254 Professor Tyndall [March 16, 



water, he determined the emissive powers of the substances thus 

 heated. These he found to difler greatly from each other. Thus, the 

 radiation from a coating of lampblack being called 100, that from the 

 uncoated metallic surface of his cube was only 12. He pointed out 

 the reciprocity existing between radiation and absor2)tion, proving 

 that those substances which emit heat copiously absorb it greedily. 

 His thermoscopic instrument was the well-known differential-ther- 

 mometer 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 undula- 

 tory theory of light, which was then on its trial, had been established. 



A figure still more remarkable than Leslie occupied the scientific 

 stage at the same time, namely, the vigorous, original, and practical 

 Benjamin Thompson, better known as Count Eumford, the originator 

 of the Royal Institution. Eumford traversed a great portion of the 

 ground occupied by Leslie, and obtained many of his results. As 

 regards priority of publication, he was obviously discontented with 

 the course which things had taken, and he endeavoured to place both 

 himself and Leslie in what he suj)posed to be their right relation to 

 the subject of radiant heat. The two investigators were unknown to each 

 other personally, and their diiferences never rose to scientific strife. 

 There can hardly, I think, be a doubt that each of them worked inde- 

 pendently of the other, and that where their labours overlap, the 

 honour 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 practising London physician. And here let it be 

 gratefully acknowledged that vast services to physics have been 

 rendered by physicians. The penetration of Wells is signalised 

 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 had distinctly recognised the principle of Natural 

 Selection, and that he was the first to recognise 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 revealed the cause of this beautiful phe- 

 nomenon. Wells knew that through the body of our atmosphere 

 invisible aqueous vapour is everywhere diffused. He proved that 

 grasses and other bodies on which dew was deposited were powerful 

 emitters of radiant heat ; that when nothing existed in the air to stop 



