322 
On Preventing the Decay of Wood, 
very frequently of fungi, has sprung from the porous 
mass ; and the decay is always attended with a wide- 
spreading exhalation, the odour of which cannot well be 
described, but which is sufficiently known. 
What then are the causes of this destruction? Precise- 
ly the same as those which I have before described; 
though their action is differently modified, and less ob- 
vious to gross observation. The decay is produced by 
the putrefactive fermentation of the component parts of 
the wood, in connection with moisture, without which, as 
I have before stated, wood cannot putrefy. 
Common air is not only capable of mixing with a con- 
siderable quantity of water in form of vapour, but during 
every state of our atmosphere is always much loaded 
with it. Water becomes vapour in consequence of be- 
ing united with a certain proportion of that substance 
which is called heat. If a sufficiently cold substance 
comes into contact with vapour, the superabundant heat, 
which was necessary to its existence in that form, passes 
into that cold substance, and the vapour is then imme- 
diately condensed or changed into water. Thus if in the 
hottest day in summer, when the vapour in our breath is 
totally invisible, we breathe on a looking-glass or plate 
of polished metal, which is colder than our breath, the 
surface is immediately dimmed; and if we continue to 
breathe on it, small drops of liquid appear, which gra- 
dually become larger and larger, and many of them at 
length uniting, run down the surface in a stream. The 
same thing takes place on the outside of a glass of water 
drawn in summer from a deep well, and of a bottle 
brought up into a warm room out of a cool cellar; and on 
the inside of our windows in frosty weather. On the 
other hand, we could not dim with our breath a plate of 
metal or glass of one hundred degrees of heat, which is 
greater than that of our breath, and no mist is observable 
