_Do the Sun's Rays put out the Fire_ (Vol. vii., p. 285.).--It is known
that solar light contains three distinct kinds of rays, which, when
decomposed by a prism, form as many spectra, varying in properties as well
as in position, viz. luminous, heating or calorific, and chemical or
actinic rays.
The greater part of the rays of heat are even less refrangible than the
least refrangible rays of light, while the chemical rays are more
refrangible than either. The latter are so called from their power of
inducing many chemical changes, such as the decomposition of water by
chlorine, and the reactions upon which photographic processes depend.
The relative quantities of these several kinds of rays in sun-light varies
with the time of day, the season, and the latitude of any spot. In general,
where the luminous and heating rays are most abundant, the proportion of
chemical rays is least; and, in fact, the two seem antagonistic to each
other. Thus, near the equator, the luminous and calorific rays being most
powerful, the chemical are feeble, as is shown by the length of time
required for the production of photographic pictures. Hence, also, June and
July are the worst months for the practice of photography, and better
results are obtained before noon than after.
It is precisely for a similar reason that the combustion of an ordinary
fire, being strictly a chemical change, is retarded whenever the sun's
heating and luminous rays are most powerful, as during bright {440}
sunshine, and that observe our fires to burn more briskly in summer than
winter; in fact, that apparently "the sun's rays put out the fire."