in various Substances. 461 



turn, coming to the sides of the bottle, there deposits its 

 Heat and its water, and afterwards bending its course 

 downwards, this circulation is continued till all the air 

 in .the bottle has acquired the exact temperature of the 

 water in the jar. 



From hence it is clear why the first appearance of con- 

 densed vapour is near the top of the bottle, as also why 

 the greatest collection of vapour is in that part, and 

 that so very small a quantity of it is found nearer the 

 bottom of the bottle. 



This experiment confirmed me in an opinion which I 

 had for some time entertained, that, though the particles 

 of air individually, or each for itself, are capable of re- 

 ceiving and transporting Heat, yet air in a quiescent 

 state, or as a fluid whose parts are at rest with respect to 

 each other, is not capable of conducting it, or giving it 

 a passage ; in short, that Heat is incapable of passing 

 through a mass of 'air ', penetrating from one particle of it 

 to another, and that it is to this circumstance that its 

 non-conducting power is principally owing. 



It is also to this circumstance, in a great measure, 

 that it is owing that its non-conducting power, or its 

 apparent warmth when employed as a covering for con- 

 fining Heat, is so remarkably increased upon its being 

 mixed with a small quantity of any very fine, light, 

 solid substance, such as the raw silk, fur, eider-down, 

 &c., in the foregoing experiments ; for as I have already 

 observed, though these substances, in the very small 

 quantities in which they were made use of, could hardly 

 have prevented, in any considerable degree, the air from 

 conducting or giving a passage to the Heat, had it been 

 capable of passing through it, yet they might very much 

 impede it in the operation of transporting it. 



