IHPKEGNATION OF TIMBER WITH ANTISEPTIC SUBSTANCES. 233 



(3). Chloride of zinc. — This is a cheap substance and is very 

 effective against decay, but does not come anywhere near creosote 

 in practical utility. 



(4). Chloride of mercury/ (corrosive sublimate). — The use of 

 this substance for impregnating wood was first made by an Eng- 

 lishman named Kyan, whence the name of Kyanizing for the 

 process invented by him. Corrosive sublimate is thoroughly 

 effective against every kind of decay and insects, but its violent- 

 ly-poisonous nature and its high cost are against its employ- 

 ment. 



(5). Carbolic acid. — TMs is used either by itself or in mixture 

 with other substances, but is too expensive for ordinary employ- 

 ment. 



(6). Tar oil, paraffin, benzene, and other carho-hydrates, derived 

 from, the dry distillation of coal and wood. — These substances are 

 injected in combination with steam, but their use has not yet be- 

 come general. 



(7). Ferric tannate. — This is a compound of iron and tannic 

 acid. It is perfectly insoluble, and hence, when it once gets inside 

 the wood, nothing will remove it. By closing the pores of the 

 wood into which it is injected, it effectually keeps out moisture. 

 As the salt is insoluble, the wood is first injected with tannic acid 

 and then with ferrous oxide, or, which comes to the same thing and 

 is very much cheaper, pyrolignite of iron. The two substances 

 combine in the wood to form the ferric tannate. 



Section II. — Methods of Impregnation. 



Impregnation may be effected either (1) by hydrostatic pressure, 

 or (2) by pneumatic pressure, or (3) by immersion, or (4) by 

 painting the surface of the wood. 



Aeticlb 1. — The Hideostatic Method. 



By this method, which was the first one ever used and was 

 invented by a Erench Doctor named Boucherie, wood can be 

 impregnated only while if is still quite green. The sap of the green 

 wood is driven out by the antiseptic liquid, which is placed at a 

 sufficient height to exert a pressure of about one atmosphere. The 

 wood to be impregnated must not be barked, otherwise much of 

 the antiseptic fluid would escape at the sides and the entrance of air 

 would interfere with the free run of th^ liquid. The pieces are 

 generally placed with the thicker end slightly raised above the 



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