JJ4 TECHNICAL PROPBRTIES OF WOOD. 



density and hardness of the wood being thereby increased to a 

 greater extent than in the formation of ebony. On the other 

 hand, situation in a moist, still, warm atmosphere, contact with 

 soil or moist masonry, and alternate submergence and exposure or 

 submergence close to the surface of the water, hasten decay. At 

 most seasons of the year the soil is moist enough for the germina- 

 tion of fungus spores, and, except at a great depth, it contains 

 sufficient air and heat for the purpose. In old posts fixed in the 

 ground, the greatest amount of decay will be found at the level 

 of the ground, and the extent to which decay has progressed in the 

 buried portion will be found to grow less as we examine the wood 

 further down. The more porous a soil is, the more rapidly does 

 the wood decay (witness the fate of railway sleepers laid deep in 

 loose ballast). Wood lasts longest in stiff clay soils, much less in 

 limestone soils (which are not only porous but also act chemically 

 on the wood), and least of all in soils containing much organic 

 matter, especially such as are themselves undergoing decay and 

 decomposition. In experimenting on the relative durability of 

 different woods, the most rigid test to apply is to bury the lower 

 ends of posts or scantlings of one and the same size in holes filled 

 with fresh cow-dung. 



Impregnation at any time with antiseptics, such as creosote, 

 sulphate of copper, &c., precludes the vegetation of fungi, provided 

 fungus spores have not already entered the substance of the wood. 

 Fungi may also be kept out indefinitely by covering the wood, 

 where it is to be in contact with a damp surface, with a coat of 

 paint impervious to moisture and therefore also to spores, or by 

 painting the surface over with creosote, tar or any other antiseptic 

 substance. Charring the surface is not always a successful method, 

 for charcoal being a highly permeable substance, may let spores 

 pass in with the water, and in the process of charring deep cracks 

 may form giving ingress to the spores. In any case the charring, 

 to be effective, must be deep, and thus detracts very considerably 

 from the strength of the wood. Woods thoroughly impregnated 

 with resin are practically imperishable. 



It is evident that the first step to rendering wood durable is to 

 season it thoroughly ; no other precautions, if this one has been 

 omitted, can save the wood from early decay. 



All woods are not equally durable, and even in the case of one 

 and the same species some specimens decay more quickly than 

 others. Greater weight is no proof of greater durability in the 

 case of woods of different species, for the lighter wood may con- 

 tain substances, such as oils, alkaloids, &c., that are poisonous to 



