ht which had passed through glasses stained red,
orange, yellow, green, blue, and violet, corresponding to definite
parts of the spectrum. They found that the blue light possessed the
greatest fading power, red light the least.

More recently (1886-88) Abney and Russell exposed water colors under
red, green, and blue glass, and came to the same conclusion.

But the chemical energy of the sun's rays is not the sole cause of the
fading of colors. There are certain contributory causes as important
as the light itself.

About fifty years ago, Chevreul showed what these accessory causes
are, by exposing to light a number of dyed colors under varied
conditions, e.g., in a vacuum, in dry and moist hydrogen, dry and
moist air, water vapor, and the ordinary atmosphere. He found that
such fugitive colors as orchil, safflower, and indigo-carmine fade
very rapidly in moist air, less rapidly in dry air, and that they
experience little or no change in hydrogen or in a vacuum. The general
conclusion arrived at was, that light, when acting alone, i.e.,
without the aid of air and moisture, exercises a very feeble
influence. Further, it was determined that the air and moisture,
without aid of light, have also comparatively little effect on dyed
colors. Abney and Russell, in their experiments with water colors,
obtained similar results.

These conclusions are exactly in accordance with our common knowledge
of the old fashioned method of bleaching cotton and linen, in which
the wetted fabric is exposed to light on the grass, and frequently
sprinkled with water. If the material becomes dry through the absence
of dew or rain, or the want of sprinkling, little or no bleaching
takes place.

The one color which Chevreul found to behave abnormally was Prussian
blue. 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