SPECIFIC GRAVITY. 29 



on the specific gravity of wood is however inconsiderable, even 

 in the case of floated wood, which after drying is not appreci- 

 ably lighter than wood which has not been floated. 



"Wood injected with creosote or metallic salts becomes heavier 

 than uninjected wood, and, according to Nordlinger, creosoted 

 Scotch pine and beechwood gains 17 to 18 per cent, in weight 

 over uninjected wood. 



4. The Different Parts of a Tree. 



The specific weight of wood varies for the different parts of a 

 tree. It may be laid-down as a general rule that branchwood is 

 heavier and rootwood lighter than stemwood. Exceptions occur 

 in the case of the branches of ring-pored wood, when these latter 

 have grown for some time in the shade, their wood then being 

 very light ; also in the case of roots of conifers * rich in resin, 

 which are often extremely heavy, rootwood of Scotch pine having 

 sometimes a specific gravity of 1*035. According to Xordlinger, 

 the specific gravity of roots is less, the finer the roots in 

 question. 



All wavy wood, burrs, knots of branches and sound growth 

 over wounds, is heavier than ordinary wood ; old knots in 

 conifers, with narrow annual zones, are frequently the heaviest 

 parts of the tree. 



As regards differences in weight between the older and 

 younger parts of the stem, a distinction must be made between 

 its upper and lower parts, and also between sapwood, heartwood 

 and imperfect heartwood. Regarding these three classes of 

 wood, no general law can be laid down. Premising that the 

 wood is air-dried, and its annual zones regular and of about 

 average breadth, in the case of oak, Scotch pine and larch, the 

 heartwood is heavier than the sapwood. In birch, the inner 

 zones are lighter than the outer ones, and in the case of many 

 species, such as spruce, no marked difference in weight exists 

 between the inner and outer zones. 



It will be easily understood that the nature and breadth of 

 the annual woody zones in different trees influence matters 



[* Termed torch-wood in India. — T::.] 



