300 THE UTILISATION OF WOODLAND PRODUCE. 



higher the specific gravity, the more durable the timber, because 

 that means a larger proportion of woody fibres, and of lignin, 

 tannic acid, resin, and other substances preservative of the 

 woody tissue. Speaking generally, wood felled in autumn, just 

 about the fall of the leaf, when there is least sap in the tree, is 

 more durable than that cut at any other time of the year ; and 

 wood felled just before the flush of the foliage in spring, when 

 the tree is fullest of sap, is usually least durable. 



The preservation of timber may take place I. By seasoning 

 1, naturally, by gradual drying in the open air; or 2, artifici 

 ally, by evaporating the sap in hot-air chambers, or by steaming 

 it in hermetically closed boxes or cylinders (with or without 

 superpressure) ; and 



II. By impregnation witli antiseptic substances 1, through 

 simple immersion, or. 2, through injection '.under pressure. Mere 

 superficial coating with paint, &c., is of little use unless the wood 

 be thoroughly seasoned, and is to be used indoors. But by far the 

 most effective way of increasing durability is to impregnate the 

 woody tissue as thoroughly as possible with antiseptic fluids, 

 rendering the albuminoid substances unfit for the food of insects 

 and fungi. 



1. Seasoning naturally. The specific gravity of timber is 

 much greater when the wood is green than when most of the 

 sap has been evaporated, and the timber has become air-dried 

 or seasoned. And though hardwoods generally lose about 30, 

 Conifers about 40, and softwoods about 45 per cent of their 

 total weight during the long, slow process of seasoning in the 

 open air, yet they all still contain on the average from 8 to 12 

 per cent of moisture, which can only be got rid of by drying 

 artificially in closed chambers. 



The best method of seasoning broad-leaved trees having a 

 distinct heartwood is to girdle or ring them by cutting clean 

 through the bark and sapwood and into the inert heartwood all 

 round the stem, just above the buttresses near the ground. 



