the sulphate of quinine and afterwards
dried, an obvious extension of the spectrum is revealed. We have, in
the first instance, a portion of the violet rendered whiter and more
brilliant; but, besides this, we have the gleaming of the colour
where, in the case of unprepared paper, nothing is seen. Other
substances produce a similar effect. A substance, for example,
recently discovered by President Morton, and named by him _Thallene_,
produces a very striking elongation of the spectrum, the new light
generated being of peculiar brilliancy.

Fluor spar, and some other substances, when raised to a temperature
still under redness, emit light. During the ages which have elapsed
since their formation, this capacity of shaking the ether into visual
tremors appears to have been enjoyed by these substances. Light has
been potential within them all this time; and, as well explained by
Draper, the heat, though not itself of visual intensity, can unlock
the molecules so as to enable them to exert their long-latent power of
vibration. This deportment of fluor spar determined Stokes in his
choice of a name for his great discovery: he called this rendering
visible of the ultra-violet rays _Fluorescence_.

By means of a deeply coloured violet glass, we cut off almost the
whole of the light of our electric beam; but this glass is peculiarly
transparent to the violet and ultra-violet rays. The violet beam now
crosses a large jar filled with water, into which I pour a solution of
sulphate of quinine. Clouds, to all appearance opaque, instantly
tumble downwards. Fragments of horse-chestnut bark thrown upon the
water also send down beautiful cloud-like strife. But these are not
clouds: there is nothing precipitated here: the observed action is an
action of _molecules_, not of _particles_. The medium before you is
not a turbid medium, for when you look through it at a luminous
surface it is perfectly clear.

If we paint upon a piece of paper a flower or a bouquet with the
sulphate of quinine, and expose it to the full beam, scarcely anything
is seen. But on interposing the violet glass, the design instantly
flashes forth in strong contrast with the deep surrounding violet.
President Morton has prepared for me a most beautiful example of such
a design which, when placed in the violet light, exhibits a peculiarly
brilliant fluorescence. From the experiments of Drs. Bence Jones and
Dupre, it would seem that there is some substance in the hum