88 TIMBER AND TIMBER TREES. [chap. 



piece of timber can be thoroughly injected, it is found 

 in practice that the penetration of the antiseptic to even 

 a few millimeters below the exposed surface of the 

 wood, enhances its durability considerably — how much, 

 depends on the nature of the antiseptic and the con- 

 ditions to which the treated wood is exposed. 



The following are the principal antiseptic substances 

 in actual use for these purposes : — 



Cupric sulphate, or sulphate of copper, is sometimes 

 used for sleepers in France. It is cheap, very soluble, 

 and easily applied ; but as it merely deposits in crystals 

 in the dried wood, it renders the timber brittle and is 

 easily soaked out in drainage water. 



Creosote, or rather, tarry oils with a large proportion 

 of creosote in them, is cheap where coal is abundant. 

 The creosote is absorbed into the substance of the walls 

 of the wood-elements and is not washed out, while the 

 oily nature of the mixture renders the wood more 

 pliable and damp-proof, and less liable to splinter, 

 etc. 



Mercuric chloride, or corrosive sublimate, is used in 

 the process of kyanising — named after the inventor, 

 Kyan — and is the most effective poison known for 

 fungi, insects, etc. ; moreover, this salt forms insoluble 

 compounds in the wood, and is therefore permanent. 

 Unfortunately, however, it is very expensive, and its 

 exceedingly dangerous characters as a poison are against 

 its general use. 



Other substances which have been employed are 

 zinc chloride, which is cheap but inferior to creosote ; 

 carbolic acid, which is, however, too expensive to com- 

 pete with creosote ; tar oils, paraffins, benzene, etc., etc. 



As to the methods of impregnating timber, the most 

 primitive is to paint the wood, preferably dry, as thickly 



