330 ON PRETEXTING THE DECAY OF WOOD. 



«r-where moi- *bove or within the ground. So also where there is an ac- 

 **" re wz y cidental hole in an exposed surface, or any artificial cavity, 

 as in a mortice and tenon, or the part where pales nearly 

 touch the rails on which they are nailed, there the wood 

 universally begins first to moulder away. The same thing 

 happens with regard to horizontal rails themselves, which, 

 when made of the same materials, rot much sooner than the 

 pales which they support. These facts are very easily ex- 

 plained. They clearly show, that the great cause of decay 

 is the constant action of water aided by air, which most af- 

 fects those points, where it is most retained, but has less 

 operation, where, as in the perpendicular pales, it chiefly 

 runs off by its own gravity, so that the little which remains 

 is easily and quickly abstracted by the cooperating power of 

 the sun and wind. 

 This owing to The change which I am describing is the consequence of 

 pu re action, putrefactive fermentation ; a chemical operation, in which 

 the component parts of the wood form new combinations 

 among themselves, and with the water which is essential to 

 the process. The precise nature of these new compounds 

 has not been ascertained ; but, so far as they are known, 

 they consist of certain gasses, or species of air, which fly off, 

 and leave behind a powder, consisting chiefly of carbon or 

 charcoal, and the earth which entered into the original com- 

 position of the wood. 

 Water acts me- Beside this chemical change depending on water, that 

 substance tends to destroy wood exposed to the open air 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 freestone 

 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 exposed wood, and thus in 

 some degree promotes its destruction. 

 Water and air It appears, then, that the contact of water and air are 

 the chief m- ^ ch j ef causes f the decay of wood. If, therefore, any 

 means can be devised, by which the access of moisture and 



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