232 IMPREGNATION OF TIMBKIl WITH ANTISBPTIC SUBSTANCES. 



CHAPTER v.— IMPREGNATION OF TIMBER WITS 

 ANTISEPTIC SUBSTANCES. 



When durable timber is not obtainable in adequate quantity and 

 at sufficiently low rates, inferior kinds have of course to be used. 

 In that case, their durability may be increased by impregnating 

 them with an antiseptic substance, that is to say, with a substance 

 that opposes decay and the attacks of insects. 



Section I. — The vabious antiseptic substances used. 



A great many kinds have been tried, but those most generally 

 in use are — 



(1). Sulphate of copper. — The cheapness and abundance of this 

 substance and the ease with which wood can be impregnated with 

 it are greatly in its favour ; but it makes the wood brittle and less 

 resisting to strains, and as it never combines with the wood fibre, 

 but is merely deposited in the interstices of the wood in the shape 

 of crystals which are readily soluble, it ultimately gets washed out 

 when the wood is placed in situations in which it is exposed to 

 heavy rain or an overflow of water. Its employment can, there- 

 fore, never become general, and actually its use is confined almost 

 solely to a few railway lines in France, the country of its origin, 

 where beech sleepers are often thus impregnated. 



(2). Creosote. — This is the creosote of commerce, and is really 

 tar oil containing a certain proportion of creosote. It is a sub- 

 stance obtained in great abundance from coal, and is cheap enough 

 in coal-producing countries. Immediately after impregnation the 

 wood is quite soft, but it soon blackens and becomes harder, but 

 rather more brittle, than it was before. The creosote is absorbed 

 into the substance of the wood-fibres, of which it therefore becomes 

 an integral part, and it can hence never be washed out. Its oily 

 nature renders the wood more or less damp-proof, so that it dimi- 

 nishes the tendency of the wood to warp and split. 



