SPECIFIC WEIGHT. 51 



fall of the barometric column, neither can silviculture nor 

 wood industries be based on the sp. weight of wood. 



Specific weight is the ratio of the weight of a given volume 

 of any substance to that of the same volume of water, so that, 

 if the sp. weight of water be 100, woods with a sp. weight over 

 100 will sink in water, and those in which it is under 100 will 

 float in water. 



According to Sachs, the cell-walls of wood have a sp. 

 weight of 156. Hartig * found this to be true for most woods, 

 especially for oak, beech, birch, and spruce, and showed that 

 there is no difference in this respect between the cell-walls of 

 sapwood and heartwood. As in the tissues of wood there are 

 innumerable air-bearing, closed luniina, it is evident that, in 

 spite of the high sp. weight of the cell-wall, the sp. weight of 

 wood is so low, that most woods float in water. 



The specific weight of green wood is that of the standing or 

 recently felled tree, but as the volume of water varies in stand- 

 ing trees, and water begins to evaporate from felled trees as soon 

 as they are felled, the weight of green wood is very variable. 

 As water ascends chiefly in the last-formed layers of sapwood, 

 that is the wettest part of a tree and usually heavier than 

 water, with which its lumina are full. Tbe next outer layers 

 of sapwood are also wetter and heavier than its inner layers. 



Heartwood in a green state is always lighter than sapwood, 

 even when, as in broadleaved trees, it is very wet. The heart- 

 wood of freshly felled conifers is always much lighter than their 

 sapwood, for there is 35 per cent, more water in the latter. 

 Fifty per cent, of the weight of sapwood is that of the con- 

 tained water, while coniferous heartwood contains only 15 per 

 cent, of water (by weight). The greater the proportion of 

 heartwood there is in a tree the lighter its wood, so that the 

 whole stern decreases in weight as it becomes older. When it 

 is assumed that there is 45 per cent, of water in green wood, 

 no account is taken of the above fact, nor of the variation in 

 the wetness of the sapwood at different seasons in a year. 



E. Hartig (<>j>. fit.) has studied the seasonal variations of 

 water in wood, but the results he has arrived at are not given 



* 11. Hiirtig, "Uber die Verteilung der organ ischen Substun/ <ics Wassers u. 

 Luftraumes in tk-ii I'iiumeii." Berlin, 1882. 



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