20 
ON COMBUSTION BY INVISIBLE RAYS. 
sixty-five years ago by Sir William Herschel. Permitting a sunbeam to pass through, 
a glass prism, he formed the coloured spectrum of the solar light; and, carrying a small 
thermometer through its various colours, he determined their heating power. He found 
this power to augment gradually from the violet to the red; but he also found that the 
calorific action did not terminate where the visible spectrum ended. Placing his ther¬ 
mometer in the dark space beyond the red, he proved the heating power there to be 
greater than in any part of the visible spectrum. 
Sir William Herschel concluded from his experiments, that besides those rays which, 
acting separately upon the retina, produce the sensation of colour, and the sum of 
which constitutes our ordinary sunshine, a vast outflow of perfectly invisible rays pro¬ 
ceeds from the sun, and that, measured by their heating power*, the strength or energy 
of these invisible rays is greater than that of all the visible rays taken together. 
This result was questioned by some and confirmed by others; but, like every natural 
truth that can be brought to the test of experiment, the verity of Sir William Herschel’s 
announcement was soon completely established. Forty years after the discovery of 
those invisible rays by his father, Sir John Herschel made them the subject of experi¬ 
ment. He made an arrangement which enabled him to estimate the heating power of 
the spectrum by its drying power. Wetting by a wash of alcohol, paper blackened on 
one side, he cast his spectrum on this paper, and observed the chasing away of the 
moisture by the heat of the rays. His drying paper presented to him a thermograph of 
the spectrum, and showed the heating power to extend far beyond the red. 
By the introduction of the thermo-electric pile, Melloni created a new epoch in re¬ 
searches on radiant heat. This instrument enables us to examine, with a precision un¬ 
attainable with ordinary thermometers, the distribution of heat in the solar spectrum. 
Melloni himself devoted some time to this subject. He had made the discovery that 
various substances, in the highest degree transparent to light, were eminently opaque to 
those invisible heat-rays. Pure water, for example, is a body of this kind. Only one 
substance did Melloni find to be equally pervious to the visible and the invisible rays— 
namely, transparent rock-salt. And though the researches of MM. De la Provostaye 
and Desains, together with some extremely suggestive experiments executed by Mr. 
Balfour Stewart, show conclusively that Melloni erred in supposing rock-salt to be per¬ 
fectly transparent, it must be admitted that, in this respect, the substance approaches 
very near perfection. 
Abandoning prisms of glass, which had been always employed previously, Melloni 
made use of a prism of rock-salt in his experiments on the solar spectrum. He was thus 
enabled to prove that the ultra-red rays discovered by Sir William Herschel formed an 
invisible spectrum, at least as long as the visible one. He also found the position of 
maximum radiant power to lie as far on one side the red as the green light of the 
spectrum on the other. 
Dr. Franz, of Berlin, subsequently examined the distribution of heat in the solar 
spectrum, employing for this purpose a flint-glass prism. He showed that the inaction 
of the ultra-red rays upon the retina did not altogether arise from the absorption of 
those rays in the humours of the eye ; at all events, he proved that a sensible portion of 
the invisible rays was transmitted across the eye-ball of an ox, and reached the back of 
the eye. Professor Muller, of Freiberg, afterwards examined very fully the heat of the 
solar spectrum; and representing, as Sir William Herschel also had approximately 
done, by lines of various lengths the thermal intensity at various points, he drew a curve 
which expressed the calorific action of the entire spectrum. 
At various intervals during the last ten years the speaker had occupied himself with 
the invisible radiation of the electric light; and to the distribution of heat in its spec¬ 
trum he now directed attention. The instruments made use of were the electric lamp of 
Duboscq and the linear thermo-electric pile of Melloni. The spectrum was formed by 
means of lenses and prisms of pure rock-salt. It was equal in width to the length of 
the.row of elements forming the pile, and the latter being caused to pass through its 
various colours in succession, and also to search the space right and left of the visible 
spectrum, the heat falling upon it, at every point of its march, was determined by the 
deflection of an extremely sensitive galvanometer. 
As in the case of the solar spectrum, the heat was found to augment from the violet 
to the red, while in the dark space beyond the red it rose to a maximum. The position 
of the maximum was about as distant from the extreme red in the one direction, as the 
green of the spectrum in the opposite one. 
