AmAi0rnr;i8^8arm*}     Paints,  Colors  and  their  Analysis.  1 57 
weigh  the  insoluble  residue  left  after  extraction  with  hyposulphite  as 
well  as  the  plumbic  sulphate  actually  dissolved  and  reprecipitated.  The 
difference  must  be  very  small. 
The  undissolved  portion  is  washed,  and  finally  treated  with  concen- 
trated hydrochloric  acid  and  absolute  alcohol.  The  result  is  a  green 
solution  containing  the  chlorides  of  iron,  aluminum,  calcium,  mag- 
nesium, chromium  (from  chromic  acid  acted  on  by  alcohol)  and  zincy 
and  a  white  insoluble  residue  consisting  of  lead  chloride,  baric  sul- 
phate and  the  silicates  of  clay.  This  insoluble  residue  is  collected  on 
a  tared  filter,  washed  with  strong  alcohol,  dried  and  weighed. 
The  filter  is  re-introduced  into  the  funnel,  the  residue  exhausted  with, 
boiling  water  in  which  plumbic  chloride  will  dissolve,  again  dried  and 
re-weighed.  The  aqueous  filtrate  is  precipitated  with  sulphuric  acid, 
and  from  the  weight  of  the  plumbic  sulphate  is  calculated  that  portion 
of  oxide  of  lead  which  has  been  combined  with  chromic  acid.  Deter- 
mination by  loss  on  washing  and  direct  estimation  agree  well  if  the 
manipulations  have  been  properly  conducted. 
The  last  insoluble  residue,  after  washing  with  hot  water,  consists  of 
baric  sulphate  and  clay.  The  quantitative  analyses  of  it  and  of  the 
alcoholic  muriatic  acid  solution  previously  obtained,  are  performed 
according  to  the  usual  methods,  and  need  no  further  comment. 
To  detect  organic  acids  in  chrome-green  the  following  process  will 
give  satisfactory  results:  About  10  grams  of  file  green  powder  are 
macerated  in  dilute  sulphuric  acid,  and  filtered.  The  clear  liquid  is 
rendered  alkaline  by  sodium  carbonate,  whereby  iron  and  chromium 
are  partly  precipitated.  After  acidulation  with  muriatic  acid  the  sulphate 
is  removed  by  the  careful  addition  of  baric  chloride,  after  which  an 
excess  of  calcic  chloride  and  sodium  acetate  is  added.  Calcium  oxa- 
late, being  insoluble  in  free  acetic  acid,  is  precipitated,  while  the  tartrate 
and  citrate  remain  in  solution.  Neutralization  of  the  free  acid  preci- 
pitates the  former,  and,  on  boiling,  also  the  latter  salt. 
To  ascertain  the  relative  "strength"  of  two  colors  of  similar  shade, 
the  following  experiment  is  made:  Half  to  1  gram  of  each  sample  is 
mixed  with  5  to  10  times  its  weight  of  a  white  pulverulent  substance, 
which  exerts  no  chemical  action  on  the  pigment,  as,  for  instance,  baric 
sulphate.  After  sufficient  mixing,  the  shades  are  compared  and  the 
darker  one  made  equal  to  the  light  one  by  further  addition  of  baric 
sulphate.  On  re-weighing  the  samples  their  weights  will  give  the  ratio 
of  their  relative  u  strength." 
