7 ° 
Sir Benjamin Thompson’s 
The next question which arises is, how air can be prevented 
from conducting heat ? and this necessarily involves another, 
which is, how does air conduct heat ? 
If air conducted heat, as it is probable that the metals and 
water, and all other solid bodies and unelastic fluids conduct 
it, that is to say, if its particles remaining in their places, 
the heat passed from one particle to another, through the 
whole mass, as there is no reason to suppose that the pro- 
pagation of heat is necessarily in right lines, I cannot conceive 
how the interposition of so small a quantity of any solid body 
as -J3- part of the volume of the air, could have effected so re- 
markable a diminution of the conducting power of the air, as 
appeared in the experiment (N° 4) with raw silk, above men- 
tioned. 
If air and water conducted heat in the same manner, it is 
more than probable that their conducting powers might be im- 
paired by the same means ; but when I made the experiment 
with water, by filling the glass globe, in the centre of which 
the bulb of the thermometer was suspended, with that fluid, 
and afterwards varied the experiment, by adding 16 grains of 
raw silk to the water, I did not find that the conducting power 
of the water was sensibly impaired by the presence of the 
silk. 
But we have just seen that the same silk, mixed with an 
equal volume of air, diminished its conducting power in a very 
remarkable degree ; consequently, there is great reason to con- 
clude that water and air conduct heat in a different manner. 
But the following experiment, I think, puts the matter be- 
yond all doubt. 
It is well known, that the power which air possesses of 
