28 TROHNIOAL PBOPKRTIBS OP 'WOOD. 



of moisture, and to presence of resins and oils, -which enable some 

 woods to burn well even in a very green condition. Decayed 

 wood, owing to its spongy texture, takes fire easily and burns until 

 it is consumed, but, as it has lost a very large proportion of its 

 carbon, its combustion is very slow and unaccompanied with flame. 



The conditions which affect the heating power of a given wood 

 are — 



(1). Quantiti/ of contained moisture. — The most highly air-dried 

 wood contains a large proportion of moisture. When the wood is 

 burnt, a certain portion of the heat produced by the combustion is 

 absorbed in converting the moisture into steam, and not only this, 

 but as the steam rushes out from the inner layers of the wood, it 

 takes up more heat from the burning outside layers. Nordlinger 

 estimates that with 45 per cent, of water present, half the heat of 

 combustion is lost, and with 60 per cent, as much as four-fifths. 

 These figures prove the great importance of drying firewood as 

 thoroughly as possible ; all large pieces should be cut into short 

 lengths and split, and the wood should be loosely arranged in long 

 narrow stacks composed of only a single row of pieces, so that 

 both ends may be exposed and air circulate freely between the 

 several pieces. 



(2). Specific weight. — This is not a safe criterion for woods of 

 different kinds, since other circumstances, such as a greener condi- 

 tion of the heavier wood, resin and oil in the lighter wood, &c., 

 may more than counterbalance the superiority possessed by the 

 heavier wood in respect of its weight alone. Thus the light, but 

 porous and quickly-dried, wood of Butea frondosa gives out more 

 heat than several much heavier woods. Nevertheless, for one and 

 the same species superior weight also means superior heating 

 power. Hence the heartwood is better than the sapwood, the 

 wood of the stem than the wood of the branches and roots (resinous 

 conifers of course excepted), the highly lignified wood of trees 

 grown in warm sunny localities and in the open than the wood of 

 trees of cooler climates and aspects and of canopied crops. 



(3). Anatomical structure. — In the case of the more porous 

 wood the moisture is expelled more quickly, during combustion, 

 from the inner mass of the wood, and hence there is less loss of 

 heat. We have already seen that the more porous wood also burns 

 more quickly. Hence in a confined place, as in a baker's oven, 

 the more porous wood will not only give out its heat more quickly, 

 but also an absolutely larger quantity of heat. For warming rooms 

 a wood of a certain minimum density is required, for if it burnt too 

 quickly most of the heat would disappear through the chimney. 



