496. Quicklime, when present in small quantities and aided by moisture, assists putrefaction; but where present in large quantities, so as to preserve the wood in a perfectly dry state, by absorbing the moisture, it hardens the sap and renders the wood durable. Chapman states that vessels employed in the Sunderland lime trade were very sound, though some of them were forty years old. ‡

497. In 1737 the first patent was obtained by Alexander Emerson for a method of preserving timber with boiled oil, mixed with poisonous substances, applied hot. Since that time numerous patents have been taken out, but we shall limit our description to those which have obtained the most success and proved the most effective.

498. Kyan's patents were obtained in 1832 and 1836 for the preservation of wood and other vegetable substances by soaking them in a solution of corrosive sublimate (bi-chloride of mercury), which in the case of wood forms a new chemical compound, with the albumen, and prevents the destructive power going on. At first the proportion used was 1 lb. of corrosive sublimate to 4 gallons of water; but on subsequent trials it was found that the wood absorbed about 6 or 7 lbs. of the salt per load, which would have rendered the process too costly for general use. Ultimately the proportions were reduced to 1 lb. of corrosive sublimate to 10 gallons of water when a maximum strength was required, and 1 lb. to 15 gallons of water when a minimum; with the latter proportion, 1 1/2 lb. was sufficient for a load of timber containing 50 cubic feet.

* ' Phytologin,' p. 519.

† Dr. Thomson's 'Chemistry,' vol. iv., p. 183.

‡' Preservation of Timber,' p. 16.

The solution was contained in a wooden tank, put together, so that no metal of any kind could come in contact with it. The salt dissolves best in tepid water. The time required to saturate the timber depends on its thickness. Twenty-four hours are usually allowed for each inch in thickness of boards and small timber. Large timber requires from a fortnight to three weeks.

So highly was this patent thought of that in 1833 it formed the subject of a lecture delivered by Professor Faraday at the Royal Institution, and in 1836 was reported most favourably of by the authorities of the Royal Carriage Department at Woolwich. In 1838 the Dutch Government adopted it in the Royal Navy, after obtaining the report of a commission appointed to inquire into its merits, and who made numerous experiments.

After a time the patent fell into the hands of a company, and the process was, in many cases, imperfectly carried out. Notwithstanding that corrosive sublimate is highly destructive to all forms of animal life, Kyan's process has not been found effective either against the worm or white ant, though it appears to have had some effect in retarding the dry rot. It is now, however, seldom used.

499. In 1837 Margary obtained a patent for preserving timber, ropes, canvas, and other substances, by soaking them in a solution of acetate, or sulphate of copper; but the only advantage which this process had over that introduced by to 5 gallons of water for canvas, cordage, etc.

Kyan was in the comparative cheapness of the salt used; and it appears to have failed even more completely in prac-;ice. Several of the sleepers on the Bristol and Exeter Railway which had been prepared according to Margary's patent were found decayed after a short time, and had to be removed.

500. The next patent which attracted public notice was taken out in 1838 by Sir William Burnett, who was Director-General of the Medical Department of the Navy, and had or its object, in common with Kyan's, Margary's, and Payne's processes, the coagulation of the albumen of the wood. He used, in a wooden tank, a solution of chloride of zinc, in the proportion of 1 lb. to 4 gallons of water for timber, and 1 lb.

Timber requires to be immersed for about two days for each inch in thickness, and afterwards taken out and left to dry from fourteen to ninety days.

Canvas, ropes, etc, require to be immersed in the solution or about forty-eight hours, and then taken out and dried. The process on wood may be more expeditiously performed by means of the hydraulic press, with which the solution of chloride of zinc is forced into the timber. The system has been introduced into several of the Government dockyards; and where the timber can be kept tolerably dry, it is no doubt beneficial, as it tends to harden the wood and renders it partially incombustible. It is also supposed to prevent he attacks of insects, which are found to commit great ravages in the interior fittings of vessels.*

501. It appears that one of the most successful means yet tried of preserving timber, whether from the effects of ex-posure to the weather, dry rot, or the attacks of worms and insects, is by impregnating its substance with the oil of tar, called creosote, which is one of the products obtained from the distillation or coal-tar, ana possessing powerful antiseptic properties. When injected into the wood creosote has the effect of coagulating the albumen, thereby preventing decomposition, and the bituminous oil with which it is combined enters the capillary tubes of the wood, closing up its pores so as to exclude both air and moisture, and the noxious properties of the oil has the effect of repelling both worms and insects.

* 'Min. Proc. Inst. C. E ' 1852-3.

