THE ELECTRIC LIGHT 471 



augment in power. The transparency of the elementary 

 gases and metalloids of oxygen, hydrogen, nitrogen, 

 chlorine, iodine, bromine, sulphur, phosphorus, and even 

 of carbon, for the invisible heat rays is extraordinary. 

 Dissolved in a proper vehicle, iodine cuts the visible ra- 

 diation sharply off, but allows the invisible free transmis- 

 sion. By dissolving iodine in sulphur, Professor Dewar 

 has recently added to the number of our effectual ray- 

 filters. The mixture may be made as black as pitch for 

 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 augmen- 

 tation 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. 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-ther- 

 mometer, 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 fu- 

 sion, they can raise it to a heat so white and luminous as 

 to yield, when analyzed, all the colors of the spectrum, 



1 See article "Radiation," vol. i. 



