. When detached and concentrated, these powerful heat
rays can produce all the effects ascribed to the mirrors of Archimedes
at the siege of Syracuse. While incompetent to produce the faintest
glimmer of light, or to affect the most delicate air-thermometer, they
will inflame paper, burn up wood, and even ignite combustible metals.
When they impinge upon a metal refractory enough to bear their shock
without fusion, they can raise it to a heat so white and luminous as
to yield, when analysed, all the colours of the spectrum. In this way
the dark rays emitted by the incandescent carbons are converted into
light rays of all colours. Still, so powerless are these invisible
rays to excite vision, that the eye has been placed at a focus
competent to raise platinum foil to bright redness, without
experiencing any visual impression. Light for light, no doubt, the
amount of heat imparted by the incandescent carbons to the air is far
less than that imparted by gas flames. It is less, because of the
smaller size of the carbons, and of the comparative smallness of the
quantity of fuel consumed in a given time. It is also less because
the air cannot penetrate the carbons as it penetrates a flame. The
temperature of the flame is lowered by the admixture of a gas which
constitutes four-fifths of our atmosphere, and which, while it
appropriates and diffuses the heat, does not aid in the combustion;
and this lowering of the temperature by the inert atmospheric
nitrogen, renders necessary the combustion of a greater amount of gas
to produce the necessary light. In fact, though the statement may
appear paradoxical, it is entirely because of its enormous actual
temperature that the electric light seems so cool. It is this
temperature that renders the proportion of luminous to non-luminous
heat greater in the electric light than in our brightest flames. The
electric light, moreover, requires no air to sustain it. It glows in
the most perfect air vacuum. Its light and heat are therefore not
purchased at the expense of the vitalising constituent of the
atmosphere.

Two orders of minds have been implicated in the development of this
subject; first, the investigator and discoverer, whose object is,
purely scientific, and who cares little for practical ends; secondly,
the practical mechanician, whose object is mainly industrial. It
would be easy, and probably in many cases true, to say that the one
wants to gain knowledge, while the