206 AXNUAL OF SCIENTIFIC DISCOVERY. 



autumn of 1861, on the possibility of sifting, by absorbents, the light 

 of a beam from its heat, that Prof. Tyndall first learned that carbon 



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was the substance which rendered Melloni's glass opaque. This fact 

 was of peculiar interest to him, for it and others seemed to extend to 

 solid bodies a law which he had detected two years previously in life 

 experiments on gases and vapors, and which showed that elementary 

 gases were highly transparent, while compound gases were all more 

 or less opaque many of them, indeed, almost perfectly opaque 

 to invisible radiant heat. The enormous differences existing between 

 elementary and compound gases, as regards their opacity to radiant 

 heat, is illustrated by the following facts : For every ray intercepted 

 in a tube four feet long, by a certain measure of air, oxygen, hydro- 

 gen, or nitrogen, transparent ammonia strikes down 7,260 rays, de- 

 fiant gas 7,'JOO, while transparent sulphurous acid destroys 8,800. 



In Prof. TyndalPs first experiments on the invisible radiation of the 

 electric light, black glass was the substance made use of. The speci- 

 mens, however, which he was able to obtain destroyed, along with the 

 visible, a considerable portion of the invisible radiation. But the dis- 

 covery of the deportment of elementary gases directed his attention 

 to other simple substances. He examined sulphur dissolved in bi- 

 sulphide of carbon, and found it almost perfectly transparent to the 

 invisible rays. He also examined the element bromine, and found 

 that, notwithstanding its dark color, it was eminently transparent to 

 the ultra-red rays. Layers of this substance, for example, which 

 entirely cut off all the light of a brilliant gas flame, transmitted its 

 invisible radiant heat with freedom. Finally, he tried a solution of 

 iodine in bisulphide of carbon, and arrived at the extraordinary result 

 that a quantity of dissolved iodine sufficiently opaque to cut off" the 

 light of the mid-day sun was, within the limits of experiment, absolutely 

 transparent to invisible radiant heat. 



This then is the substance by which the invisible rays of the electric 

 light may be almost perfectly detached from the visible ones. Con- 

 centrating by a small glass mirror, silvered in front, the rays, emitted 

 by the carbon points of the electric lamp, we obtain a convergent 

 cone of light. Interposing in the path of this concentrated beam a 

 cell containing the opaque solution of iodine, the light of the cone is 

 utterly destroyed, while its invisible ravs are scarcely., if at all, med- 



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died with. These converge to a focus, at which, though nothing can 

 be seen even in the darkest room, the following series of effects may 

 be produced : 



When a piece of black paper is placed in the focus, it is pierced by 

 the invisible rays, as if a white-hot spear had been suddenly driven 

 through it. The paper instantly blazes, without apparent contact with 

 any thing hot. 



A piece of brown paper placed at the focus soon shows a red-hot 

 burning surface, extending over a considerable space of the paper, 

 which finally bursts into flame. 



The wood of a hat-box similarly placed, is rapidly burnt through. 

 A pile of wood and shavings, on which the focus falls, is quickly ig- 

 nited, and thus a fire may be set burning by the invisible rays. 



A cigar or a pipe is immediately lighted when placed at the focus of 

 invisible rays. 



