272  Adulteration  of  White  Lead.  {kVTiVm™* 
these  substances  ought  to  be  present ;  they  possess  no  covering  power 
and  needlessly  absorb  oil.  Pure  white  lead  ought  to  be  perfectly  soluble 
in*very  dilute  nitric  acid,  and  in  the  resulting  clear  solution  caustic 
potassa  should  not  produce  a  precipitate,  for  if  it  does  chalk  is  pre- 
sent. An  insoluble  residue  in  the  dilute  nitric  acid  indicates  the 
presence  of  gypsum,  heavy  spar  or  sulphate  of  lead.  The  sulphate 
of  lead  may  be  recognized  by  reducing  the  lead  with  the  blowpipe. 
Sulphate  of  baryta  can  be  made  evident  by  ignition  with  charcoal  in 
the  blowpipe  flame,  treating  the  residue  with  dilute  hydrochloric  acid, 
and  adding  a  solution  of  gypsum,  which  again  yields  a  precipitate  of 
sulphate  of  baryta.  Gypsum  does  not  yield  an  insoluble  precipitate 
with  dilate  nitric  acid,  but  does  so  with  a  solution  of  oxalate  of  am- 
monia. According  to  Dr.  Stein,  the  most  simple  method  of  estimat- 
ing quantitatively  a  mixture  of  white  lead  and  sulphate  of  baryta  is 
to  heat  the  weighed  sample  in  a  piece  of  combustion  tube,  and  to  col- 
lect the  carbonic  acid  in  a  Liebig's  potassa  bulb,  a  chloride  of  calcium 
tube  being  fastened  by  a  perforated  cork  to  the  combustion  tube  to 
absorb  the  moisture.  The  quantity  of  carbonic  acid  given  off  stands 
in  direct  proportion  to  the  quantity  of  carbonate  of  lead  present. 
Pure  white  lead  of  good  quality  gives  off  about  14*5  per  cent,  of  the 
gas,  and,  according  to  Dr.  Stein's  researches,  the  undermentioned 
series  of  mixture  gave  off  the  quantities  of  carbonic  acid  indicated : 
33*3  parts  of  white  lead  and  66*6  parts  of  heavy  spar  lost  by  ignition 
4-5 — 5  per  cent. 
66*6  parts  of  white  lead  and  33*3  parts  of  heavy  spar  lost  by  ignition 
6*5 — 7  per  cent. 
80*0  parts  of  white  lead  and  20*0  parts  of  heavy  spar  lost  by  ignition 
13*0  per  cent. 
50*0  parts  of  white  lead  and  50*0  parts  of  heavy  spar  lost  by  ignition 
10 — 10  per  cent. 
The  extensive  applications  of  this  material  as  a  constituent  of 
paints,  uto  give  body,"  as  the  term  runs,  and  as  putty,  and  for  va- 
rious chemical  operations,  are  well  known.  It  has  been  experiment- 
ally proved  by  Dr.  G.  J.  Mulder  in  his  treatise  "  On  the  Chemis- 
try of  Drying  Oils  and  the  Practical  Applications  to  be  Drawn  There- 
from," that  the  quantity  of  white  lead  used  in  proportion  to  linseed 
oil  for  painting  purposes  is  far  too  great,  being  on  an  average  from 
250  to  280  parts  of  white  lead  to  100  parts  of  oil,  while  the  author 
found  that  52  parts  of  unadulterated  white  lead,  or  44  parts  of  oxide 
