This faded even in a vacuum; but, strange to say, on keeping the
faded color in the dark, and exposed to air, the color was restored.
It was shown that, during the exposure to light, the color lost
cyanogen, or hydrocyanic acid, while in the dark and exposed to the
air, oxygen was absorbed. Chevreul concluded, therefore, that the
fading of Prussian blue was due to a process of reduction.
The prevailing opinion, however, is that the fading of colors is a
process of oxidation, caused by the ozone, or hydrogen peroxide, which
is probably formed in small quantity during the evaporation of the
moisture present, and both these substances are powerful bleaching
agents.
It would be extremely convenient to have some rapid method of testing
colors for fastness to light, and I believe it is the custom with some
to apply certain chemical tests with this object in view. The results
of my own experiments in this direction lead me to the conclusion that
at present we have no sufficient substitute for sunlight for this
purpose, since I have not found any oxidizing or reducing substance
which affects dyed colors in all respects like the natural
color-fading agencies; further, I am inclined to the opinion that the
action of light varies somewhat with the different coloring matters,
according to their chemical constitution and the fiber upon which they
are applied.
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