THE ELECTEIC LIGHT. 449 



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. 1 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 im- 

 pinge 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 im- 

 pression. 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 fs 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 

 See article 'Radiation,' vol. i. 



