570 THE POPULAR SCIENCE MONTHLY. 



tute about 95 per cent, of the total radiation from the white-hot plati- 

 num wire. They make up nearly 90 per cent, of the emission from a 

 brilliant electric light. You can by no means have the light of the 

 carbons without this invisible emission as an accompaniment. The 

 visible radiation is, as it were, built upon the invisible as its necessary 

 foundation. 



It is easy to illustrate the growth in intensity of these invisible 

 raj'S as the visible ones enter the radiation and augment in power. The 

 transparency of the simple gases and metalloids — of oxygen, hydro- 

 gen, 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 radiation sharply off, but allows 

 the invisible free transmission. By dissolving iodine in sulphur. Pro- 

 fessor 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. Yiy such filters it is 

 possible to detach the invisible rays from the total radiation, and to watch 

 their augmentation 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 one, the invisible radiation from 

 the same wire raised to a white heat may be five hundred 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 Syra- 

 cuse. While incompetent to produce the faintest glimmer of light, or to 

 effect 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 red- 

 ness 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 can not penetrate the carbons as it penetrates a flame. The tem- 

 perature of the flame is lowered by the admixture of a gas which con- 

 stitutes four fifths of our atmosphere, and which, while it appropriates 

 and diffuses the heat, does not aid in the combustion ; and this lower- 

 ing of the temperature by the inert atmospheric nitrogen renders 

 necessary the combustion of a greater amount of gas to produce the 



