SPECIFIC GRAVITY. 31 



5. Determination of the Specific Gravity of Wood. 



The determination of the specific gravity of a piece of wood 

 consists merely in measuring its absolute weight in grams 

 and its volume in cubic centimeters, and dividing the former 

 by the latter.* The absolute weight of a piece of wood is 

 measured by the chemical balance, and its volume by the 

 xylometer.t Owing to the important proportion that the con- 

 tained water always bears to even air-dried wood, the determination 

 of its degree of moisture is highly important in ascertaining its 

 weight. This is most variable in forest-dried wood, although 

 air-dried wood also varies in this respect ; as wood is usually 

 employed in this latter condition, figures of the specific gravity 

 of wood, especially when averages are employed, usually refer to 

 air-dried wood. 



The determination of the specific gravity of woods is usually 

 undertaken for small pieces only. More recently, however, 

 larger pieces have been used for this purpose, and the specific 

 gravity determined for different parts of a tree. If the object is 

 to ascertain the average specific gravity of a whole stem, the 

 simplest method is to divide it into a number of transverse 

 pieces of equal length and ascertain the specific gravity of each, 

 after it has been thoroughly air-dried, and then take the aver- 

 age specific gravity of them all as that of the stem. 



From a consideration of the preceding paragraphs regarding 

 the specific gravity of wood, it is evident that, in dealing with 

 different species of trees, only average figures can be taken ; for 

 the specific gravity of any kind of wood ranges within wide 

 limits, independently of the difference between that of heartwood 

 or sapwood, the upper and lower parts of stems, &c. 



The following table gives the highest and lowest specific 

 gravity of diff'erent woods, as well as its average value. Although 

 such figures can only be of relatively small utility, they give the 

 approximate order of the species as regards the weight of their 

 stem-wood. The results are derived from observations made 

 by Nordlinger, Baur, R. Hartig, Exner, v. Seckendorff and 



[* Or by dividing its weight in thmisand ounces by its volume in cubic feet. — 

 Tu.] 



t TiV^c Schlich's Man. of Forestry, Vol. III., p. 27. 



