The necessity, therefore, is obvious for removing, in limine, the infected materials; and, as these are very often "invisible or dimly seen," concealing themselves in minute inter-, stices, a penetrating and accustomed eye is wanted for their detection.

For the purpose of securing the outer walls of buildings from that permanent moisture which is so dangerous, it is suggested that open spaces might be left, at intervals of a few feet, through the whole wall, from, top to bottom. Thick and close walls are constructed for strength: but it is well observed that the strength of an edifice can never exceed that of its weakest parts, and consequently that partial strength may produce general weakness. Such walls, long remaining at a lower temperature than that of the atmosphere, are kept moist by condensing the air; every brick conducts the moisture which it has absorbed as far into the wall as that brick goes; the mortar conducts it to another brick; and so on till the inside of the wall is often retained in a state of constant moisture. When walls, from their situation and aspect, must be exposed to a great degree of moisture or cold, Mr. McW. has accordingly built them half a brick thicker than the support of the mere perpendicular pressure required, and left the thickness of that half brick hollow within the wall; laying the bricks so that every other header may form a tie, and every stretcher on the outside, leave the hollow space in the wall. The moisture which is absorbed then finds its way to the inside of the brick or stone as before: but, as there is no conductor, it is intercepted; and, not being able to proceed farther, it either falls down the wall or is evaporated by the wind from the outside.; Many of those massy walls that were built by the antients as boundaries were crusts, or shells, with interstices left for the circulation of air: the great Chinese wall, and the walls of Pekin, are so constructed; and, though such enormous edifices as these are not required in modern times, the principle may be reduced to any scale, and adopted with advantage.

It often happens that an entire story of a house, containing kitchen, cellars, and other offices, are under ground: in this case the walls are damp; and those apartments which require the freest current of air obtain the least. If frequent fires are made, the predisposing causes of Dry Rot are put into double activity: the air of the room is decomposed, its oxygen is consumed, and its nitrogen, being specifically lighter, flies up the chimney; while the carbon both of the fuel and of the air, being heavier than the latter, falls down, and gets to the floors and joists. The fresh air, which finds its way from the upper apartments and supplies the vacuum occasioned by conREV. AUG. 1818. sumption,

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sumption, pressing straight towards the fire, will leave the moist and damp air in the corners of the room, at the foot of the walls, and under the floòr; and, assisted by warmth, frequent change of temperature, and a large supply of carbonic acid-gas from the combustion, it nourishes the fungus. Hence, according to the author, basement-floors are destroyed much sooner in apartments which have fires than in those which have none. To remedy this evil, it is proposed to pass flues under the floors through the different apartments; carrying the mouths of them out at such places as may freely supply them with pure air; and bringing the other ends into one spot near the hearth, so that they may be carried up behind the grate, and thence into a sort of chimney-flue made in the wall, which may pass out at the top or any other part of the building. The construction of this flue is a matter of considerable nicety, for which we must refer to the volume: the principle of its operation, however, (and it may be applied to ships as well as houses,) is that the air behind the grate becomes rarified by the heat, and ascends up the chimney, while a volume of denser air from below presses in to fill up the space; this in its turn, becoming rarified, likewise ascends, and a current is produced which brings a stream of pure air through the flue under the floor.

Water is likewise a powerful agent in preventing and remedying the ravages of the rot; and many instances have been recorded that prove the antiseptic properties of water acting on wood entirely submersed in it. The piles driven into the Thames by Julius Caesar were undecayed a few years ago; and those on which London Bridge is built have been immersed more than 600 years, and are yet uninjured; while vast quantities of timber, which may have lain buried for centuries, have been found in the bogs and peat-mosses of England, Ireland, Scotland, Holland, France, &c., quite sound, and some of it fit for the purposes of building. When water is applied as a substitute for air in cellars, vaults, &c., care must be taken that it does not stagnate, but flow regularly through the drains; in which case it will carry off with it much of the carbonic acid-gas which is so essential a pabulum to fungi.

It is scarcely necessary to say that timber, taken down in the spring when in a succulent state, or used before it has been properly seasoned, is peculiarly subject to the Dry Rot; and yet Parliament, which is rather too fond o' interfering in private concerns, and rarely so interferes without doing mischief, prohibited, in the first year of James I., every person from cutting oak-timber except in the barking season, under a

severe penalty! For the sake of the fleece, they sacrificed the carcase! If we strip off the bark in the spring of the year, and let the trees stand till the following winter, having cut a ring an inch deep through the alburnum, near the roots; then the sap and saccharine mucilage already in the tree in the spring will be expended during the summer in buds and leaves, while the ring through the alburnum will intercept any fresh supply from the roots. In whatever period of the year trees are felled, however, some portion of matter will yet remain not converted into woody fibre, which it is of importance to expel: but those that are cut down in the winter, particularly if the supply of sap has been intercepted in the summer, will give much less trouble in seasoning than others which have been felled at an improper time.

Mere desiccation is insufficient; for, although the succus communis, the aqueous particles, may be driven off, much saline, saccharine, and other matter remains which is soluble in water, and thus at once points out the remedy. When timber is felled, the sooner it is sawn into squares or planks the better; let these be then immersed in a pond of tolerably clear and soft water, which will extract their juices from the capillary tubes of the timber, and exhibit the activity of its agency by becoming discoloured in a few days. When they have remained some months in water, they should be piled up, protected from sun and rain, and exposed to wind; the butt ends of the timber being uppermost. It is a custom with the ship-builders in the royal arsenal at Genoa to steep their timber for about three years in fresh water, before they use it; and the Spaniards also soak it; as do the Swedes and Norwegians.

