ture by having closed his shop at the usual hour.
The young man telephoned in the morning and told the proprietor what had
happened, but on being asked to make certain of the facts he went to the
window and looked at his boat and behold! it was white. It had regained
whiteness during the night but would turn black again during the day.
Although pigments and dyes are not generally as peculiar as lithopone,
much uncertainty is eliminated by systematic tests under constant,
continuous, and controllable artificial light.

The sources of so-called chemical rays are numerous for laboratory work,
but there is a need for highly efficient powerful producers of this kind
of energy. In general the flame-arcs perhaps are foremost sources at the
present time, with other kinds of carbon arcs and the quartz mercury-arc
ranking next. One advantage of the mercury-arc is its constancy.
Furthermore, for work with a single wave-length it is easy to isolate
one of the spectral lines. The regular glass-tube mercury-arc is an
efficient producer of the actinic rays and as a consequence has been
extensively used in photographic work and in other photochemical
processes. An excellent source for experimental work can be made easily
by producing an arc between two small iron rods. The electric spark has
served in much experimental work, but the total radiant energy from it
is small. By varying the metals used for electrodes a considerable
variety in the radiant energy is possible. This is also true of the
electric arcs, and the flame-arcs may be varied widely by using
different chemical compounds in the carbons.

There are other effects of light which have found applications but not
in chemical reactions. For example, selenium changes its electrical
resistance under the influence of light and many applications of this
phenomenon have been made. Another group of light-effects forms a branch
of science known as photo-electricity. If a spark-gap is illuminated by
ultra-violet rays, the resistance of the gap is diminished. If an
insulated zinc plate is illuminated by ultra-violet or violet rays, it
will gradually become positively charged. These effects are due to the
emission of electrons from the metal. Violet and ultra-violet rays will
cause a colorless glass containing manganese to assume a pinkish color.
The latter is the color which manganese imparts to glass and under the
influence of these rays the color is augmented. Certain ultra-violet
ray