introduced into a wide glass tube, and let a concentrated beam of solar
light be sent through the tube along its axis. Prior to the entry of the
beam, the vapor is as invisible as the purest air. When the light
enters, a bright cloud is immediately precipitated on the beam. This is
entirely due to the waves of light, which wreck the nitrite of amyl
molecules, the products of decomposition forming innumerable liquid
particles which constitute the cloud. Many other gases and vapors are
acted upon in a similar manner. Now the waves that produce this
decomposition are by no means the most powerful of those emitted by the
sun. It is, for example, possible to gather up the ultra-red waves into
a concentrated beam, and to send it through the vapor, like the beam of
light. But, though possessing vastly greater energy than the light
waves, they fail to produce decomposition. Hence the justification of
the statement already made, that a suitable relation must subsist
between the molecules and the waves of ether to render the latter
effectual.

A very impressive illustration of the decomposing power of the waves of
light is here purposely chosen; but the processes of photography
illustrate the same principle. The photographer, without fear,
illuminates his developing room with light transmitted through red or
yellow glass; but he dares not use blue glass, for blue light would
decompose his chemicals. And yet the waves of red light, measured by the
amount of energy which they carry, are immensely more powerful than the
waves of blue. The blue rays are usually called chemical rays--a
misleading term; for, as Draper and others have taught us, the rays that
produce the grandest chemical effects in nature, by decomposing the
carbonic acid and water which form the nutriment of plants, are not the
blue ones. In regard, however, to the salts of silver, and many other
compounds, the blue rays are the most effectual. How is it then that