The charring of timber is of very antient use, and against external infection is an admirable preservative: but, when the principles of decomposition are within, it is of very little advantage in resisting them. Paint, when the timber is properly seasoned and dry, is likewise very beneficial; and the following "hints on this subject may be useful:

The oil first used for this purpose should be such as has an affinity with the essential oil of the species of timber to which it is applied. For fir-timber, the best is oil of turpentine, with a small portion of linseed oil and bees' wax, and some metallic oxyd, particularly that of lead. The first coat ought to consist principally of the oils, in order that they may penetrate into the cavities of the timber, and fill them up. Thus they will form a better hold for the succeeding coats, which should have a greater proportion of the metallic oxyd, and a proportion of charcoal powdered and properly mixed with any of the drying oils. This will form-a solid close body: and, if properly executed, will protect the

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timber

timber from external contamination for a considerable length of time: as the oil, &c. will give the charcoal-powder a considerable degree of elasticity, and prevent its cracking by the expansion of the wood when damp. For oak-timber, the first coat should be principally linseed oil and oxyd of iron, copper, or zinc. The second should have the addition of bees' wax and litharge; and the following coat should have powdered charcoal, with any other of the drying oils. This I have often known to preserve wood externally, not only from the dry rot, fungus, &c., but likewise from the action and re-action of air and moisture. Timber, however, that is exposed to the alternate action of the respective agents of decomposition, will be effected by internal decay in spite of all external protection. And if the timber be not seasoned and dry in the middle, painting or varnishing will in many cases accelerate the decay; as was the case with the gun-carriages inade in England of unseasoned timber, painted, and sent out to Nova Scotia, in the year 1799. Though the painting preserved the outside crust apparently sound, yet in a few months after they were placed in the fort, they crumbled to pieces by internal decay. Numberless similar instances might be mentioned, from which the inefficacy of external antidotes is apparent; but, if thoroughly dry, good painting is a very great protection for all sorts of timber.'

The impregnation of timber with oleaginous or resinous matter may be deemed a security against the Dry Rot. When it has been well seasoned, if the capillary vessels, from which the aqueous, saccharine, and other matter has been expelled, are filled up with substances insoluble in water, and not readily agitated by the change of temperature, timber will resist the great agents of decomposition, air and moisture. The Americans impregnate their masts by hollowing out the tops in the form of cups, and pouring oil into them, which soon finds its way to the interior; and many similar experiments have been tried in our own dock-yards. The various sorts of timber will absorb that oil the most freely which has the greatest affinity to their native juices. Linseed-oil is well calculated for most sorts; it is not easily soluble in water; and it penetrates with great facility into the vesicular cavities of the wood, where it concretes into solid masses, and unites so closely as to form a substance in many instances, says Mr. MacWilliam, not distinguishable from the timber itself. Salts, minerals, and metallic oxyds have also been applied as antidotes against the Dry Rot: but with respect to common salt, its power of attracting moisture from the atmosphere must more thian counterbalance any advantages that it may possess; and the dampness which it would occasion in our houses would be intolerable. Various methods of applying iron as a preserv ative have been adopted, and they are simple and generally successful.

successful. The oxyd of iron is destructive of vegetation. If the disease originates in the walls, we should brush away the fungus, clear out the joints of the brick-work, point them again with common blue mortar, made of lime with the oxyd of iron, and wash the whole surface with a saturated solution of sulphat of iron (green vitriol); iron-filings, however, are soluble in water with almost any of the acids; and this mixture will produce an oxyd answering the same purpose.

Mr. McWilliam observes that different sorts of timber absorb some oils with more reluctance than they absorb others, which latter are thence said to have a greater affinity for: them: thus turpentine or resinous matter is inapplicable to oak; and oil combined with metallic oxyd is not well adapted to fir. All those timbers, likewise, which contain the astringent principle, tannin and gallic acid, will be readily impregnated with a solution of iron; while those which do not, having no affinity to the metal, absorb it only because it is moisture, and refuse it if they are previously saturated with water. On the contrary, if we apply a solution of iron to oaktimber already saturated with water, it will still absorb the solution, not because it is moisture but because it has an affinity with the metal itself. If the solution, however, be applied to timber, the native juices of which have no such affinity with the metal, and if it be absorbed only because the timber is dry, the water will filter through the woody fibres to the centre, while the particles of metal will be intercepted, and a super-oxydation succeed on the surface, assuming the appearance of rust; and a double injury will follow. A solution of iron may in all cases be used with the greatest advantage in the form of a wash to destroy the roots and seeds of fungi; and those trees which will absorb it, Oak, Elm, Spanish and Horse Chesnut, Ash, Birch, Poplar, Larch, Willow, and others, may thus be effectually armed against the Dry Rot. Several instances are here enumerated of the entire conversion of some woods, oak particularly, into a solid substance of iron Wherever oak-timber is found in bogs, and iron occurs near, it invariably attracts a portion of the metal; and, uniting with the acid of the timber, it becomes black: in Staffordshire, it is usually found as black and hard as ebony, and, being under ground, it is prevented from superoxydation. Where this danger exists, oil or some grease must be used to prevent corrosion.

ore.

When the solution of this metal is to be used, care should be taken to apply it as soon as the iron is dissolved, while it has a black appearance'; for by this means the timber will have an opportunity of saturating the gallic acid with iron, without so much

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oxygen.