370 Of the Propagation of Heat 



that the intense Heat of a large smelting furnace, such 

 as is necessary for melting the most refractory metals, 

 actually exists in the feeble flame of the smallest candle ; 

 and what may appear still more extraordinary, this in- 

 tense degree of Heat often exists in the air of the at- 

 mosphere, where no visible signs of Heat appear^ as I 

 shall presently show. 



Iron is fully red-hot by daylight at the temperature of 

 about looo*^ of Fahrenheit's scale ; brass melts at 3807°, 

 copper at 4587°, silver at 4717°, and gold at 52,37° ; and 

 nothing is more certain than that the Heat must be at 

 that intensity which corresponds to the 5237th degree 

 of Fahrenheit's scale, where gold is found to melt. But 

 very fine gold, silver, or copper wire, flatted, (such as is 

 used to cover thread to make lace,) melts instantaneously 

 on being held in the flame of a candle. It will even be 

 melted if it be held a few seconds over the flame of a 

 candle, at the distance of more than an inch from the top 

 of the flame ^ in a place where there is no appearance of 

 fire, or of anything red-hot. 



From the important information which we acquire 

 from the result of these experiments, we see how much 

 we ought to be on our guard in forming an opinion with 

 respect to the intensity of the Heat which may exist in 

 the invisible insulated particles of matter of any kind 

 that may be scattered about in a given space, or which 

 may float in any Fluid, where neither our feeling nor 

 our thermometers can possibly be sensibly affected by it. 



A thermometer can do no more than indicate the 

 mean of the different temperatures of all those bodies or 

 particles of matter which happen to come into contact with it. 

 If it be suspended in air, it will indicate the mean of 

 the temperatures of those particles of air which happen to 



