376 
kochleder's  proximate  analysis. 
after  exact  neutralization  with  ammonia,  is  mixed  with  a  tenth  part  of  the 
quantity  of  solution  of  sugar  of  lead  or  subacetate  of  lead  which  would  be  re- 
quired for  its  entire  precipitation.  The  precipitate  is  filtered  off,  and  the  se- 
cond tenth  of  the  solution  of  sugar  of  lead  or  subacetate  of  lead  is  added  to 
the  filtrate,  whereon  the  precipitate  is  again  collected  on  a  filter.  This  pro- 
cess is  continued  until  ten  precipitates  are  obtained  fram  th^  fluid,  all  of 
which  are  washed  with  water,  diffused  in  the  same  medium,  and  decom- 
posed with  sulphuretted  hydrogen,  whereby  ten  parts  result  from  the  fluid, 
which  are  subj acted  to  a  more  minute  examination  when  the  sulphuretted 
hydrogen  has  been  driven  off. 
I  have  convinced  myself  that  this  fractional  precipitation  is  not  so  suita- 
ble to  effect  the  separation  of  various  substances  as  another  method,  which 
I  will  caU /radio nal  solution.  This  is  executed  thus  : — It  is  determined  how 
much  acetic  acid  is  required  to  dissolve  the  whole  quantity  of  a  precipi- 
tate which  has  been  diffused  through  water  to  a  smooth  paste.  Now,  if  a 
tenth  part  of  the  acetic  is  added,  a  tenth  part  of  the  precipitate  is  dissolved. 
This  solution  is  filtered  off,  the  precipitate  found  on  the  filter  is  transferred 
to  a  beaker  glass  by  means  of  a  stream  from  a  washing  l)ottle,  and  then 
treated  with  the  second  tenth  of  the  acetic  acid,  &c.  The  ten  solutions  are 
precipitated  with  subacetate  of  lead,  the  ten  precipitates  collected,  each 
washed  on  a  filter  with  water,  diffused  in  water,  and  decomposed  with  sul- 
phuretted hydrogen.  The  sulphuret  of  lead  is  separated  by  a  filter,  the 
excess  of  sulphuretted  hydrogen  expelled,  and  each  of  the  ten  fluids  tested 
with  reagents,  Tt  ia  evident  that  only  lead  compounds  soluble  in  acetic 
acid  can  be  in  this  manner  treated,  and  not  the  parts  of  precipitates 
insoluble  in  that  menstruum  which  are  produced  by  sugar  of  lead  in  the 
watery  decoctions,  and  remain  undissolved  by  treatment  with  alcohol. 
Before  T  pass  to  the  examination  of  the  precipitates  which  are  produced 
in  a  portion  of  the  watery  decoction  by  alum  and  ammonia,  I  have  still  to 
mention  the  means  by  which  a  separation  of  the  substances  in  some  cases 
is  effected,  which  are  obtaiued  from  the  precipitates  developed  by  acetate  and 
subacetate  of  lead,  in  a  condition  free  from  lead,  according  to  the  method 
previously  given. 
There  are  substances  \\h\Q\\,  per  se,  or  by  the  agency  of  other  bodies  with 
which  they  are  mixed,  are  soluble  in  water  in  rather  considerable  quanti- 
ty. Many  of  the  bodies  lose  their  solubility  in  considerable  degree  lohen 
they  are  completely  dried,  or  then  dissolve  only  very  slowly  in  water  ;  while 
drying,  on  many  other  bodies,  exercises  no  influence  of  that  kind  on  their 
solubility.  Consequently  it  is  often  a  good  method,  to  obtain  the  greatest 
part  of  a  substance  in  a  state  of  separation  from  the  other  bodies  accom- 
panying it,  to  allow  the  mixture  to  thoroughly  dry  in  vacuo;  and  best  over 
sulphuric  acid,  to  treat  the  dried  mass  with  a  little  cold  water,  and  then 
by  stirring  and  trituration  to  bring  the  dried  mass  into  close  contact  with 
(To  be  continued.) 
