368 Of the Propagation of Heat 



the generated water to steam, before it has time to freeze 

 again ; or by supposing that this is effected by intense 

 Heat generated by light absorbed by small projecting 

 points of the ice ? As ice is a very bad conductor of 

 Heat, that circumstance renders it more likely that the 

 event in question should actually take place in either of 

 these ways. 



If the metals were very bad conductors of Heat, in- 

 stead of being very good conductors of it, I think it 

 more than probable that even they would be found to 

 evaporate when exposed to the action of the direct rays 

 of the sun ; and perhaps also in situations in which such 

 an event would appear still more extraordinary. 



MERCURY has been actually found to evaporate under the 

 mean temperature of the atmosphere ! What a striking 

 proof is this that fluid mercury is a non-conductor of 

 Heat, and also, that very intense Heat may be gener- 

 ated, or exist where it would not naturally be expected 

 to be found ! And does not the evaporation of water 

 under the mean temperature of the atmosphere afford 

 another proof of this last fact ? 



That the most intense heat is often excited in very 

 small particles of solid bodies dispersed about in the 

 midst Oi~ masses of cold liquids is not to be doubted. 

 It is well known what an intense Heat the rays of the 

 sun are capable of exciting ; and it seems to be highly 

 probable that Heat actually excited by them is always 

 the same, that is to say, intense in the extreme : but when 

 the rays are few, and when circumstances are not favour- 

 able to the accumulation of the Heat they generate, it is 

 often so soon dispersed that it escapes the cognizance 

 of our senses and of our instruments, and sometimes 

 leaves no visible traces of its existence behind it. 



