sodium is greenish when newly cut, and at sixty
or seventy degrees centigrade is just as intensely
luminous as phosphorus. Each substance has
its own light, though the typical light of
phosphorescence is the greenish yellow of the
glow-worm. Some marbles and amber give a golden
yellow shine; some specimens of fluor-spar,
arseniate of lime and chloride of calcium are
greenish; other fluor-spars are blue violet;
chlorophane is green; the shine of the Oriental
garnet is reddish; harmotome or crosstone
(zeolite) is green-yellow; dolomite, a white
marble or magnesian carbonate of lime,
aragonite, and some diamonds give a white light;
oxide of zinc is blue, and copper a green-yellow,
like a glow-worm.

Some of the gases are phosphorescent. If
rarefied oxygen be put into a chain of glass
globes, and a stream of electricity passed
through, all the globes become illuminated if
the stream is suddenly cut off. Sulphurous acid
gas is also a light-bearer; and mercury can be
played off with marvellous effect of fiery mimic
rain and softly-falling glowing snow, when acted
on by the atmosphere in an exhausted receiver.

There are many accounts of luminous rain and
snow and fog. M. de Saussure, travelling on
the summit of the Breven in the midst of a storm,
felt a strange creeping sensation in his fingers
when he raised his hand, and in a short time
saw that the rain was luminous, and that an
electric spark was drawn from a gold button in
his companion's hat. On the 25th of January,
1822, M. de Thielaw, on his way to Freyburg
during a heavy fall of snow, saw that the
branches of the trees glowed with a bluish light,
and on the same day the Freyburg miners noticed
that a shower of sleet which fell there was
luminous when it struck the earth. On the
3rd of June, 1731, one Hallai, a priest near
Constance, saw a rain which glowed like red-hot
liquid metal. This was during a thunder-storm:
and in 1761 Bergman wrote to the Royal Society
of London, concerning a luminous rain which
sparkled as it fell, and covered the earth with
waves of fire. On the 3rd of May, 1768, M.
Pasumot was overtaken by a violent storm, when
on an open plain near Arnay-le-Duc; when he
shook off the rain which had collected on the
brim of his hat, it was luminous and sparkled as
it fell. There are many records of luminous
mists. The luminous fog of 1783, the year of
the great Calabrian earthquake, is a well-known
historical fact. It was a dry fog which spread
from the North of Africa up to Sweden, passing
over North America too, which rose higher than
the highest mountains, and was dispersed by
neither wind nor rain. It was so luminous that
things could be plainly seen at six hundred
yards' distance at night, giving as much light as
the moon when behind a cloud; it had an evil
smell; and in the same year came the disastrous
earthquake of Calabria, and many of the most
remarkable eruptions of Mount Hecla. There
was another luminous fog in 1831, when whole
nights were so light that the smallest print
could be read at midnight, in Italy and the north
of Germany; and again in 1859, reported to M.
Elie de Beaumont by M. Wartmann of Geneva,
and which was so bright, he said, that he could
distinguish things on his table. Again, one in
1861, just before the great comet which came
so unexpectedly: the fog was in the day and the
comet appeared at night. Had we passed through
its tail unawares? Luminous zones of cloud
have been often noticed. Beccaria reports one at
Turin, which cast such a strong reddish glare
that ordinary print could be read by it; and
General Sabine saw a permanent luminous cloud
—a cloud by day, but a pillar of fire by night—
resting on the top of one of the mountains round
Loch Scavig in the Isle of Skye. It was not only
self-illuminated at night, but also gave out
frequent jets of phosphoric light, which was not the
Aurora Borealis. In July, 1797, a shining cloud
first red and then blue, was observed during a
storm; though these luminous zones are more
generally observed in winter between successive
falls of snow. Of the same class of phenomena is
that faint diffused light which Arago notices as to
be seen in autumn and winter, even in cloudy,
moonless, starless nights, and with no snow on
the ground. There is always a little light in
the atmosphere, a phosphorescence gathered
from the sun during the day, which perhaps
accounts for the saying, "the darkest hour is
the hour before dawn;" as that is the moment of
longest exposure, and consequently of greatest
weakness. This theory has lately received a
strange confirmation in that curious experiment
of "bottling up light." Card-board steeped
in a solution of tartaric acid or a salt of
uranium, was rolled into a cylinder and put into
a tin tube, opened at the end, so as to line it.
The mouth of the tube was then held up to
receive the full rays of the sun: after a quarter
of an hour, it was hermetically closed, and not
opened until many weeks after. Some of the
tubes experimented on were opened a week,
some two, some a month, some several mouths
after; but all, when placed mouth downward
on to prepared photographic paper, left a
distinct impression of the orifice: those which
had been sealed up the longest gave the weakest;
those which had been sealed up the shortest
time, the strongest; but all gave a clear and
complete impression of the orifice, like any other
photograph taken by the light.

Water-spouts are luminous at night; and a
luminous meteoric dust is on record as having
fallen during the great eruption of Vesuvius
in 1794, when a shower of fine dust gave out a
pale phosphorescent light, like that of countless
glow-worms in the air. Shooting-stars often
leave streams of light behind them; and
Admiral Krusenstern saw an aërolite leave a
phosphoric band of light behind it, which lasted a
whole hour. General Sabine and Captain Ross
once sailed into an immense belt of light on the
Greenland seas, about four hundred and fifty
yards broad, which lighted up the ship like
noonday—a belt that was sailed into and sailed
out of, and remained for long like an arc of
light between the sea and sky; and Loch Scavig,