Wood, Dr. Parry supposes to be subject to destruction from two causes—rotting, and the depredations of insects. Of rot there are two supposed kinds : the first takes place in the open air; the second under cover.
When perfectly dry, and in a certain degree of tempera ture, both animal and vegetable matters seem scarcely eapa ble of spontaneous decay. On this principle, fish and other animal matter is often preserved.
"Similar causes produce the same effect 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, Westminste•-hall, built by Richard II. in 1807 ; and the more extraordinary instance 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 Em peror 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 im mersed in water, or even when deeply 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 atmos pherical air is not necessary to the decay of wood, it is at least highly conducive to it.'' Putrefitetion is the cause of rotting ; and putretlietion is occasioned by stagnant air and moisture. The moisture of the air, coming in contact with wood of a lower temperature, is condensed in the same manner, as is more visible in our !*lass 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 advantages of this, the Gothic architects seemed aware; for it was common with them to leave openings for this purpose ; a practice which We would strongly recommend in cellars. &e.
" It ill• that the contact of water and air are the chief causes of the decay of wood. If', therefore, any means can be devised by Nvhiell 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 ease which it exactly tills, and the two ends of which arc, as it is called, hermeti cally that is, entirely closed, by uniting the melted sides of each end of the tube. Who will doubt that such a piece of wood inight remain in the open air unchanged ! Or let us take a little more apposite illustration of this fact, 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, are seen imbedded, having been preserved aroma decay much longer probably than a thousand years, and with no apparent tendency to change i'or ten times that period." These observations lead to the theory of painting timber, filr the purpose of preserving it.
Mr. Batson of Limehouse is of opinion, that the dry-rot proceeds from a plant, called boletus luc/uryuoins, one of the fungus tribe, and is one of the few that have leaves, as the mistletoe. But Dr. Parry justly observes, that these plants " begin merely because decayed wood is their proper soil." •• The smell which we perceive in going into vaults or cel lars, 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 decomposition of the substance of the wood, from moisture deposited on it by condensation, to the action of which it is more disposed in certain situations than in others; and that this moisture ope rates most quickly on wood which most abounds with the sac charine ur fermentable principles of the sap." See DRY-ROT.