ON COMBUSTION BY INVISIBLE BAYS. 
21 
The augmentation of temperature beyond the red in the spectrum of the electric light 
is sudden and enormous. Representing the thermal intensities by lines of proportional 
lengths, and erecting these lines as perpendiculars at the places to which they correspond, 
when we pass beyond the red these perpendiculars suddenly and greatly increase in 
length, reach a maximum, and then fall somewhat more suddenly on the opposite side 
of the maximum. When the ends of the perpendiculars are united, the curve beyond 
the red, representing the obscure radiation, rises in a steep and massive peak, which 
quite dwarfs by its magnitude the radiation of the luminous portion of the spectrum. 
Interposing suitable substances in the path of the beam, this peak may be in part cut 
away. Water, in certain thicknesses, does this very effectually. The vapour of water 
would do the same; and this fact enables us to account for the difference between the 
distribution of heat in the solar and in the electric spectrum. The comparative height 
and steepness of the ultra-red peak, in the case of the electric light, are much greater 
than in the case of the sun, as shown by the diagram of Professor Muller. No doubt 
the reason is, that the eminence corresponding to the position of maximum heat in the 
solar spectrum has been cut down by the aqueous vapour of our atmosphere. Could a 
solar spectrum be produced beyond the limits of the atmosphere, it would probably show 
as steep a mountain of invisible rays as that exhibited by the electric light, which is 
practically uninfluenced by atmospheric absorption. 
Having thus demonstrated that a powerful flux of dark rays accompanies the bright 
ones of the electric light, the question arises, “Can we not detach the former, and ex¬ 
periment on them alone ? ” 
One way of doing this would be to cut off the luminous portion of the decomposed 
beam by an opaque screen, allowing the non-luminous portion to pass by its edge. We 
might then operate at pleasure upon the latter:—reflect it, refract it, concentrate it. 
This, in fact, was done by Sir William Herschel, but a quantity of heat could not thus 
be obtained sufficient to produce the results intended to be exhibited before the con¬ 
clusion of the discourse. Another plan consists in permitting the total radiation to pass 
through some substance transparent to the heat rays, but opaque to the light rays. 
Melloni discovered that lampblack, and also a kind of black glass, while perfectly opaque 
to light, transmitted a considerable quantity of radiant heat. In the ‘ Lectures on Heat,’ 
given at the Royal Institution in 1862, and since made public, experiments with these 
bodies are described. It was while conversing with his friend Mr. Warren De la Rue, in 
the autumn of 1861, on the possibility of sifting, by absorbents, the light of a beam 
from its heat, that the speaker first learned that carbon was the substance which rendered 
Melloni’s glass opaque. This fact w r as 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 
his experiments on gases and vapours, 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. 
In the speaker’s first experiments on the invisible radiation of the electric light, black 
glass was the substance made use of. The specimens, however, which he was able to 
obtain destroyed, along with the visible, a considerable portion of the invisible radiation.* 
But the discovery of the deportment of elementary gases directed his attention to other 
simple substances. He examined sulphur dissolved in bisulphide of carbon, and found 
it almost perfectly transparent to the invisible rays. He also examined the element 
bromine, and found that, notwithstanding its dark colour, 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 extra¬ 
ordinary 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. Concentrating 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 
* The glass in thin layers had a greenish hue; I have since found black glass more dia¬ 
thermic.—J. T. 
