138 



PHILOSOPHICAL TRANSACTIONS. 



The bulb of the thermometer being sur- 

 rounded by Eider down. 



[anno 1792. 



Suspecting that this might ari.se from the volumes or solid contents of the 

 substances being different (though their weights were the same,) arising from 

 the difference of their specific gravities; and as it was not easy to determine the 

 specific gravities of these substances with accuracy, in order to see how far any 

 known difference in the volume or quantity of the same substance, confined al- 

 ways in the same space, would add to, or diminish, the time of cooling, or the 

 apparent warmth of the covering, I made the three following experiments. In 

 the first, the bulb of the thermometer was surrounded by 16 grains of Eider 

 down; in the second by 32 grains; and 

 in the third by 64 grains; and in all 

 these experiments the substance was made 

 to occupy exactly the same space, viz. 

 the whole internal capacity of the glass globe, 

 in the centre of which the bulb of the ther- 

 mometer was placed; consequently the thick- 

 ness of the covering of the thermometer 

 remained the same, while its density was varied 

 in proportion to the numbers 1, 2, and 4. 

 The results of these experiments were as an- 

 nexed : 



Finding, by the last experiments, that the density of the covering added so 

 considerably to its warmth, its thickness remaining the same, I was now de- 

 sirous of discovering how far its internal structure contributed to render it more 

 or less pervious to heat, its thickness and quantity of matter remaining the 

 same. By internal structure, I mean the disposition of the parts of the sub- 

 stance which forms the covering; thus they may be extremely divided, or very 

 fine, as raw silk as spun by the worms, and they may be equally distributed 

 through the whole space they occupy; or they may be coarser, or in larger 

 masses, with larger interstices, as the ravelings of cloth or cuttings of threads. 



Having, in the experiment N° 4, ascer- 

 tained the warmth of 16 grains of raw silk, 

 I now repeated the experiment with the same 

 quantity, or weight, of the ravelings of white 

 taffety, and afterwards with a like quantity of 

 common sewing silk, cut into lengths of 

 about two inches. The annexed table shows 

 the results of these 3 experiments: 



Here, though the quantities of the silk 

 were the same in the 3 experiments, and 

 though in eacii of them it was made to oc- 

 cupy the same space, yet the warmth of the 



