VII.] IMPREGNATION. 89 



as possible, and trust to soakage ; tarred timber is very 

 commonly employed on this principle. The chief draw- 

 backs are that the liquid soaks in a very little way, and 

 any crack opened after treatment exposes the raw sur- 

 face of the wood to the agents of decay. 



The next simplest method is to submerge the logs, 

 poles, etc., in a large bath and leave them there as long as 

 practicable ; in certain cases the bath is heated, even to 

 boiling, with more rapid results. The principle of this 

 method is exactly the same as that concerned in water- 

 logging ; the air in the cavities of the wood-elements is 

 gradually displaced, more or less as the case may be, by 

 the liquid, and obviously this displacement is hurried 

 and rendered more complete if the liquid is hot enough 

 to cause the imprisoned air to expand and escape. 

 Experience shows that long submergence may render 

 the timber brittle, and the results differ with different 

 species of wood. Tar, sulphate of iron or of copper, 

 chloride of zinc, and creosote have been used in this way 

 often with excellent results, though the liquid only 

 enters a very little way into the sleepers, poles, etc., 

 treated. 



A somewhat more complicated and more costly 

 process has been employed with great success of late 

 years. This consists in placing the sleepers, telegraph 

 poles, etc., in air-tight chambers, which are then partially 

 exhausted, so that some of the air in the wood escapes. 

 Then the chamber is filled with the solution — usually 

 creosote, but salts of copper or zinc, or tar, ferric tan- 

 nate, etc., have been used — which is allowed to soak in, 

 either at the ordinary atmospheric pressure, or under 

 pressures applied by force-pumps. 



Even more effective, with creosote, is this method 

 combined with the heating of the whole apparatus ; or 



