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CYCLOPEDlA OF PAINTING
then be replaced in their respective positions, and from time to time are compared together. Any change which may have been brought about by the action of sunlight and air on the exposed card will be observable; some colors will be changed in a few weeks' exposure, other colors require months of exposure to produce any effect.
By placing a card painted in the manner described, with different pigments in a closed cupboard, in which is placed a vessel containing some ferrous sulphide and diluted sulphuric acid, the action of sulphuretted hydrogen on the colors can be tested; if any are affected by this test it is certain that they will be similarly affected when exposed to the action of impure air.
