen,  and  then 
filtered  ;  the  sulphuret  of  lead  is  washed  on  the  filter  with  anhydrous  al- 
cohol as  long  as  anything  is  taken  up,  afterwards  the  sulphuret  of  lead  is 
treated  with  water  on  the  filter,  as  it  ofien  happens  that  a  substance  quite 
or  almost  insoluble  in  anhydrous  spirit  is  contained  in  the  sulphuret  of 
lead  which  dissolves  in  water,  and  can  be  thus  extracted.  By  evapora- 
tion the  filtrate  spirituous  as  well  as  aqueous  is  reduced  to  a  sm;ill  volume, 
and  this  residue  set  aside  to  afford  an  opportunity  for  crystals  to  form; 
which  sometimes  happens  when  crystallizable  substances  are  present. 
The  part  of  the  precipitate  thrown  down  hij  sugar  of  lead  from  the  watery 
decoction,  and  insoluble  in  alcohol  or  spirit,  is,  after  having  been  washed  with 
alcohol  or  weaker  spirit,  formed  with  water  into  a  thick  smooth  paste,  and  a 
small  portion  of  this  paste  is  mixed  with  a  great  excess  of  acetic  acid.  It  is 
observed  whether  the  whole  or  a  part  of  the  precipitate  disappears.  Some- 
times very  little  or  nothing  dissolves  in  the  acetic  acid.  When  a  partial 
or  complete  solution  is  noticed,  the  whole  quantity  of  the  precipitate, 
mixed  with  water,  is  treated  with  acetic  acid  as  long  as  a  decrease  of  the 
precipitate  results.  Now,  either  a  complete  solution  is  obtained,  or  in  the 
resulting  solution  an  insoluble  portion  is  suspended.  In  the  latter  case 
the  solution  is  filtered  from  the  undissolved  portion.  Either  this  filtered 
solution,  or  the  entire  liquid  when  all  the  precipitate  dissolves  in  the  ace- 
tic acid,  is  mixed  with  subacetate  of  lead  in  a  very  concentrated  condition. 
With  continued  agitation  small  quantities  of  the  subacetate  of  lead  are  add- 
ed. It  often  happens  that  a  precipitate  forms  after  a  very  small  quantity  of 
the  subacetate  of  lead  has  been  added,  although  the  fluid  is  still  strongly 
acid.  After  this  precipitate  has  been  removed  by  a  filter,  much  subacetate  of 
lead  can  be  added  before  a  fresh  precipitate  forms,  which  first  appears 
when  the  free  acetic  acid  has  been  neutralized  by  the  subacetate  of  lead.  If 
a  second  precipitate  ensues  by  the  addition  of  much  subacetate  of  lead,  this 
is  collected  on  a  filter  by  itself.    Both  precipitates  which  are  thrown  down 
