284 Of the Propagation of Heat 



been called), or by the calorific rays from the hot water ; 

 then, in that case, as some of these rays must probably 

 have been reflected downwards at the surface of the wa- 

 ter in attempting to pass upwards into the air, I thought 

 that by preventing this part of them from reaching the 

 ice, which I endeavoured to do by causing them to be 

 absorbed by a light black body (a circular piece of deal 

 board covered over with black silk), which I caused to 

 swim on the surface of the water, their effects in melting 

 the ice might perhaps be sensibly diminished. Had this 

 really been the case, it would certainly have afforded 

 strong grounds to suspect that these rays were in fact the 

 cause of the appearances in question ; but on making 

 the experiment with the greatest care, I could not per- 

 ceive that the covering of the surface of the hot water 

 with a black body produced any difference whatever in 

 the result of the experiment as it was first made (experi- 

 ment No. 17), or when this black covering was not 

 used. 



After some meditation on the subject, it occurred to 

 me that this melting of the ice at its upper surface could 

 be accounted for in a manner which appeared to me to 

 be perfectly satisfactory ; without supposing either that 

 water is a conductor of Heat or that the effect in ques- 

 tion was produced by calorific rays. 



Though it is one of the most general laws of nature 

 with which we are acquainted, that all bodies, solids as 

 well as fluids, are condensed by cold, yet in regard to 

 water there appears to be a very remarkable exception to 

 this law. Water, like all other known bodies, is indeed 

 condensed by cold at every degree of temperature which 

 is considerably higher than that of freezing, but its con- 

 densation, on parting with Heat, does not go on till it 



