202 



ON THE NATURE OF HEAT, 



but, by a patient and careful examination of the fubjefl, he 

 is convinced t}>at the efficacy of radiation is much greater than 

 he had fuppofed ; from the refalt of the experiment No, 31, 

 it appears that a very fraall part of the heat of a body cooled 

 in tl)e air, is in fadt communicated to that fluid; much the 

 greateft portion being communicated to furrounding bodies at 

 a diftance; and, in one of his former experiments, a hot body 

 was cooled, though it was placed in a torricellian vacuum. 

 Air heated only He confiders the heat which air receives by coming in con- 

 st Its furface fa6l with a hot body, to be communicated by radiation, in the 

 and by radiation. ....,,,,. ,.r 



lame manner as it is received by other bodies at a greater diU 



tance ; and he apprehends that the contiguous particle receives 

 the heat in preference for no other reafon than becaufe it is at 

 the furface of the fluid, this being the place where refledlion, 

 Tefradion, and increafe of temperature, take place ; and, 

 from thefe confiderations, he explains what has been called 

 the non-conduding power of tranfparent fluids. 

 Analogy hz- By extending the analogy of tho(e fadls which we know 



tween light and concerning the effed of polifaed furfaces on light, to the ra- 

 diations of heat alfo, the preceding fads are eafily explained. 

 The frigorific rays are refleded externally, and a large por- 

 tion of the calorific rays, which would have itfued forth through 

 a rough furface, are, in the other cafe, turned inwards by 

 refledion, 

 A drop of water The poliflied furface of a drop of water, which rolls about 

 refifts the heat at a diftance from the face of a piece of red-hot iron, enables 

 by Us'polilhr* '^ ^^ refled the calorific rays ; the water acquires little heat, 

 and evaporates flowly. 



With a lefs heat the water enters the pores of the oxide 

 upon the metal, lofes its polifh, acquires heat very rapidly, 

 and is foon evaporated. 

 It retains its po- If the metal be lefs oxidable, as, for example, a filver- 

 lifh longer in fpoon, the drop of water will fupport or refift a lower heat. 

 flfts^more. ^ ' I" f^^l it does not fo foon lofe its poiifti ; but~at a flill lower 

 heat, that is to fay, a little above boiling-water, a drop of 

 water is inftantaneoufly evaporated. 

 Exp. 34< Water Exp. 34. A clean poliftied fpoon, rendered black by hold- 

 rolling in a fpoon i,-jg j^ over the flame of a wax pandle, will receive a large 

 fniok-e, cannot ^''^^P ^^ water, which will roll about without wetting the 

 ^e maic to boil, blackened furface. This drop cannot be made to boil by- 

 holding the fpoon over the flame of a candle. When the 



fpoon 



