When perfectly dry, and in a certain degree of tem perature, both animal and vegetable matters seem scarcely capable of spontaneous decay. On this princi ple, fish and other animal matter is often preserved.
" Similar causes produce the same effects on wood. Even under less rigid circumstances of this kind, as in the roofs and other timber of large buildings, it continues for an astonishing length of time unchanged. Witness the timber of that noble edifice, Westminster-hall, built by Richard II. in 1397; and the more extraordinary in stance quoted by Dr Darwin, in his ingenious work the Phytologia, of the gates of the old St Peter's church in Rome, which were said to have continued without rotting, from the time of the Emperor Constantine to that of Pope Eugene IV. a period of eleven hundred years. On the other hand, wood will remain for ages, with little change, when continually immersed in water, or even when deep ly buried in the earth, as in the piles and buttresses of bridges, and in various morasses. These latter facts seem to show, that if the access of atmospherical air is not necessary to the decay of wood, it is at least highly con ducive to it." Putrefaction is the cause of rotting, and putrefaction is occasioned by stagnant air and moisture. The moisture of the air, coming in contact with wood of a lower tem perature, is condensed in the same manner, as is more visible in our glass windows. In order to prevent the bad effects of this condensation, currents of dry air ought to be made to pass in contact with the timber. Of the ad vantages of this, the Gothic architects seemed aware ; for it was common with them to leave openings for this purpose—a practice which we could strongly recommend 'in cellars, &c.
" It appears 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 cud of the tube. Who will doubt that such a piece of wood
might remain in the open air a thousand years unchan ged? Or let us take a little more apposite illustration of this that of amber, a native bitumen or resin, in which a variety of small flies, filaments of vegetables, and others of the most fragile substances, arc seen im bedded, having been preserved from decay much longer probably than a thousand years, and with no apparent tendency to change for ten times that period." These observations lead to the theory of painting tim ber, for the purpose of preserving it.
Mr 13atson of Limehouse is of opinion, that the dry rot proceeds from a plant, called Boletus Lacryn2ans, one of the fungus tribe, and is one of the few that have leaves as the mislctoe. But Dr Parry justly observes, that these plants " begin merely, because decayed wood 18 their pro per soil." "The smell which we perceive in going into vaults or cellars, where this process is going on, arises partly from the extrication of certain gases, mingled perhaps with some volatile oil, and partly from the effluvia of those vegetable substances, which have already been said to grow on it, and which, though they begin merely because the decayed wood is their proper soil, yet afterwards tend probably to the more speedy decomposition of the wood itself.
" The following, then, appears to be the whole theory of the dry rot, that it is a more or less rapid decomposi tion of the substance of the wood from moisture, depo sited on it by condensation, to the action of which it is more disposed in certain situations than in others ; and that this moisture operates most quickly on wood, which most abounds with the saccharine or fermentable princi ples of the sap." Thus far Dr Parry.
Charring of wood is known to be a most effectual mode of preservation against rotting.
The incorruptibility of charcoal is attested by numer ous unquestionable facts. At the destruction of the fa mous temple at Ephesus, it was found to be erected on piles that had been charred ; and the charcoal in Hercu laneum, after almost 200 years, was entire and undimi nished.