Experiments upon Heat. 73 
having deposited a part of its water upon the surface of the 
glass, at the same time that it communicates to it its heat, slides 
downwards by the sides of the bottle in consequence of its 
increased specific gravity, and, taking its place at the bottom 
of the bottle, forces the whole mass of hot air upwards ; which, 
in its turn coming to the sides of the bottle, there deposites its 
heat and its water, and afterwards bending its course down- 
wards, 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 condensed 
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 in- 
dividually, or each for itself, are capable of receiving 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 capa- 
ble 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 circum- 
stance 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 confining heat, is so remark- 
ably increased upon 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, 
mdccxcu, L 
