344 ^f ^'^^ Propagation of Heat 



was made to project about -^-^ of an inch below the flat 

 end of the iron. This precaution was likewise used, and 

 for a similar reason, in the preceding experiment, when 

 oil was used in the place of the mercury ; as was men- 

 tioned, though without being explained, in giving an ac- 

 count of that experiment. 



As the cake of ice, on which the mercury reposed, was 

 at that temperature precisely at which ice is disposed to 

 melt with the smallest additional quantity of Heat, if any 

 Heat had found its way downwards through the mercury 

 to the ice in this experiment, water would most un- 

 doubtedly have been formed, and this water would as 

 undoubtedly have appeared on the surface of the mer- 

 cury on taking away the iron ; but there was not the 

 smallest appearance of any ice having been melted. 



To find out whether the cake of ice was really at that 

 temperature at which it was disposed to melt with any 

 additional Heat, I thrust down the end of my finger 

 through the mercury, and touched the ice ; and this ex- 

 periment removed all my doubts, for I found that, 

 however expeditiously I performed that operation, it was 

 hardly possible for me to touch the ice without evident 

 signs of water having been produced being left behind, 

 on the clean and bright surface of the mercury, on tak- 

 ing away my finger. 



From the results of all these experimental investiga- 

 tions it appears to me that we may safely conclude that 

 water, oil, and mercury are perfect non-conductors of Heat ; 

 or, that when either of those substances takes the form 

 of a Fluid, all interchange and communication of Heat 

 among its particles, or from one of them to the other, di- 

 rectly, becomes from that moment absolutely impossible. 



That this is also the case with respect to the particles 



