any compounds such as silver salts, even
to the extent of liberating the metal. Oxygen is converted partially
into ozone under the influence of certain rays and there are many
examples of polymerization caused by light.

Various allotropic changes of the elements are due to the influence of
light; for example, a sulphur soluble in carbon disulphide is converted
into sulphur which is insoluble, and the rate of change of yellow
phosphorus into the red variety is greatly accelerated by light.
Hydrogen and chlorine combine under the action of light with explosive
rapidity to form hydrochloric acid and there are many other examples of
the synthesizing action of light. Carbon monoxide and chlorine combine
to form phosgene and the combination of chlorine, bromine, and iodine,
with organic compounds, is much hastened by exposing the mixture to
light. In a similar manner many decompositions are due to light; for
example, hydrogen peroxide is decomposed into water and oxygen. This
suggests the reason for the use of brown bottles as containers for many
chemical compounds. Such glass does not transmit appreciably the
so-called actinic or chemical rays.

There is a large number of reactions due to light in organic chemistry
and one of fundamental importance to mankind is the effect of light on
the chlorophyll, the green coloring matter in vegetation. No permanent
change takes place in the chlorophyll, but by the action of light it
enables the plant to absorb oxygen, carbon dioxide, and water and to use
these to build up the complex organic substances which are found in
plants. Radiant energy or light is absorbed and converted into chemical
energy. This use of radiant energy occurs only in those parts of the
plant in which chlorophyll is present, that is, in the leaves and stems.
These parts absorb the radiant energy and take carbon dioxide from the
air through breathing openings. They convert the radiant energy into
chemical energy and use this energy in decomposing the carbon dioxide.
The oxygen is exhausted and the carbon enters into the structure of the
plant. The energy of plant life thus comes from radiant energy and with
this aid the simple compounds, such as the carbon dioxide of the air and
the phosphates and nitrates of the soil, are built into complex
structures. Thus plants are constructive and synthetic in operation. It
is interesting to note that the animal organism converts complex
compounds into mechanical and heat e