vapor becomes hot enough to give off light (when it is incandescent),
it does not give off white light but light of different colors. An
experiment will let you see this for yourself.

EXPERIMENT 53. Sprinkle a little copper sulfate (bluestone)
in the flame of a Bunsen burner. What color does it make the
flame?

Copper vapor always gives this greenish-blue light when it is heated.
The photographer's mercury-vapor light gave a greenish-violet glow.
When you burn salt or soda in a gas flame, you remember that you get a
clear yellow light. By breaking up these lights, somewhat as you broke
up the sunlight with the prism, chemists and astronomers can tell what
kind of gas is glowing. The instrument they use to break up the light
into its different colors is called a _spectroscope_, and the band
of colors formed is called the _spectrum_. With the spectroscope they
examine the light that comes from the sun and stars and by the colors
of the spectra they can tell what these far-distant bodies are made
of.

_APPLICATION 39._ If you were going to the tropics, would it
be better to wear outside clothes that were white or black?

_APPLICATION 40._ A dancer was to dance in a spotlight on the
stage. The light was to change colors constantly. She wanted
her robe to reflect each color that was thrown on it. Should
she have worn a robe of red, yellow, white, green, or blue?

_APPLICATION 41._ If you looked through a red glass at a
purple flower (purple is red mixed with blue), would the
flower look red, blue, purple, black, or white?

INFERENCE EXERCISE

Explain the following:

241. Mercury is separated from its ore by heating the ore so
strongly that the mercury rises from it as a vapor.

242. Hothouses are built of glass.

243. A "rainbow" is sometimes seen in the spray of a garden
hose.

244. Your feet become hot when your shoes are being polished.

245. Doors into offices usually have windows of ground glass
or frosted glass.

246. Opera glasses are of value to those sitting at a distance
from the stage.

247. In order to see clearly through opera glasses, you have
to adjust them.

248. It is warm inside an Eskimo's hut although it is built of
ice and snow.

249. It is usually cooler on a lawn than on dry ground.

250. Black clothes are warmer in the sunlight than clothes of
any other c