“Thus, the colouring matters of the coloured glasses, while they so powerfully affect the relations of quantity which the different rays of ordinary light bear to each other, exercise no elective action on the concomitant calorific rays. This curious phenomenon is the more remarkable as the colouring matters absorb almost always a very considerable portion of the heat naturally transmitted by the glass. The following are, in fact, the calorific transmissions of the seven coloured glasses referred to; the transmission of the common glass being represented by 100; red glass, 82·5; orange, 72·5; yellow, 55; bluish-green, 57·5; blue, 52·5; indigo, 30; violet, 85. The quantity of heat absorbed through the action of the colouring substances is, therefore, 17·5 in the red glass, 27·5 in the orange, 45 in the yellow, 42·5 in the green, 47·5 in the blue, 70 in the indigo, and 15 in the violet. Now, as these absorptions extinguish a proportional part of each of the rays which constitute the calorific stream transmitted by common glass, they may be compared, as we said before, with the absorbent action exercised on light by matters more or less deeply brown or dark, when they are immersed in water, or some other colourless liquid which dissolves, but does not affect them chemically.”—Annales de Chimie et de Physique, tom. xl. p. 382.
Guided by these principles, the author selected the glass employed in glazing the Royal Palm-House, at Kew Botanical Gardens, where it was desired to obstruct the passage of those rays which have a particular scorching influence. Of this glass a description was given at the meeting of the British Association at Oxford, which appears in the Transactions for that year. The result has been all that could be desired—not a single instance of scorching having occurred during the three years which have elapsed.
[45] In the Philosophical Transactions, vol. xc., the following papers, by Sir William Herschel, may be consulted:—
Investigation of the powers of the prismatic colours to heat and illuminate objects; with remarks that prove the different refrangibility of radiant heat. To which is added, an inquiry into the method of viewing the sun advantageously, with telescopes of large apertures and high magnifying powers, p. 255. Experiments on the refrangibility of the invisible rays of the sun, p. 284. Experiments on the solar and on the terrestrial rays that occasion heat; with a comparative view of the laws to which light and heat, or rather the rays which occasion them, are subject; in order to determine whether they are the same or different, pp. 293, 437.
In connection with this inquiry, Sir William Herschel remarks, that since a red glass stops no less than 692 out of 1,000 such rays as are of the refrangibility of red light, we have a direct and simple proof, in the case of the red glass, that the rays of light are transmitted, while those of heat are stopped, and that thus they have nothing in common but a certain equal degree of refrangibility, which by the power of the glass must occasion them to be thrown together into the place which is pointed out to us by the visibility of the rays of light.
On the same subject, a Memoir, by Sir Henry Englefield, in the Journal of the Royal Institution for 1802, p. 202, may be consulted; and Researches on Light, by the Author.
[46] Dr. Draper, On the production of light by heat, in the Phil. Mag. for 1847.
Sir Isaac Newton fixed the temperature at which bodies become self-luminous at 635°; Sir Humphry Davy at 812°; Mr. Wedgewood at 947°; and Mr. Daniell at 980°; whilst Dr. Draper from his experiments gives 977°; and Dr. Robinson 865°.
In a review of the above paper by Melloni, entitled Researches on the Radiations of Incandescent Bodies, and on the Elementary Colours of the Solar Spectrum, translated for Silliman’s Journal for August, 1847, he remarks:—
“I say that they conduct, as do others heretofore known on light and radiant heat, to a perfect analogy between the general laws which govern these two great agents of nature. I will add that I regard the theory of their identity as the only one admissible by the rules of philosophy; and that I consider myself obliged to adopt it, until it shall have been proved to me that there is a necessity of having recourse to two different principles, for the explanation of a series of phenomena which at present appear to belong to a solitary agent.”