OY RADIATION. 457 



his own apparatus, and applied it in bis own way. To produce radiat- 

 ing surfaces, be employed metallic cubes, wbicb to tbe present bour 

 are known as Leslie's cubes. Tbe different faces of tbese cubes be 

 coated witb different sixbstances, and, filling tbe cubes witb boiling 

 water, be determined tbe emissive powers of tbe substances tbus 

 heated. Tbese be found to differ greatly from each other. Thus, tbe 

 radiation from a coating of lamp-black being called 100, tbat 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 tbermoscopic instrument was the well-known differential-thermom- 

 eter 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 witb so much ability are con- 

 fused and incorrect. Indeed, he could hardly have formed any clear 

 notion of tbe 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 prac- 

 tical Benjamin Thompson, better known as Count Rumford, tbe origi- 

 nator of tbe 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. Tbei - e can hardly, I think, be a doubt that each of them 

 worked independently of tbe other, and that, where their labors over- 

 lap, the honor of discovery belongs equally to botb. 



The results of Leslie and Rumford were obtained in the labora- 

 tory ; but tbe 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. Tbe 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 ren- 

 dered by physicians. Tbe 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 Lon- 

 don doctor bad distinctly recognized tbe 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 tbe 

 experiments of Leslie and Rumford, and worked out by the most re- 

 fined and conclusive observations on tbe part of Wells himself, first 



