IMPREGNATION-METHODS. 87 



insoluble compounds in the wood, and is, therefore, permanent ; 

 but its costliness and dangerously poisonous character are against 

 it. Zinc chloride, introduced by Sir William Burnett, is cheap 

 and effective against both insects and fungi, but less so than 

 creosote. Copper sulphate, sometimes used for sleepers in France, 

 is cheap ; but is deposited in crystals in the wood, rendering it 

 brittle, and, owing to its solubility, is as easily washed out as 

 it is injected. Creosote, originally suggested by Bethell, and now 

 very largely employed in various ways, is cheap, lasting in its 

 effects, and useful in rendering the wood damp-proof. The more 

 expensive carbolic acid and ferric tannate have also been used. 



To force the antiseptic solution into the wood, M. Boucherie, 

 who first employed copper sulphate, proposed placing it in an 

 elevated reservoir connected by a pipe with the lower end of a 

 log ; but this requires the log to have its bark on, and is thus a 

 wasteful process. 



A more complicated and costly, but very successful, process 

 consists in the use of air-tight chambers, in which the converted 

 timber is placed. The air is then partially exhausted, so as to 

 draw out some of that in the vessels of the wood, and the anti- 

 septic solution is then forced in by pumps, preferably with steam 

 or heat, the whole process occupying less than an hour. About 

 75 Ibs. of creosote, however, are required for the impregnation of 

 an ordinary railway sleeper, and various attempts to reduce this 

 quantity by the use of some liquid solvent have failed. Herr F. 

 Seidenschnur proposes that the timber be first steamed under 

 pressure, the air then exhausted by reduced pressure, and then 

 an emulsion of 15 per cent, of creosote, in a resin soap to which 

 water is added, forced in under a pressure of seven atmospheres. 

 The latest process, known as the Nodon-Bretonneau method, is 

 electrical. The timber is placed on a lead plate, connected with 

 the positive pole of a dynamo, in a tank filled with a solution of 

 10 per cent, of borax, 5 per cent, of resin, and 75 per cent, of 

 carbonate of soda, a second lead plate, connected with the 

 negative pole, being on top. The circuit is completed through 

 the wood ; and, within from 5 to 8 hours, the sap rises to the 



