and the Economy of FueL 49 



harm than good, as I have found by experience. 

 When, instead of making use of eider-down, fur, or 

 fine silk for impeding the internal motion of the con- 

 fined air, I used an equal volume of exceedingly fine 

 silver-wire flatted (being the ravellings of gold or silver 

 lace), the passage of the heat through the barrier, so far 

 from being impeded, was remarkably facilitated by this 

 addition, the heat passing through this compound of 

 air and fine threads of metal much sooner than it would 

 have made its way through the air alone. 



Another circumstance to be attended to in the choice 

 of a substance to be mixed with air, in order to form a 

 covering or barrier for confining heat, is the fineness or 

 subtilty of its parts ; for the finer they are, the greater 

 will be their surface in proportion to their solidity, and 

 the more will they impede the motions of the particles 

 of the air. Coarse horse-hair would be found to answer 

 much worse for this purpose than the fine fur of a 

 beaver, though it is not probable that there is any 

 essential difference in the chemical properties of those 

 two kinds of hair. 



But it is not only the fineness of the parts of a sub- 

 stance, and its being a non-conductor, which render it 

 proper to be employed in the formation of covering to 

 confine heat; there is still another property, more 

 occult, which seems to have great influence in render- 

 ing some substances better fitted for this use than 

 others : and this is a certain attraction which subsists 

 between certain bodies and air. The obstinacy with 

 which air adheres to the fine fur of beasts and to the 

 feathers of birds is well-known ; and it may easily be 

 proved that this attraction must assist very powerfully 

 in preventing the motion of the air concealed in the 



VOL. III. 4 



