i»S TECHNICAL PROPERTIES OF WOOD. 



woody substance is the same for all species of trees, and the 

 rxperimeuts as to its heating-power made hy Brix, attest its equal 

 lieatiug-power in all cases. The circumstances which affect the 

 heating-powers of the different woods will therefore be considered 

 here. 



1. The Specific Gravity of Wood. 



The heating-power is usually proportional to the amount of 

 solid woody substance, and [unless, as in certain Indiati icoods, 

 mineral matter is deposited in the lamina of the vessels. — Tr.] 

 this is directly proportional to the specific gravity of the wood; 

 so that for a given volume, heavy woods give out more heat than 

 light woods. This is not, however, always the case, and it cannot, 

 therefore, be laid-down as a general rule that the heating-power 

 of wood is proportional to its specific gravity; there are excep- 

 tions to this rule which may be explained, partly, by the am.ount of 

 mineral matter some woods contain, partly, by the fact that wood 

 of any species used for burning is frequently of a different quality 

 to that used for determining its specific gravity, and also owing 

 to the varying amount of resin contained in different woods. 



Oakwood forms a well-known exception to this rule, for 

 although it is usually heavier than the wood of beech, birch, and 

 maple, it possesses less heating-power than these woods. It 

 should, however, be noted that we use only inferior qualities of 

 oakwood for burning, good oak being always used for timber. 

 The specific gravity of oakwood is, however, always calculated for 

 perfectly sound wood, and varies between 0*53 and 1'05, show- 

 ing differences up to 100 per cent., so that much oakwood is 

 really lighter than the above-named woods, which may explain 

 the apparent anomaly. 



[In the W. Himalayas, the best fuel is supplied by the heavy wood of 

 the evergreen oaks, (J. Ilej-,semicarpl folia, dilatata, &c., the maxinuuii 

 weights of which per cubic foot are given in Gamble's Manual of 

 Indian Timbers as G8, 54, and 61 lbs. corresponding to specific gravi- 

 ties of 1-08, -80 and -98.— Tu.] 



Although the average specific gravity of the different species 

 of woods is no conclusive guide to their heating-power, it may 

 be aflSrmed that the heating-power of wood of the same species 



