On Preventing the Decay of Wood . 30 i 
new compounds has not been ascertained ; but, so fur as 
they are known, they consist of certain gases, or species 
of air, which fly off, and leave behind a powder, consist- 
ing chiefly of carbon or charcoal, and the earth which en- 
tered into the original composition of the wood. 
Beside this chemical change depending on water, that 
substance tends to destroy wood exposed to the open aii v 
by a mechanical operation. Every farmer is acquainted 
with the power of winter in mouldering down the earth of 
his fallows. It is equally well known, that porous free- 
stone splits and shivers during severe winters. These 
effects are produced by frost, which, acting on the water 
in the pores or interstices of these substances, expands it 
by conversion into ice, and thus bursts the minute cells in 
which it was contained. There can be no doubt, that 
a similar operation takes place to a certain extent in ex- 
posed wood, and thus in some degree promotes its de- 
struction. 
It appears, then, that the contact of water and air are 
the chief causes of the decay of wood. If, therefore, any 
means can be devised, by which the access of moisture 
and air can be prevented, the wood is so far secure against 
decay. This principle may be illustrated by supposing 
a cylinder of dry wood to be placed in a glass tube or 
case, which it exactly fills, and the two ends of which 
are, as it is called, hermetically sealed, that is, entirely 
closed by uniting the melted sides of each end of the 
tube. Who will doubt that such a piece of wood might 
remain in the open air a thousand years unchanged ? Or 
let us take a still more apposite illustration of this fact; 
that of amber, a native bitumen, or resin, in which a varie- 
ty of small flies, filaments of vegetables, and others of the 
most fragile substances are seen imbedded, having been 
preserved from decay much longer probably than a thou 
sand years, and with no apparent tendency to change 
