reater quantities of active rays, and many still
remain interesting scientific facts without any apparent practical
applications at the present time. Visible and ultra-violet rays are the
radiations almost entirely responsible for photochemical reactions, but
the most active of these are the blue, violet, and ultra-violet rays.
These are often designated chemical or actinic rays in order to
distinguish the group as a whole from other groups such as ultra-violet,
visible, and infra-red. Light is a unique agent in chemical reactions
because it is not a material substance. It neither contaminates nor
leaves a residue. Although much information pertaining to photochemistry
has been available for years, the absence of powerful light-sources
emitting so-called chemical rays in large quantities inhibited the
practical development of the science of photochemistry. Even to-day,
with vast applications of light in this manner, mankind is only
beginning to utilize its chemical powers.

[Illustration: In a moving-picture studio

In a portrait studio

ARTIFICIAL LIGHT IN PHOTOGRAPHY]

[Illustration: Swimming pool

City waterworks

STERILIZING WATER WITH RADIANT ENERGY FROM QUARTZ MERCURY-ARCS]

Although it appears that the chemical action of light was known to the
ancients, the earliest photochemical investigations which could be
considered scientific and systematic were those of K. W. Scheele in 1777
on silver salts. An extract from his own account is as follows:

I precipitated a solution of silver by sal-ammoniac; then I
edulcorated (washed) it and dried the precipitate and exposed
it to the beams of the sun for two weeks; after which I stirred
the powder and repeated the same several times. Hereupon I
poured some caustic spirit of sal-ammoniac (strong ammonia) on
this, in all appearance, black powder, and set it by for
digestion. This menstruum (solvent) dissolved a quantity of
luna cornua (horn silver), though some black powder remained
undissolved. The powder having been washed was, for the greater
part, dissolved by a pure acid of nitre (nitric acid), which,
by the operation, acquired volatility. This solution I
precipitated again by means of sal-ammoniac into horn silver.
Hence it follows that the blackness which the luna cornua
acquires from the sun's light, and likewise the solution of
silver poured on chalk, is _silver by reduction_. I mix