It is thus more closely analogous to what is observed
in the element phosphorus itself, which owes its name (meaning
"light-bearer") to the fact that when exposed to the air at ordinary
temperatures it glows in the dark, in consequence of its becoming
slowly combined with oxygen.

At one time it was believed that the presence of oxygen was not
necessary to the exhibition of phosphorescence in organic forms, but
it has now been placed beyond doubt that this is a mistake. Oxygen has
been proved to be indispensable, and hence we see a reason for the
luminous organs in the glow-worm being so intimately connected, as
above mentioned, with the air-tubes that ramify through the insect.

This fact of itself might be taken as a strong indication of the
chemical nature of the process to which phosphorescence is due. But
the problem has been made the subject of further investigations which
have thrown more light upon it. It was long known that there were
various inorganic bodies besides phosphorus which emitted a
phosphorescent light in the dark, at least after being exposed to the
rays of the sun; but it was not till quite recently that any organic
compound was known to phosphoresce at ordinary temperatures.

This discovery was made by a Polish chemist, named Bronislaus
Radziszewski, who followed it up with a long series of experiments on
the phosphorescence of organic compounds, by which he was able to
determine the conditions under which that phenomenon was exhibited. In
all the substances investigated by him in which phosphorescence was
introduced he found that three conditions were essential to its
production: (1) that oxygen should be present; (2) that there should
be an alkaline reaction in the phosphorescing mixture--that is, a
reaction such as is produced on acids and vegetable coloring matters
by potash, soda, and the other alkalies; and (3) that some kind of
chemical action should take place.

He found, moreover, that among the organic compounds that could be
made to phosphoresce under these conditions were nearly all the fixed
and ethereal oils. With reference to the phosphorescence of animals,
this observation is important, for it has been shown in a great many
cases that a fatty substance forms the main constituent in their
luminous organs. This has long been known to be the case in the
luminous insects belonging to the Lampyridae and Elateridae, as well as
in the luminous centipedes; and the researches of Pance