492. Coal-tar, when deprived of a portion of its naphtha by boiling, is a valuable protection to timber exposed to the weather; but in order to give it "body," dry chalk powdered fine is frequently mixed with it.

The Dutch, for the preservation of their gates, sluices, drawbridges, and other large works of timber exposed to the weather, coat them with a mixture of pitch and tar, upon which they sprinkle small pieces of cockle and other shells, ground almost to a powder, and then mixed with sea-sand, or the scales of iron beaten small and sifted.†

Semple describes a method of protecting timber from the effects of the weather, by thoroughly heating, and even scorching it all over, and while hot, applying linseed oil and tar which have been well boiled together and kept boiling while being applied. He says that if the wood be tolerably well seasoned, the mixture will sink into the wood one inch or more, and, by closing all the pores, make it exceedingly hard and durable. ‡

Vegetable tar, which is obtained from pine timber, is thought by some to be preferable to coal-tar, in consequence of its being less evanescent.

* 'Preservation of Timber,' p. 147. † Evelyn's ' Silva,' vol. ii., p. 219. ‡'Building in Water,' p. 85.

493. Silicate of Soda in solution has been recommended by Mr. Abel, the eminent chemist to the War Department, as giving to wood, when applied to it like paint, a hard coating which is durable for several years, and is also a considerable protection against fire. The silicate of soda, which is prepared for use in the form of a thick syrup, is diluted in water in the proportion of 1 part by measure of the syrup to 4 parts of water, which is added slowly, until a perfect mixture is obtained by constant stirring. The wood is then washed over two or three times with this liquid by means of an ordinary whitewash brush, so as to absorb as much of it as possible. When this first coating is nearly dry, the wood is painted over with a wash made by slaking good fat lime diluted to the consistency of thick cream. Then, after the limewash has become moderately dry, another solution of the silicate of soda, in the proportion of 1 part by measure to 2 parts of water, is applied in the same manner as the first coating. The preparation of the wood is then complete; but if the lime coating has been applied too quickly, the surface of the wood may be found, when quite dry after the last coat of the silicate, to give off a little lime when rubbed with the hand; in which case it should be once more coated over with a solution of the silicate of the same strength as in the first operation.

494. Common Salt (muriate of soda) is found to protect timber when the proportion of salt is considerable. The large wooden props which support the roofs of the salt-mines in Hungary, and which are perpetually moistened with saltwater trickling down them, are said to last for centuries;* and the timber of vessels employed in carrying salt fish are preserved for a great number of years by the incrustations of salt upon them.* The use of salt for preserving timber is objectionable in most situations, owing to its affinity for moisture.

* Darwin's ' Phylologia,' p. 520.

Timber has been cleared of fungus by immersing it for several months in sea-water. A ship called the 'Eden' was cleared of every trace of fungus by remaining eighteen months under water, †

It is known, however, that a small quantity of common salt rather assists the decomposition of vegetable matter than otherwise.

495. Sulphate of Iron (commonly called green copperas) has been thought of to prevent decay. Wood becomes so hard and compact when boiled in a solution of it, and kept for some days in a warm place to dry, that moisture cannot penetrate it. In the 'Swedish Transactions' it is recommended for preserving the wood of wheel carriages. ‡

Chapman observes that the wooden vessels in which copperas is crystallized become exceedingly hard, and are not subject to decay. He also recommends that timber for shipbuilding should be immersed in a solution of the salt.

Sulphate of iron is the principal substance used in the process patented by Mr. Payne in 1841, which consisted in using two solutions in succession, which mutually decompose each other, and form an insoluble substance in the pores of the wood, one being sulphate of iron, and the other carbonate of soda. The first is introduced into the timber under pressure, then drawn off, and the latter forced in, the combination formed in the cellular vessels of the wood by the process being oxide of iron. The process has been extensively adopted, and with tolerable success where well performed.

* Bowden on 'Dry Rot,' p. 162.

† 'Trans, of the Soc. of Arts,' vol. xxxvi., p. 54.

‡ Neuman, quoted by Chapman ' On Preservation of Timber.'

Dr. Darwin, many years before Payne obtained his patent, suggested soaking timber in lime-water, and afterwards, when dry, in a weak solution of sulphuric acid in water, with the view of forming a compound of sulphate of lime in the pores of the wood, but it does not appear to have been of any practical utility; sulphate of lime being soluble in water, the timber is required to be kept dry.*

Boiling in alkalies has been proposed; but as the alkalies dissolve and decompose the woody fibre, † the process cannot be attended with advantage.