e
tube, copious precipitation occurred there also.

Solar light also effects the decomposition of the nitrite-of-amyl
vapour. On October 10, 1868, I partially darkened a small room in the
Royal Institution, into which the sun shone, permitting the light 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 darkened 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 vapour at the distant end of the
tube was in part shielded by that in front, and was also more feebly
acted on through the divergence of the rays. On reversing the tube, a
second and similar cone was precipitated.

Physical Considerations.

I sought to determine the particular portion of the light which
produced the foregoing effects. When, previous to entering the
experimental tube, the beam was caused to pass through a red glass,
the effect was greatly weakened, but not extinguished. This was also
the case with various samples of yellow glass. A blue glass being
introduced before the removal of the yellow or the red, on taking the
latter away prompt precipitation occurred along the track of the blue
beam. Hence, in this case, the more refrangible rays are the most
chemically active. The colour of the liquid nitrite of amyl indicates
that this must be the case; it is a feeble but distinct yellow: in
other words, the yellow portion of the beam is most freely
transmitted. It is not, however, the transmitted portion of any beam
which produces chemical action, but the absorbed portion. Blue, as
the complementary colour to yellow, is here absorbed, and hence the
more energetic action of the blue rays.

This reasoning, however, assumes that the same rays are absorbed by
the liquid and its vapour. The assumption is worth testing. A
solution of the yellow chromate of potash, the colour of which may be
made almost, if not altogether, identical with that of the liquid
nitrite of amyl, was found far more effective in stopping the chemical
rays than either the red or the yellow glass. But of all substances
the liquid nitrite itself is most potent in arresting the rays which
act upon its vapour. A layer one-eighth of an