XIX

ARTIFICIAL LIGHT AND CHEMISTRY

Some one in an early century was the first to notice that the sun’s rays tanned the skin, and this unknown individual made the initial discovery in what is now an extensive branch of science known as photo-chemistry.  The fading of dyes, the bleaching of textiles, the darkening of silver salts, the synthesis and decomposition of compounds are common examples of chemical reactions induced by light.  There are thousands of other examples of the chemical effects of light some of which have been utilized by mankind.  Others await the development of more efficient light-sources emitting greater 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.