448 Professor Tyndall [May 3, 10, and 17, . 



latter. ITe pictures his frigorific rays as procliicecl by vibrations 

 of a special kind. In Pictet's celebrated experiment of conjugate 

 mirrors, and in many other experiments, chilling by a cold body 

 showed itself to be so exactly analogous to heating by a warm one, 

 that Eumford never could shake from his mind the notion of rays of 

 cold. The fall of the thermometer in one focus when a lump of ice 

 was placed in the other, was in his view caused by a positive emission 

 of cold rays from the ice, and not to its absorption of the heat radiated 

 against it by the thermometer. These frigorific rays, he says, were 

 suspected by Bacon. Their existence was actually established by 

 the academicians of Florence, but these learned gentlemen were so 

 " blinded by their prejudices respecting the nature of heat, that they 

 did not believe the report of their own eyes." 



Eumford indulges in various untenable speculations and erroneous 

 notions regarding the part played by clothing, by the blackness of 

 the negro's skin, and by the oiled surface of the Hottentot. We are, 

 he contends, kept warm by our clotliing, not so much by confining 

 our heat as by keeping off the frigorific rays which tend to cool us. 

 He reverts to the respective cases of a black and a white man, and 

 describes an experiment which elucidates his views. He covered two 

 of his vessels with goklbcater's skin, and painted one of them l)lack 

 wdth Indian ink, leaving the other of its natural white colour. Filling 

 both vessels witli hot water, he left tliem to cool in the air of a large, 

 quiet room. The vessel covered with the black skin represented a 

 nef^ro, the other vessel a white man ; and the result was that while 

 the black required only 23.V minutes to cool, the white man required 

 28 minutes. The practical issue of the experiment is thus stated : — 

 " All I will venture to say on the subject is, that were I called to 

 inhabit a very hot country, nothing should prevent me from making 

 the experiment of blackening my skin, or, at least, of wearing a black 

 shirt, in the shade, and especially at night, in order to find out if, by 

 those means, I could not contrive to make myself more comfortable." 



There w^as at times a headstrong element, if I may use the term, 

 in Eumford's scientific reasoning. He here overlooks the fact that in 

 a former experiment be found scarcely an appreciable difference 

 between white and black as regards their powers of cooling. He also 

 forgets the possible influence of a second coating, which his former 

 experiments had revealed. As regards the negro and the white man, 

 Eumford's first experiment illustrated the case more correctly than 

 any subsequent ones. There are, moreover, transparent substances 

 which, used as varnishes, would not have impaired the whiteness of 

 the goldbeater's skin, but w^hich would have hastened the cooling 

 quite as much as the Indian ink. 



Those who are acquainted with Sir John Leslie's experiments on 

 radiant heat will not fail to notice that he and Eumford travelled 

 over common ground. With a view of setting this matter right 

 Eumford wrote a paper entitled " Historical Eeview of Experiments 

 on the Subject of Heat," in which he shows that his experiments 



