26o Of the Propagation of Heat 



I have shewn In another place, and I believe I may 

 venture to say I have proved,* that Heat is actually prop- 

 agated in air in the same manner I here suppose it to 

 be propagated in water, and if the conducting powers of 

 both these fluids are found to be impaired by the same 

 means, it affords very strong grounds to conclude that 

 they both conduct Heat in the same manner ; but this 

 has been found to be actually the case. 



Eider-down, which cannot affect the specific qualities 

 of either of those fluids, and which certainly does no more 

 when mixed with them than merely to obstruct and em- 

 barrass their internal motions, has been found to retard 

 very much the propagation of Heat in both of them : 

 on comparing these experiments with those I formerly 

 made on the conducting power of air, it will even be 

 found that the conducting power of water is nearly, if 

 not quite, as much impaired by a mixture of eider-down 

 as that of air. 



In the course of my experiments on the various sub- 

 stances used in forming artificial clothing for confining 

 Heat, I found that the thickness of a stratum of air, 

 which served as a barrier to Heat, remaining the same, 

 the passage of Heat through it was sometimes rendered 

 more difficult by increasing the quantity of the light 

 substance which was mixed with it to obstruct its internal 

 motion. 



To see if similar effects would be produced by the 

 same means when Heat is made to pass through water, 

 I repeated the experiments with eider-down, reducing the 

 quantity of it mixed with the water to 48 grains, or one 

 quarter of the quantity used in the experiments No. 1 1 

 and No. 12. 



* See Philosophical Transactions, 1792. 



