236 IMPREGNATION OF TIMBER WITH ANTISEPTIC SUBSTANCES. 



servoir above. The timber-yard is accordingly intersected with a 

 well-devised system of masonry or asphalte gutters. 



The substances injected in this manner are principally sulphate 

 of copper and chloride of zinc. The strength of the-sulphate solu- 

 tion is 1 of salt to 100 of water. 



One important advantage of the hydrostatic method is that it 

 involves only a very small capital outlay and requires no special 

 mechanical skill to work. On the other hand, it has two disadvan- 

 tages, which are great enough to militate against its general adop- 

 tion. In the first place, wood in the round has to be used, so that 

 all the portions (at least 30 per cent.), which fall off in conversion, 

 are wasted, and thus a very large proportion of the antiseptic sub- 

 stance is lost. In the second place, as the wood must be green and 

 also have its bark on, no conversion in the forest is possible, and 

 thus cost of carriage is made a very heavy item. 



Article 2.-.-TgB Pneumatic Method. 



This method is of English origin, The wood, fully converted, 

 and seasoned or unseasoned (the former the better), is placed in an 

 air-tight chamber. This chamber is completely exhausted with an 

 air-pump, an operation which draws off all the moisture from the 

 wood. This result is aided either by heating the chamber or by 

 filling it, previous to working the air-pump, with steam raised to 

 a temperature of 112^° 0. and then condensing the steam to form 

 a vacuum. Into the exhausted chamber the antiseptic solution is 

 allowed to flow in, and, with the aid of a forcing pump, the pres- 

 sure of the liquid is raised to that of nearly seven atmospheres. At 

 the end of from 45 to 75 minutes the impregnation is complete. 

 The liquid filling the chamber is then drawn off through a pipe at 

 the bottom, and the chamber is opened and the wood taken out. 



The substances injected in this way are creosote, chloride of 

 zinc, sulphate of copper, tar, and ferric tannate. Carbolic acid, 

 added in small proportions, increases the effectiveness of chloride 

 of zinc Creosote is the substance most largely injected by the 

 pneumatic method. In using it the temperature in the chamber 

 is raised to 130° C, and in order that the wood may become per- 

 fectly dry, it is kept inside the chamber for about two days before 

 the creosote is let in. The chamber is large enough to hold seve- 

 ral tons of wood, and the wood is brought into it on trucks moved 

 on rails. 



Another form of the pneumatic method, which is daily gaining 

 on public favour, consists in injecting steam saturated' with tho 



