the calorific Poivers of the different prismatic Bays. 403 



its size was considerable, yet he could not set a piece of wood 

 •Ml fire with it; whereas a far less speculuin, cf the same form, 

 made out of a more reflecting substance, would presently have 

 made it flame. He took also a large broad tile, and having 

 made one half of its surface white, and the other black, he ex- 

 posed it to the summer sun for some time, when he found that 

 while the whited part remained cool, the part that had been 

 blacked had grown very hot. For his further satisfaction he 

 left part of the tile of its native red ; and, after exposing it to 

 the sun, observed that this part grew hotter than the white, but 

 was not so hot as the black part. 



Mr. Melville supposed that, since there is no reflection of light 

 but at the surface of a medium, the greatest quantity of rays, 

 though crowded ii;to the smallest space, would not of themselves 

 produce any heat. If this be true, it follows that the portion , 

 of air which lies in the focus of the most powerful speculum is 

 not at all affected by the passage of light through it, but con- 

 tinues of the same temperature with the surrounding air : whereas, 

 if an opaque body denser than air be put into this place, it would 

 be intensely heated in an instant. Hence he observes, the at- 

 mosphere is not much warmed by the passage of the sun's light 

 through it, but ratlier by its contact with the surface of the 

 earth. This he thought furnished a simple and plausible reason 

 why it is coldest, in all climates, on the tops of high mountains; 

 they being furthest removed from the general surface of the 

 earth. On this principle we may also account for the difficulty 

 which the Abbe Nollet met with in endeavouring to fire spirit of 

 wine, oil of turpentine, olive oil, and ether ; as these fluids trans- 

 mit the rays, instead of reflecting or absorbing them in a sufficient 

 degree to produce the heat necessary for their combustion. 



It was remarked by Scheele, that "the thermometer when filled 

 with alcohol of a deep red colour, rose more rapidly when ex- 

 ))osed to the sun's rays than another filled with the same kind 

 of spirit uncolo\ired; but that the fluid rose equally in both when 

 di))pcd together in the same vessel of warm water. 



Dr. Franklin found that the hand when applied alternately to 

 the black ])art of any one's dress, and tiien to a white ])art, whilst 

 walking in the sun, would feel a great diff^^rence in their warmth. 

 He observed also that black paper exposed to the concentrated 

 rays of the sun from a lens, would take fire much sooner than 

 white paper ; and that a fluid of any kind becomes warm in 

 less time, when placed before the fire in a black vessel, than in 

 a white one, or in a bright silver tankard. But the most cu- 

 rious experiment he made was, by exposing pieces of broad-cloth 

 of various colours to the sun, by laying them upon snovv. In a 

 few hours the black, being warm'ed most by the sun, was sunk so 



C c 2 low 



