his own apparatus, and applied it in bis own way. To produce radiating surfaces, be employed metallic cubes, which to the present hour are known as Leslie's cubes. The different faces of these cubes be coated with different substances, and, filling the cubes with boiling water, be determined the emissive powers of the substances thus heated. These be found to differ greatly from each other. Thus, the radiation from a coating of lamp-black being called 100, that from the uncoated metallic surface of his cube was only 12. He pointed out the reciprocity existing between radiation and absorption, proving that those substances which emit heat copiously absorb it greedily. His thermoscopic instrument was the well-known differential-thermometer invented by himself. In experiment Leslie was very strong, but in theory he was not so strong. His notions as to the nature of the agent whose phenomena he investigated with so much ability are confused and incorrect. Indeed, he could hardly have formed any clear notion of the physical meaning of radiation before the undulatory theory of light, which was then on its trial, bad been established.
A figure still more remarkable than Leslie occupied the scientific stage at the same time namely, the vigorous, penetrating, and practical Benjamin Thompson, better known as Count Rumford, the originator of the Royal Institution. Rumford traversed a great portion of the ground occupied by Leslie, and obtained many of his results. As regards priority of publication, be was obviously discontented with the course which things had taken, and he endeavored to place both himself and Leslie in what he supposed to be their right relation to the subject of radiant heat. The two investigators were unknown to each other personally, and their differences hardly rose to scientific strife. There can hardly, I think, be a doubt that each of them worked independently of the other, and that, where their labors overlap, the honor of discovery belongs equally to both.
The results of Leslie and Rumford were obtained in the laboratory; but the walls of a laboratory do not constitute the boundary of its results. Nature's hand specimens are always fair samples, and, if the experiments of the laboratory be only true, they will be ratified throughout the universe. The results of Leslie and Rumford were in due time carried from the cabinet of the experimenter to the open sky, by Dr. Wells, a practicing London physician. And here let it be gratefully acknowledged that vast services to physics have been rendered by physicians. The penetration of Wells is signalized among other things by the fact recorded by the late Mr. Darwin, that, forty-five years before the publication of the "Origin of Species," the London doctor bad distinctly recognized the principle of Natural Selection, and that he was the first who recognized it. But Wells is principally known to us through his "Theory of Dew," which, prompted by the experiments of Leslie and Rumford, and worked out by the most refined and conclusive observations on the part of Wells himself, first