48 Of the Management of Fire 



Now it appears, from the result of a number of experi- 

 ments which I made with a view to the investigation of 

 this subject, and which are published in a paper read 

 before the Royal Society,* that though the particles of 

 air, each particle for itself, can receive heat from other 

 bodies, or communicate it to them, yet there is no com- 

 munication of heat between one particle of air and 

 another particle of air. And from hence it follows 

 that though air may, and certainly does, carry off heat 

 and transport it from one place or from one body to 

 another, yet a mass of air in a quiescent state, or with 

 all its particles at rest, could it remain in that state, 

 would be totally impervious to heat, or such a mass of 

 air would be a perfect non-conductor. 



Now if heat passes in a mass of air merely in conse- 

 quence of the motion it occasions in that air ; if it be 

 transported, not suffered to pass, in that case, it is 

 clear that whatever can obstruct and impede the inter- 

 nal motion of the air must tend to diminish its con- 

 ducting power. And this I have found to be the case 

 in fact. I found that a certain quantity of heat which 

 was able to make its way through a wall, or rather 

 a sheet of confined air, \ an inch thick in 9! minutes, 

 required 2 if minutes to make its way through the same 

 wall, when the internal motion of this air was impeded 

 by mixing with it -^ part of its bulk of eider-down, of 

 very fine fur, or of fine silk, as spun by the worm. 



But in mixing bodies with air, in order to impede its 

 internal motion and render it more fit for confining 

 heat, such bodies only must be chosen as are themselves 

 non-conductors of heat, otherwise they will do more 



* See the Philosophical Transactions, 1792. See also Vol. I., pp. 401 and 

 following. 



