148  Paints,  Colors  and  their  Analysis.  {^ApX^"" 
class,  which  are  extensively  used  in  the  arts  and  manufactures,  are 
ultramarine  and  Prussian  blue.  The  former  is  a  combination  of  sodic 
sulphide  with  alumina  silicate.  Lapis  lazuli,  the  native  ultramarine, 
is  a  very  beautiful,  though  rare  mineral,  and  resists  the  action  af  acetic 
acid  and  alum,  while  the  artificial  product  is  more  or  less  acted  on  by 
these  chemicals,  and  entirely  destroyed  by  strong  mineral  acids,  with 
evolution  of  sulph-hydric  gas.  There  are  several  shades  of  ultramarine 
in  the  market,  viz. : 
a.  Green  or  bluish-green  ultramarines. 
b.  Light  blue  shades  of  great  intensity. 
c.  Pure  deep  blue  shades. 
d.  Reddish  or  violet  ultramarines. 
They  all  have  qualitatively  the  same  composition,  merely  differing 
in  the  percentage  of  sulphur  and  oxygen,  and  are  most  generally  manu- 
factured by  ignition  of  a  mixture  of  China  clay,  sodic  sulphate  and 
charcoal.  The  product  is  green,  and  is  sometimes  brought  into  the 
market  after  grinding,  washing  and  drying.  On  mixing  this  green 
powder  with  sulphur,  and  heating  till  the  latter  burns  out,  forming  sul- 
phurous anhydrid,  a  light-blue  shade  is  obtained,  containing  from  6  to 
8  per  cent,  of  sulphur. 
Beautiful  deep  blue  shades  are  formed  by  addition  of  sodic  carbonate 
and  sulphur  to  the  charge  previously  mentioned,  and  subsequent  reheat- 
ing with  sulphur.  The  product  of  this  process  contains  from  10  to 
12  per  cent,  of  sulphur.  Mixtures  of  China  clay,  infusorial  earth, 
coal,  sodic  carbonate  and  sulphur  yield  reddish-blue  and  violet  shades 
on  ignition. 
The  more  silica  an  ultramarine  contains  the  better  it  resists  the  acids. 
These  pigments  are  extensively  used  in  fresco  painting. 
As  to  Prussian  blue — ferrocyanide  of  iron — I  refer  to  my  article  on 
"  Wash-Blue  and  its  Analysis,"  in  the  "  American  Journal  of  Phar- 
macy," Feb.,  1877.  The  lighter  shades  of  blue  are  obtained  by  grind- 
ing white  pigments,  as  chalk,  whitening,  barytes,  zinc,  clay,  etc.,  with 
a  certain  quantity  of  pure  Prussian  or  Chinese  blue.  It  is  astonishing 
what  an  immense  colorific  power  this  pigment  possesses,  particularly 
if  very  finely  distributed. 
The  following  experiment,  which  may  be  worth  while  trying,  proves 
the  latter  assertion.  Take  9/6  grams  of  whitening  and  0*4  of  pure 
blue.    After  mixing  them  in  a  mortar  and  rubbing  with  a  pestle  for 
