﻿Properties 
  of 
  Artists' 
  Colours. 
  729 
  

  

  peroxide 
  or 
  ozone, 
  and 
  that 
  the 
  pure 
  samples 
  exposed 
  to 
  

   sunlight 
  were 
  also 
  unaffected. 
  

  

  In 
  conclusion, 
  it 
  may 
  be 
  pointed 
  out 
  that 
  if 
  the 
  fading 
  of 
  

   paints 
  is 
  due 
  to 
  hydrogen 
  peroxide, 
  much 
  may 
  be 
  done 
  

   towards 
  their 
  preservation 
  by 
  the 
  employment 
  of 
  a 
  suitable 
  

   medium. 
  Water-colours 
  are 
  known 
  to 
  be 
  much 
  less 
  stable 
  

   than 
  oils, 
  and 
  by 
  finding 
  a 
  medium 
  very 
  impermeable 
  to 
  air 
  

   and 
  moisture, 
  it 
  should 
  be 
  possible 
  to 
  prolong 
  almost 
  indefi- 
  

   nitely 
  the 
  life 
  even 
  of 
  a 
  fugitive 
  colour. 
  

  

  Part 
  II. 
  — 
  The 
  Transparency 
  of 
  Paints. 
  

  

  The 
  transparency 
  of 
  a 
  paint, 
  or 
  its 
  inverse 
  the 
  opacity, 
  is 
  

   of 
  more 
  importance 
  to 
  the 
  technical 
  painter 
  than 
  to 
  the 
  artist, 
  

   for 
  the 
  greater 
  the 
  opacity 
  of 
  a 
  paint, 
  the 
  greater 
  is 
  its 
  

   efficiency 
  as 
  a 
  covering 
  agent, 
  or 
  in 
  other 
  words, 
  the 
  greater 
  

   the 
  opacity, 
  the 
  less 
  paint 
  will 
  be 
  required 
  to 
  produce 
  a 
  given 
  

   effect. 
  

  

  The 
  following 
  experiments 
  were 
  performed 
  with 
  a 
  view 
  of 
  

   comparing 
  the 
  opacity 
  of 
  various 
  paints. 
  The 
  method 
  

   employed 
  was 
  to 
  cover 
  a 
  bright 
  metal 
  surface 
  crossed 
  by 
  

   black 
  lines 
  with 
  a 
  suspension 
  of 
  the 
  colouring 
  material 
  in 
  

   water. 
  After 
  drying 
  in 
  a 
  level 
  position 
  the 
  effect 
  was 
  

   observed. 
  If 
  the 
  black 
  lines 
  were 
  visible, 
  more 
  paint 
  was 
  

   put 
  on, 
  but 
  if 
  not, 
  a 
  little 
  was 
  washed 
  off. 
  The 
  whole 
  

   was 
  weighed 
  after 
  each 
  operation, 
  and 
  in 
  this 
  way 
  two 
  

   weights 
  were 
  obtained 
  fairly 
  close 
  together; 
  in 
  one 
  case, 
  

   however, 
  the 
  black 
  lines 
  were 
  just 
  visible, 
  and 
  in 
  the 
  other 
  

   they 
  were 
  not. 
  By 
  taking 
  the 
  mean 
  of 
  these, 
  the 
  weight 
  of 
  

   paint 
  which 
  would 
  just 
  cover 
  the 
  metal 
  was 
  approximately 
  

   obtained. 
  

  

  These 
  experiments 
  cannot 
  claim 
  any 
  great 
  degree 
  of 
  

   accuracy, 
  but 
  they 
  afford 
  at 
  least 
  some 
  idea 
  of 
  the 
  relative 
  

   transparency. 
  The 
  results 
  below 
  (p. 
  730) 
  show 
  the 
  number 
  

   of 
  milligrams 
  of 
  paint 
  required 
  to 
  just 
  cover 
  one 
  square 
  

   centimetre. 
  

  

  In 
  a 
  few 
  cases 
  the 
  paints 
  were 
  so 
  transparent 
  that 
  the 
  thick 
  

   coating 
  necessary 
  to 
  hide 
  the 
  black 
  lines 
  could 
  not 
  be 
  applied 
  

   evenly, 
  and 
  in 
  such 
  cases 
  it 
  usually 
  cracked 
  and 
  peeled 
  off. 
  

   Beyond 
  this, 
  the 
  only 
  point 
  of 
  interest 
  is 
  that 
  gamboge 
  when 
  

   painted 
  on 
  metal 
  dries 
  as 
  a 
  perfectly 
  transparent 
  yellow 
  

   varnish, 
  and 
  in 
  this 
  appears 
  to 
  differ 
  from 
  all 
  other 
  paints, 
  

   the 
  nearest 
  approach 
  to 
  it 
  being 
  sap 
  green. 
  

  

  