SOLAR CHEMISTRY INVOLVED IN THIS EXPLANATION
FOUCAULT'S EXPERIMENT
PRINCIPLES OF ABSORPTION
ANALOGY OF SOUND AND LIGHT
EXPERIMENTAL DEMONSTRATION OF THIS ANALOGY
RECENT APPLICATIONS OF THE SPECTROSCOPE
SUMMARY AND CONCLUSION.

We have employed as our source of light in these lectures the ends of
two rods of coke rendered incandescent by electricity. Coke is
particularly suitable for this purpose, because it can bear intense
heat without fusion or vaporization. It is also black, which helps the
light; for, other circumstances being equal, as shown experimentally
by Professor Balfour Stewart, the blacker the body the brighter will
be its light when incandescent. Still, refractory as carbon is, if we
closely examined our voltaic arc, or stream of light between the
carbon-points, we should find there incandescent carbon-vapour. And if
we could detach the light of this vapour from the more dazzling light
of the solid points, we should find its spectrum not only less
brilliant, but of a totally different character from the spectra that
we have already seen. Instead of being an unbroken succession of
colours from red to violet, the carbon-vapour would yield a few bands
of colour with spaces of darkness between them.

What is true of the carbon is true in a still more striking degree of
the metals, the most refractory of which can be fused, boiled, and
reduced to vapour by the electric current. From the incandescent
vapour the light, as a general rule, flashes in groups of rays of
definite degrees of refrangibility, spaces existing between group and
group, which are unfilled by rays of any kind. But the contemplation
of the facts will render this subject more intelligible than words can
make it. Within the camera is now placed a cylinder of carbon hollowed
out at the top; in the hollow is placed a fragment of the metal
thallium. Down upon this we bring the upper carbon-point, and then
separate the one from the other. A stream of incandescent
thallium-vapour passes between them, the magnified image of which is
now seen upon the screen. It is of a beautiful green colour. What is
the meaning of that green? We answer the question by subjecting the
light to prismatic analysis. Sent through the prism, its spectrum is
seen to consist of a single refracted band. Light of one degree of
refrangibility--that corresponding to this particular green--is
emitted by the thallium-vapour.

We will now remove the thallium and put a bit of