a triangular prism of glass, the
light is bent or refracted out of its course instead of passing
straight through the glass. It thus comes out of the prism at
some angle to the line at which it entered. Yellow light is
bent more than red, and violet more than yellow. When light
made up of the yellow of sodium and the violet of potassium
shines through a slit upon such a prism, the yellow and the
violet lights come out at somewhat different angles, and so two
colored lines of light--a yellow line and a violet line--are
seen on looking into the prism in the proper direction. The
instrument used for separating the rays of light in this way is
called a _spectroscope_ (Fig. 79). The material to be tested is
placed on a platinum wire and held in the colorless Bunsen
flame. The resulting light passes through the slit in the end
of tube B, and then through B to the prism. The resulting
lines of light are seen by looking into the tube A, which
contains a magnifying lens. Most elements give more than one
image of the slit, each having a different color, and the
series of colored lines due to an element is called its
spectrum.

[Illustration: Fig. 79]

The spectra of the known elements have been carefully studied, and any
element which imparts a characteristic color to a flame, or has a
spectrum of its own, can be identified even when other elements are
present. Through the spectroscopic examination of certain minerals a
number of elements have been discovered by the observation of lines
which did not belong to any known element. A study of the substance then
brought to light the new element. Rubidium and caesium were discovered in
this way, rubidium having bright red lines and caesium a very intense
blue line. Lithium colors the flame deep red, and has a bright red line
in its spectrum.

EXERCISES

1. What is an alkali? Can a metal itself be an alkali?

2. Write equations showing how the following changes may be brought
about, giving the general principle involved in each change: NaCl -->
Na_{2}SO_{3}, Na_{2}SO_{3} --> NaCl, NaCl --> NaBr, Na_{2}SO_{4} -->
NaNO_{3}, NaNO_{3} --> NaHCO_{3}.

3. What carbonates are soluble?

4. State the conditions under which the reaction represented by the
following equation can be made to go in either direction:

Na_{2}CO_{3} + H_{2}O + CO_{2} <--> 2 NaHCO_{3}.

5. Account for the f