THE SUN’S ENERGY—DANIELS 247 
is used to vaporize fresh water, but the cost of such stills is very 
high. Premium prices can be paid in some areas for domestic and 
household water and for drinking water for animals. Here the solar 
distillation of salt water is particularly attractive. Egypt, Asia, 
Australia, and parts of the United States have significant regions for 
practical tests. 
Solar-distilled water cannot easily compete with irrigation water 
because the investment costs of the necessarily large areas of water- 
containing vessels and glass or plastic are so high. New cheap, 
weather-resistant plastics are needed which are impervious to water 
vapor, but are wetted by liquid water and able to withstand years 
of bright sunlight. Also needed is a solar-operated circulating fan 
which will circulate air where electrical power is not available. 
PHOTOCHEMISTRY 
The long-range hope for the direct utilization of solar energy lies 
in photochemistry and photoelectricity. The research goal in photo- 
chemistry is to find a suitable reaction which can be produced by 
sunlight with the absorption of energy, and then allowed to reverse 
itself at will in the dark with the evolution of energy. Many endo- 
thermic photochemical reactions reverse themselves so rapidly that 
they merely convert light energy into heat and are not suitable for 
storing energy. ‘There is a chance that some of the photoproducts 
will give up electrons to a wire and thus produce an electric current. 
Most photochemical reactions are spontaneous reactions in which the 
light merely accelerates the reaction rate, and the reaction does not 
reverse itself in the dark. Again, many of the promising endothermic 
reactions respond only to ultraviolet light, which does not exist in 
sunlight to an appreciable extent. The photochemical challenge is a 
difficult one. 
The fact that photosynthesis exists and carbon dioxide and water 
combine photochemically in the presence of chlorophyll in the growing 
plant gives encouragement to those who are trying to use the sun 
photochemically. The end products of photosynthesis are carbohy- 
drates and other organic materials which, on combustion, will give 
back the carbon dioxide and water and release the stored energy. ‘The 
first chemical step in photosynthesis is the release of hydrogen atoms. 
In trying to copy and improve on nature, it may be easier to store 
the solar energy in hydrogen rather than in carbohydrates and other 
products produced by further reduction of the carbon dioxide. 
Although hydrogen cannot be used for food, it can readily be used 
for fuel. 
Photosynthesis is a remarkable process which we are just beginning 
to understand. In the laboratory under special conditions it can be 
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