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vapors under the action of the unsq'fted beam, you have in-
stantl y this fine luminous cloud precipitated.

The light of the sun also effects the decomposition of
the nitrite-of-amyl vapor. A small room in the Royal
Institution, into which the sun shone, was partially dark-
ened, the light being permitted to enter through an open
portion of the window-shutter. In the track of the beam
was placed a large plano—convex lens, which formed a fine
conVergent cone in the dust of the room behind it. The
experimental tube was filled. in the laboratory, covered
With a black cloth, and carried into the partially-dark-
ened room. On thrusting one end of the tube into the
cone of rays behind the lens, precipitation within the cone
was copious and immediate. The vapor at the distant end
of the tube was shielded by that in front; but on revers-
ing the tube, a second and similar splendid cone was pre-

Now let us pause for a moment and glance at the
ground over which we have passed. We have~defined a
Vapor as an aggregate of molecules mutually repellent,
but hindered from indefinitely retreating from each other
by an external pressure. We have defined a molecule as
an aggregate of atoms maintained in positions of equi-
librium by the equalized action of two opposing forces,
and always oscillating to and fro across those positions.
We have defined a beam of light as a train of innumerable
waves, and have illustrated their chemical action. We
have learned that it is not the magnitude or power of the
waves, so much as their periods of recurrence, that renders
them effectual as chemical agents. We have also seen
how the luminous beam is sifted by the vapor which it
decomposes, and deprived of those rays which are com-
petent to effect the decomposition. The effects, moreover,
obtained with the electric beam are also produced by the
beams of the sun.

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