are not perfectly black. A pair of shoes, for instance, has
particles of gray dust on them; or if they are very shiny they reflect
part of the light that strikes them as a white high-light. But the
really black part of your shoes would be invisible against an equally
black background.

A black thing absorbs the light that strikes it and turns it to heat.
Here is an experiment that will prove this to you:

EXPERIMENT 52. (a) On a sunny day, take three bottles, all of
the same size and shape, and pour water out of a pitcher or
pan into each bottle. Do not run the water directly from the
faucet into the bottle, because sometimes that which comes
out of the faucet first is warmer or colder than that which
follows; in the pitcher or pan it will all be mixed together,
and so you can be sure that the water in all three bottles is
of the same temperature to begin with. Wrap a piece of white
cotton cloth twice around one bottle; a piece of red or
green cotton cloth of the same weight twice around the second
bottle, and a piece of black cotton cloth of the same weight
twice around the third bottle, fastening each with a rubber
band. Set all three bottles side by side in the sunlight, with
2 or 3 inches of space between them. Leave them for about an
hour. Now put a thermometer into each to see which is warmest
and which is least warm.

From which bottle has most of the light been reflected back
into the air by the cloth around it? Which cloth absorbed most
of the light and changed it into heat? Does the colored cloth
absorb more or less light than the white one? than the black
one?

[Illustration: FIG. 93. Which color is warmest in the sunlight?]

(b) On a sunny day when there is snow on the ground, spread
three pieces of cotton cloth, all of the same size and
thickness, one white, one red or green, and one black, on top
of the snow, where the sun shines on them. Watch them for a
time. Under which does the snow melt first?

The white cloth is white because it reflects _all_ colors back
at once. It therefore absorbs practically no light. But the
reason the black cloth looks black is that it reflects almost
none of the colors--it absorbs them all and changes them to
heat. The colored cloth reflects just the red or the green
light and absorbs the rest.

Maybe you will understand color better if it