191 1.] Increasing the Durability of Timber. 283 



air freely enters from above, and the conditions of decay are 

 at their best. The lower extremity of the post is in contact 

 with more moisture, but air is excluded by the mass of soil ; 

 while the upper part of the post is comparatively dry, 

 although it is in free contact with air. After being for some 

 years in position, if such a post be removed from the ground 

 and examined, it will be found to be practically sound in 

 every part, except at the place corresponding with the position 

 of the surface of the ground. This, then, is the weakest part 

 in the wood, and one way in which an attempt is made to 

 strengthen it is by charring it. The effects of charring are 

 due to the destruction of wood to the depth of half an inch 

 or so, the resins, gums, tannin, &c, which this wood contains 

 being driven in front of the heat, until they saturate a layer 

 which then acts as a protecting mantle to the deeper wood. 

 To be effective charring must proceed so far as to convert 

 a considerable amount of wood into charcoal — a mere singe- 

 ing or scorching of the wood will do more harm than good, 

 as it will cause the wood to crack and thus form openings 

 for the entrance of fungi, but will not have proceeded so far as 

 to saturate a layer of wood with resin, &c. The destruction 

 of the surface wood is necessarily accompanied by a weaken- 

 ing of the post, and it is doubtful whether, on the whole, 

 the charring of posts is a profitable process. 



Of the various methods that are practised for increasing 

 the durability of timber, that which at present occupies the 

 foremost place is the application of creosote. This substance 

 owes its efficacy to the fact that it is a virulent plant poison, 

 so that wood which contains a considerable quantity of 

 creosote is more or less completely protected against the attack 

 of decay-inducing organisms such as fungi. Wood, when 

 in its natural state, holds certain substances (starch, pro- 

 teins, &c), which are the special food of fungi, but when 

 these are saturated by creosote they are incapable of sustain- 

 • ing fungoid life. Creosote also acts as a preservative to some 

 extent owing to the fact that it displaces air and water in 

 the tissues of the wood, and these are essential to the process 

 of decay. Many other substances besides creosote have been 

 used as preservatives, such as copper sulphate, zinc chloride, 

 and corrosive sublimate, but on account of their cost, or 



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