nergy. The animal organism depends
upon the synthetic work of plants, consuming as food the complex
structures built by them under the action of light. For example, plants
inhale carbon dioxide, liberate the oxygen, and store the carbon in
complex compounds, while the animal uses oxygen to burn up the complex
compounds derived from plants and exhales carbon dioxide. It is a
beautiful cycle, which shows that ultimately all life on earth depends
upon light and other radiant energy associated with it. Contrary to most
photochemical reactions, it appears that plant life utilize yellow, red,
and infra-red energy more than the blue, violet, and ultra-violet.

In general, great intensities of blue light and of the closely
associated rays are necessary for most photochemical reactions with
which man is industrially interested. It has been found that the white
flame-arc excels other artificial light-sources in hastening the
chlorination of natural gas in the production of chloroform. One
advantage of the radiation from this light-source is that it does not
extend far into the ultra-violet, for the ultra-violet rays of short
wave-lengths decompose some compounds. In other words, it is necessary
to choose radiation which is effective but which does not have rays
associated with it that destroy the desired products of the reaction. By
the use of a shunt across the arc the light can be gradually varied over
a considerable range of intensity. Another advantage of the flame-arc in
photochemistry is the ease with which the quality or spectral character
of the radiant energy may be altered by varying the chemical salts used
in the carbons. For example, strontium fluoride is used in the red
flame-arc whose radiant energy is rich in red and yellow. Calcium
fluoride is used in the carbons of the yellow flame-arc which emits
excessive red and green rays causing by visual synthesis the yellow
color. The radiant energy emitted by the snow-white flame-arc is a close
approximation to average daylight both as to visible and to ultra-violet
rays. Its carbons contain rare-earths. The uses of the flame-arcs are
continually being extended because they are of high intensity and
efficiency and they afford a variety of color or spectral quality. A
million white flame-carbons are being used annually in this country for
various photochemical processes.

Of the hundreds of dyes and pigments available many are not permanent
and until recent years sunlight