in lighting for photography and ignite it in a
Bunsen flame. If it is held carefully while burning, a ribbon of ash
(magnesia) will be obtained intact. Placing this in the faintly luminous
flame, he will be surprised at the brilliance of its incandescence when
it has become heated. The simple experiment indicates the possibilities
of light-production in this direction. Naturally, metals of high
melting-point such as platinum were tried and a network of platinum
wire, in reality a platinum mantle, came into practical use in about
1880. The town of Nantes was lighted by gas-burners using these
platinum-gauze mantles, but the mantles were unsuccessful owing to their
rapid deterioration. This line of experimentation finally bore fruit of
immense value for from it the gas-mantle evolved.

A group of so-called "rare-earths," among which are zirconia, thoria,
ceria, erbia, and yttria (these are oxides of zirconium, etc.) possess a
number of interesting chemical properties some of which have been
utilized to advantage in the development of modern artificial light.
They are white or yellowish-white oxides of a highly refractory
character found in certain rare minerals. Most of them are very
brilliant when heated to a high temperature. This latter feature is
easily explained if the nature of light and the radiating properties of
substances are considered. Suppose pieces of different substances, for
example, glass and lime, are heated in a Bunsen flame to the same
temperature which is sufficiently great to cause both of them to glow.
Notwithstanding the identical conditions of heating, the glass will be
only faintly luminous, while the piece of lime will glow brilliantly.
The former is a poor radiator; furthermore, the lime radiates a
relatively greater percentage of its total energy in the form of
luminous energy.

The latter point will become clearer if the reader will refresh his
memory regarding the nature of light. Any luminous source such as the
sun, a candle flame, or an incandescent lamp is sending forth
electromagnetic waves not unlike those used in wireless telegraphy
excepting that they are of much shorter wave-length. The eye is capable
of recording some of these waves as light just as a receiving station is
tuned to record a range of wave-lengths of electromagnetic energy. The
electromagnetic waves sent forth by a light-source like the sun are not
all visible, that is, all of them do not arouse a sensation of light.