Several attempts have been made from time to time to introduce this substance into notice, as a preservative of wood; and although it has been used in one form or another since the time of the Egyptians, as seen in the mummy cases, some of which have remained sound to this day, it was not until Mb. Bethell obtained his patent in 1838 that it became extensively used. The opinions of engineers, who have used it for the preservation of railway sleepers in all climates, both at home and abroad, have been strongly in its favour, and its power of enabling timber to resist putrefaction, and to a considerable extent of repelling the attacks of the sea-worm and white ant, when properly applied and in sufficient quantity, has been placed beyond doubt.

It was found, however, by Mr. Stevenson to have failed in repelling the attacks of the Limnoria terebrans at Invergorden in Scotland, where the piles of a jetty, erected in 1858 and which had been thoroughly creosoted, "were very much eaten and perforated" in about four years after being fixed; and Mr. Stevenson, in a paper "On the Ravages of the Limnoria terebrans," read before the Royal Society in 1862, gave it as his opinion that the process of creosoting preserved timber from the attacks of marine insects, only so long as the oil existed as a film, or coating, on the outside of the timber. When the attrition caused by the motion of the sea removed this film or coating, and exposed the fibrous surface of the timber, the insects would then attack and perforate it, whether it was creosoted or not. its search being for a fibrous substance in which to burrow.*

The mode of impregnating wood with creosote adopted by Mr. Bethell is to dry out all the moisture from the pores of the timber, by passing all the smoke and products of combustion from the burning fuel through the drying-house so as to pass between the different pieces of wood, thereby drying and smoking them at the same time, after the manner that hams, bacon, and fish are smoked and cured. By this mode of drying, wood that has been cut down for several months loses in ten hours about 8 lbs. in weight per cubic foot; and if immersed in hot creosote oil in open tanks directly after it leaves the drying-house, and while warm, it quickly absorbs the oil to the extent of 8 or 9 lbs. per cubic foot. Another method is to place the timber (after it leaves the drying-house) in a wrought-iron cylinder with closed ends, and to force in the heated oil at a pressure of about 170 lbs. to the square inch. The heat is kept up in order to prevent the creosote from crystallizing in the pores of the wood during the process. Under this system, pine, fir, or other soft wood easily absorbs from 10 to 12 lbs. of oil per cubic foot. For railway works, Mr. Bethell considered 7 lbs. per cubic foot sufficient, but for marine works he recommended that 10 lbs. of the oil per cubic foot, at least, should be forced into the wood, and some engineers have required even 12 lbs. Into oak and other hard woods, particularly those of India, it is sometimes difficult to force more than 2 or 3 lbs. of the oil, even by the heaviest pressure. † The Saul-wood of India was seldom penetrated more than 1/8 of an inch from the surface. 502. Another method which is applicable to the preserva-on of straight-grained or porous timber was introduced same years ago by M. Boucherie, a French chemist. Instead of using great pressure to impregnate the timber, as in creo-soting, he applied a moderate pressure only to one end of the log or tree, which had the effect of expelling the sap, and permitted the pores of the timber to be filled with the preserving fluid, which consisted of a solution composed of 1 part of sulphate of copper to 100 parts of water by weight. The specific gravity of the solution at 60° Fahr., when of proper strength, being 1.006 or nearly so. The process is as follows: - A water-tight cap is placed on one end of the log to be saturated, and the solution is introduced within it by a flexible tube. The pressure required not being more than from 15 to 20 lbs. on the square inch, it may be obtained in a very simple way by raising the tank which contains the solution to the height of 30 or 40 feet from the ground. When the pressure is applied, the sap runs in a stream from the opposite end of the log; and a ready means exists of discovering when it is exhausted and the whole length of the timber penetrated by the solution, by simply rubbing the end with a piece of prussiate of potash, which will leave a deep brown mark when brought into contact with the sulphate of copper.

* ' Civ. Eng. and Arch. Journal,' vol. xxv., p. 206.

† Tract on Bethell's ' Improvements in Preserving Timber,' 1850.

There are certain kinds of timber which are impenetrable by the solution applied in the manner described. It answers best with newly-felled beech, birch, larch, Scotch pine, alder, elm, poplar, etc. Trees felled any time between November and May can be prepared in the latter month, but when cut down in May, or any month between that and November, they should be prepared within three weeks of the time of felling. .

It was found, during the preparation of vast quantities of timber for the French navy and railways, that the time necessary for the operation depends both on the length of the tree and on the description of timber. Trees of 40 feet in length, prepared at Fontainebleau for the French navy, equired from eight to ten days to become sufficiently impregnated; whereas for a length of 9 feet only, the process was accomplished in twenty-four hours. One great advantage attending M. Boucherie's method is the small cost of the apparatus required.

M. Boucherie also used the impure pyrolignite of iron, which was found not only to preserve the wood from decay, but to harden it to a very high degree.