WEIGHING  OF  MOIST  PRECIPITATES. 
357 
In  case  the  precipitate  settles  but  slowly  it  may  be  collected 
on  a  filter,  and  together  with  the  filter,  after  washing,  be  intro- 
duced into  the  bottle,  in  which  case  the  weight  of  the  filter  and 
its  specific  gravity,  supposing  any  difference  should  exist  be- 
tween its  own  and  that  of  water,  is  to  be  taken  in  account. 
Precipitates  soluble  in  or  affected  by  water  may  be  weighed  in 
some  other  liquid. 
This  method,  of  which  the  above  are  the  outlines,  is  spoken 
of  in  the  Jahresbericht  der  Chemie  for  1858  in  rather  dispar- 
aging terms,  and  I  consider  it  not  more  than  justice  to  the 
method,  if  not  also  to  Mr.  Mene,  to  prove  its  correctness,  the 
more  so  as  I  have  applied  the  principle  on  a  large  scale  as  far 
back  as  1855. 
I  engaged  in  that  year  in  the  manufacture  of  carbonate  of 
lead  from  refuse  sulphate  of  lead,  by  treating  the  latter  in  a 
pulpy  condition  with  carbonate  of  soda.  The  sulphate  of  lead  I 
used  contained  very  varying  proportions  of  water  and  soluble 
impurities,  from  which  latter  it  had  first  to  be  freed  by  washing. 
It  was  then  in  the  state  of  a  thin  pulp,  and  the  difficulty  was  to 
find  the  amount  of  dry  sulphate  of  lead,  as  it  was  a  matter  of 
importance  to  use  as  little  carbonate  of  soda,  and  to  obtain  as 
pure  a  carbonate  of  lead  and  sulphate  of  soda  as  possible.  This 
could  only  be  done  by  weighing  it  as  whole,  or  in  portions ;  but 
as  the  drying  of  a  tubful  of  sulphate  of  lead  (from  500  to  1200 
pounds)  was  impracticable,  and  sampling  not  less  so,  since  the 
upper  strata  contained  a  much  larger  proportion  of  water  than 
the  lead  at  the  bottom ;  I  contrived  the  following  method, 
which  enabled  me  to  leave  the  management  of  the  process  in  the 
hands  of  a  workman. 
I  took  a  strong  oaken  pail,  weighing  eight  pounds  when  empty, 
and  caused  a  black  mark  to  be  burnt  in  horizontally  around  the 
inside  of  the  pail,  two  inches  below  the  rim,  up  to  which  mark 
it  held  twenty  pounds  of  water.  I  reasoned  as  follows :  The 
specific  gravity  of  sulphate  of  lead  being  6-3,  the  pail  if  filled  up 
to  the  mark  would  hold  126  pounds  of  pure  sulphate  of  lead. 
The  specific  gravity  of  water  being  5.3  less  than  that  of  sulphate 
of  lead,  it  followed  that  if  there  was  one  pound  of  water  in  the 
pailfull  of  moist  sulphate,  the  pail  would  weigh  5-3  pounds  less 
than  126  (+8,  the  tare  of  the  pail)  =  120-7  (+8) ;  if  there 
