DRY ROT. This is the name given to a disease affecting timber, and particularly the oak, employed for naval purposes. When dry rot is produced by the attacks of fungi, the first sign of it consists in the appearance of small white points, from which a filamentous substance radiates parallel with the surface of the timber. This is the first stage of growth of the seeds of the fungus, and the filamen tous matter is their thallus, or spawn. As the thallus gathers strength it insinuates its filaments into any crevice of the wood, and they, being of excessive fineness, readily pass down and between the tubes from which the wood is organised, forcing them asunder, and completely destroying the cohesion of the tissue. When the thalli of many fungi inter lace, the radiating appearance can no longer be remarked ; but a thick tough leathery white stratum is formed wherever there is room for its development, and from this a fresh supply of the destructive filamentous thallus is emitted with such constantly increasing ra pidity and force, that the total ruin of timber speedily ensues where circumstances are favourable for the growth of the fungi.
There is reason to believe that any of the fungi that are commonly found upon decaying trees in woods are capable of producing dry rot. The circumstances that are most favour able to the development of the dry rot fungi are damp unventilated situations, and a sub acid state of the wood. Whatever other causes may combine to promote the decompo sition of wood by dry-rot, or other forms of decay, there can be no doubt that imperfect seasoning, by leaving in the pores of the timber a large portion of the fermentable juices always found in recently-felled timber, is one of the most important, and therefore that good seasoning is as essential in promo ting the durability of wood as it is in lessening the tendency to those changes of form and bulk which so greatly increase the difficulties of the carpenter and joiner. The process of seasoning usually consists simply in the ex posure of the timber to the action of air in a dry situation, in stacks or piles so constructed as to allow the free circulation of air in contact with as much as possible of the surface of each piece of timber, until the sap or vege table juices shall have dried up so far as to offer no facility for the germination of the microscopic fungi which constitute various kinds of rot. In order to the success of this operation it is important that the pile of timber be so far elevated from the ground as to allow the circulation of air beneath as well as through and around it ; and also that, if ex posure to rain be not entirely avoided, care be taken to prevent the lodgment of moisture in any place where it would be likely to remain long.
The protecting power of metallic oxides, when applied to the surface of wood in the form of paint, is well known ; and many abortive schemes for the preservation of timber have been devised to act upon the same principle, which is that of excluding such ex ternal influences as might promote decay. To imperfectly-seasoned timber, however, such applications are worse than useless, because by filling up the pores they impede the natural drying of the vegetable juices, and therefore rather promote than check internal decay. Far more efficient than these are the numerous modes of protection which involve the impreg nation of the timber with some antiseptic substance, or with such matters as, by pre occupying the pores, may render the reception and germination of destructive fungi mecha nically impossible.
Of plans for protecting timber by impreg nation, perhaps none has attained such general celebrity as Mr. Kyan's, which was patented in 1832, and has since been very extensively used. The preservative agent in this process is bi-chloride of mercury, com monly called corrosive sublimate, which- is dissolved in water, and forced into the pores of the timber, in closed tanks, by means of forcing-pumps, and which combines with the albumen of the wood, and converts it into a compound capable of resisting the ordinary chemical changes of vegetable matter. Chlo ride of zinc, creasote obtained from the dis tillation of tar, oil of tar, and other bituminous matters containing creasote, and pyrolignite of iron, have all been successfully used. Coal tar is stated to be very superior to vegetable tar, and its efficacy in resisting the worm is attributed to its containing sulphocyanie or sulpho-prussic acid, which is highly destruc tive to animal and vegetable life. Piles pro tected with coal-oil are stated to have resisted the attacks of the teredo better than those protected by Kyan's process. It is necessary however to observe that the coal must be deprived of its ammonia, which would pro duce immediate decay if thrown into the timber.
Another process, called Paynisiug, from the name of the inventor, consists in first filling the pores of the wood with a solution of chlo. ride of lime, and next forcing in a solution of sulphate of iron, by which an insoluble sul phate of iron is formed in the body of the wood, rendering the latter extremely hard. Timber so prepared has been recently much used.