4juJn°eU?;iP873RM-}        Adulteration  of  White  Lead.  273 
•of  lead  to  100  parts  of  raw  or  boiled  oil  are  amply  sufficient  quanti- 
ties. White  lead,  however  useful,  is  very  sensitive  to  the  action  of 
sulphuretted  hydrogen,  by  which  it  is  blackened  and  discolored,  caus- 
ing not  only  all  the  white  paint  to  be  spoiled,  but  also  all  pigments 
•and  paints  of  which  white  lead  is  a  constituent,  as  may  be  seen  to  a 
very  large  extent  every  summer  at  Amsterdam,  where  from  the  stag- 
nant canals  sulphuretted  hydrogen  is  abundantly  given  off.  The  ac- 
tion, however,  of  the  sea  air  in  autumn  has  the  effect  of  somewhat  re- 
storing the  blackened  and  discolored  painted  surfaces  to  their  primi- 
tive hue.  The  late  Professor  Thenard  suggested  that  pictures  which 
had  become  blackened  should  be  cleaned  by  means  of  peroxide  of 
hydrogen,  the  oxygen  of  which  present  as  ozone  converts  the  black- 
ened lead  colors  into  white  sulphate  of  lead. 
In  this  country  it  has  become  an  almost  universal  custom  to  sell 
white  lead  ready  ground  with  linseed  oil  into  a  thick  paste.  This 
practice  certainly  saves  painters  a  great  deal  of  trouble,  but  is  also 
pregnant  with  the  difficulty  of  detecting  adulteration,  while  there  is  a 
chance  of  inferior  oil-^resin  oil — being  added.  The  oil  almost  en- 
tirely prevents  the  action  of  any  acid  upon  the  paste  ;  even  if  very 
strong  nitric  acid  be  taken,  and  heat  applied,  the  decomposition  and 
disintegration  are  very  slow  and  incomplete,  and,  besides,  owing  to 
the  insolubility  of  nitrate  of  lead  in  nitric  acid,  the  action  of  strong 
nitric  acid  upon  oil  thus  mixed  gives  rise  to  a  variety  of  compounds, 
which  interfere  with  the  usual  modes  of  testing  the  white  lead.  To 
remove  the  oil  in  order  to  test  white  lead,  the  best  plan  is  to  thor- 
oughly incorporate  some  of  the  sample  with  a  mixture  of  chloroform 
and  strong  alcohol  in  equal  parts,  and  to  wash  the  mass  by  decanta- 
tion  or  on  a  filter  with  a  fluid  composed  of  alcohol.*  The  quantity 
of  the  oil  may  then  be  ascertained  by  the  evaporation  of  this  solvent. 
After  washing  once  or  twice  with  boiling  alcohol  and  then  drying, 
the  white  lead  can  be  readily  tested  by  any  of  the  known  methods. — 
Journ.  Applied  Chemistry,  April,  1873. 
*In  the  examination  of  white  lead  ground  in  oil,  we  have  successfully  used 
t)oth  petroleum  benzin  and  ether,  as  suitable  solvents  for  removing  the  linseed 
•oil.  The  white  lead  must  be  thoroughly  incorporated  with  the  solvent,  of 
which,  after  decantation,  fresh  portions  should  be  used,  until  the  residue  after 
-drying,  becomes  pulverulent,  when  the  washing  maybe  completed  upon  a  filter. 
On  treating  the  lead  now  with  dilute  nitric  acid,  a  little  oxidized  fat  separates 
readily. — Ed.  Am.  Jour.  Phar. 
15 
