684 FRA QMENTti OF SCIKNCE. 



the visible, while 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 transformation 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." 



