WOOD PRESERVATION. If proper ly seasoned, timber, in a dry shim• non with a free circulation of air round it, is very durable, and has been known to last for several hundred years without apparent deterioration. Tins is not, how ever, the case when exposed to moisture, which is always more or less prejudicial to its durability.
When timber is constantly under water, the action:of the "water dissolves a portion of its substance, which is made apparent by its becoming covered with a coat of slime. If it be exposed to alter nations of dryness and moisture, as in the case of piles in tidal waters, the dis solved parts being continually removed by evaporation and the action of the water, new surfaces are exposed, and the wood rapidly decays.
Where timber is exposed to heat and moisture, the albumen or gelatinous mat; ter in the sapwood speedily putrefies and decomposes, causing what is called rot. The rot in timber is commonly divided into two kinds, the we and the dry; but the chief difference between them is, that where the timber is exposed to the air, the gaseous products are freely evaporat ed ; whilst, in a confined situation, they combine in a new form, viz., the dry-rot fungus, which, deriving its nourishment from the decaying timber, often grows to a length of many feet, spreading in every direction, and insinuating its delicate fi bres even through the joints of brick walls.
In addition to the sources of decay above mentioned, timber placed in sea water is very liable to be completely de stroyed by the perforations of the worm, unless protected by copper sheathing.
The best method of protecting wood work from decay, when exposed to the weather, is to paint it thoroughly, so as to prevent its being affected by moisture.
It is, however, most important not to apply paint to any wood-work which has not been thoroughly seasoned ; for in this case the evaporation of the sap being prevented, it decomposes, arid. the wood rapidly decays.
Many plans have been proposed for the prevention of the rot. Kyan's process consists in impregnating the timber with corrosive sublimate, thus converting the albumen into an indecomposable sub stance. This method, although not al ways successful, is undoubtedly of great use, particularly where inferior or imper fectly seasoned timber has to be used.
It is, however, said to render the wood brittle.
Payens's process consists in impregnat ing the wood with metallic oxides, alka lies, or earths, as may be required, and decomposing them in the wood, forming new and insoluble compounds. The usual preparation was first injecting into the timber a solution of sulphate of iron, and then one of chloride of calcium. An air pump was used to draw the liquids into the pores of the wood. Timber thus prepared will not burn, but only smoulders.
A process invented by a Mr. Bethell, and very good in railway works, is to impregnate the timber with oil of tar : this appears to be very successful in pre venting decay, but the danger of acci dents from fire is much increased.
Dr. Boucherie's process of imprepat ing timber was with a solution of sul phate of copper. After the varnishing, the appearance of the wood is rich, and is said to be permanent. The Dr. con fines his application of it to the soft wood generally ; and lie exhibited at a French meeting a work-box so impregnated, made of a tree within three months after it was cut. He showed a block sawed into three sections, but not disconnected, which had been buried for six years in a fungus pit. It is of pine, and immediate ly after being felled, the two side sections were impregnated by means of the natu ral action of the sap vessels of the wood, the one with the dento-chloride of mer cury (corrosive sublimate, as recommend ed by Kyan), 800 grammes of 1-5 per cent, strength ; the other with 800 grammes of sulphate of copper, of 1-5 per cent. The centre section was left in its natural state. The block shows the por tions which were left in a natural state, and that impregnated with the corrosive sublimate, equally and completely rot ten, the fibre destroyed, and the wood crumbling into dust, while the section marked as impregnated with the sul phate is perfectly sound and good. It is sai4, that traversers and sleepers on rail wags so impregnated have been used six years, and are still sound.