ess. In iron dishes a mixture of dilute alcohol and salt
was placed, and warmed so as to promote vaporization. The vapour was
ignited, and through the yellow flame thus produced the beam from the
electric lamp was sent; but a faint darkening only of the yellow band
of a projected spectrum could be obtained. A trough was then made
which, when fed with the salt and alcohol, yielded a flame ten feet
thick; but the result of sending the light through this depth of flame
was still unsatisfactory. Remembering that the direct combustion of
sodium in a Bunsen's flame produces a yellow far more intense than
that of the salt flame, and inferring that the intensity of the colour
indicated the copiousness of the incandescent vapour, I sent through
the flame from metallic sodium the beam of the electric lamp. The
success was complete; and this experiment I wish now to repeat in your
presence.[25]

Firstly then you notice, when a fragment of sodium is placed in a
platinum spoon and introduced into a Bunsen's flame, an intensely
yellow light is produced. It corresponds in refrangibility with the
yellow band of the spectrum. Like our tuning-fork, it emits waves of a
special period. When the white light from the electric lamp is sent
through that flame, you will have ocular proof that the yellow flame
intercepts the yellow of the spectrum; in other words, that it absorbs
waves of the same period as its own, thus producing, to all intents
and purposes, a dark Fraunhofer's band in the place of the yellow.

In front of the slit (at L, fig. 56) through which the beam issues is
placed a Bunsen's burner (_b_) protected by a chimney (C). This beam,
after passing through a lens, traverses the prism (P) (in the real
experiment there was a pair of prisms), is there decomposed, and forms
a vivid continuous spectrum (S S) upon the screen. Introducing a
platinum spoon with its pellet of sodium into the Bunsen's flame, the
pellet first fuses, colours the flame intensely yellow, and at length
bursts into violent combustion. At the same moment the spectrum is
furrowed by an intensely dark band (D), two inches wide and two feet
long. Introducing and withdrawing the sodium flame in rapid
succession, the sudden appearance and disappearance of the band of
darkness is shown in a most striking manner. In contrast with the
adjacent brightness this band appears absolutely black, so vigorous is
the absorption. The blackness, however, is but relative, for upon