Note 46 — Chapter VI

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.”

Reference should also be made to a paper by Dr. Robinson, On the effects of Heat in lessening the Affinities of the Elements of Water, in the Transactions of the Royal Irish Academy, 1848, where he says that “when a platinum wire is traversed by a current gradually increased till it produces ignition, the first gleam that appears is not red, but of a colour which, when I first saw it, I compared to the ‘lavender ray’ discovered by Sir John Herschel beyond the violet, though I was surprised at seeing the tint of that most refrangible ray preceding the ray which is least so. It is quite conspicuous at about 865°; and as the mode in which it makes its appearance presents nothing abrupt or discontinuous, it seems likely that it is merely a transition from invisible rays excited at a lower temperature to ordinary light.”—p. 310.