825 
On Preventing the Decay of Wood . 
inside a thin layer of brick, with an interval of one or two 
inches from the outer and thicker layer of brick or stone.* 
to which it must be united by proper binders. The po 
rous structure of the bricks, added to the impermeableness 
of the intermediate stratum of air, would so ill conduct* 
heat, that such walls would necessarily tend to keep a 
house dry and warm in the winter, as well as cool in the 
summer. This end would be still further promoted by 
filling the interval between the two layers with dry sand, 
fresh sifted coal-aslies, or pow dered charcoal. In fact, 
when the common external means before described have 
succeeded in curing dampness, it has been either by af- 
fording a varnish, which has diminished evaporation by 
preventing absorption, or by increasing the space or 
changing the quality of the materials of the wail through 
which the heat was to pass, so as in either of these cases 
to retain it more forcibly : And when the dampness has 
been remedied by removing the paper to some distance 
from the wall by means of strained canvass, that effect 
has been produced by rendering the paper a worse com 
ductor of heat ; and therefore indisposing it to condpise 
the vapour in the room so readily as when it was in con- 
tact with the colder wall. 
It has been suggested, that it would be possible to keep 
out cold, or, in more accurate language, prevent the egress 
of heat from the inside of a room, and therefore from the 
walls surrounding it, by shutting it closely up, and pre- 
venting any admission of the cold external air. This 
has arisen from the supposition that air is not a good con- 
ductor or transmitter of heat through its substance or 
pores, but that it merely carries it by changing place 
with some other portion which was less charged with it 
If there w ere no other mode of abstracting the heat from 
the wails of a room, and if it were possible wholly to 
prevent any change of its air, this theory might perhaps 
