684 FRAGMENTS OF SCTKNVE. 
the visible, wliile remaining transparent for the invisible 
rays. By such filters it is possible to detach the invisible 
rays from the total radiation, and to watch their augmenta- 
tion as the light increases. Expressing the radiation from 
a platinum wire when it first feels warm to the touch 
when, therefore, all its rays are invisible by the number 
1, the invisible radiation from the same wire raised to a 
white heat may be 500 or more.* It is not, then, by the 
diminution or tniusformation of the non-luminous emission 
that we obtain the luminous; the heat rays maintain their 
ground as the necessary antecedents and companions of 
the light rays. 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 
analyzed, all the colors of the spectrum. In this way the 
dark rays emitted by the incandescent carbons are converted 
into light rays of all colors. 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 pen- 
etrates 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 
*See article " Radiation." 
