1 01- TECHNICAL IMIOPKKTIP^S OF WOOD. 



woods by comparing their prices forms no absolute criterion of 

 their heating effect, for many other factors come into the 

 question. 



The average ratios of the heating-powers of coal, lignite, 

 and wood may be placed as the figures 2*6 : 2*14 : 0*5. The 

 results of the physical experiments, even more those of the 

 chemical experiments, as regards the heating-power of wood are 

 of doubtful value, contradicting as they do every-day experience, 

 and average results taken from many repeated experiments are 

 therefore desirable. Even were the exact heating-power of the 

 woods to be correctly determined, these results would only be of 

 limited use in actual practice, where the practical heating-power 

 is always found to be much less than theory states it to be, and 

 the loss of theoretical heating-power varies with the diflferent stoves 

 used. This loss is chiefly due to the defects of ordinary stoves 

 as compared with those used for determining the heating-power 

 of combustibles, and the absence in ordinary use of precautions 

 taken in experimental burning. The loss of heat which escapes 

 with the varying draughts of air up the chimneys also varies 

 greatly, and so does the dampness of the wood actually used for 

 fuel. As a matter of fact about half the heating-power of 

 combustibles is lost owing to the way in which they are 

 burned. 



The groups below represent the relative heating-powers of 

 equal volumes of different woods, arranged according to com- 

 mon experience by burning them in stoves used for heating 

 rooms. 



i. Best heating Woods. — Hornbeam, beech, birch, Turkey-oak, 

 mountain-pine from high altitudes, robinia, old resinous ^^cotch 

 pine, black pine. 



ii. Good heating Woods. — Sycamore, ash, common elm, resinous 

 birch, ordinary Scotch pine, oak. 



iii. Moderately heating Woods. — Wych-elm, spruce and silver- 

 fir, sweet chestnut, Ceuibraii ]niiv. 



iv. Badly heating Woods. — Wcymouth-pine, lime, alder, fallen 

 oak-branches, aspen, poplar, willow. 



Woods vary also in the manner with which they burn ; some 

 give out much smoke and soot, as resinous coniferous wood 



