50 ECONOMIC WOODS OF THE UNITED STATES 



reason any wood floats in water is because of the buoyancy of the 

 air imprisoned in its elements and spaces. When this air is dis- 

 placed by water the wood becomes "waterlogged," and will no 

 longer float. The greater the proportion of cell wall the greater 

 the density; consequently late wood is denser and of higher specific 

 gravity than early wood, and the greater the proportion of late 

 wood the denser the wood as a whole. Woods composed largely 

 of thick-walled, narrow-lumined fibres are always dense and 

 heavy. Other things being equal, the weight of wood is a good 

 criterion of its hardness and strength. 



In practice the weight of wood is calculated from small, 

 sound specimens which have been oven-dried at a temperature of 

 100° C. (the boiling-point of water) until they reach a constant 

 weight. Since weight is subject to wide variations, the single 

 value usually assigned to a species is really the average of a large 

 number of determinations and is applicable only in a general way. 

 If a wood weighs less than thirty pounds per cubic foot it is con- 

 sidered light; if between thirty and forty pounds, medium light 

 or medium heavy; and if more than forty pounds, heavy. 



The lightest wood in the United States is that of Leitneria 

 floridana, the specific gravity of which is 0.21 for body wood and 

 0.15 for root wood. The wood of Condalia ferrea has a specific 

 gravity of 1.3; that of Guaiacum sanctum 1.14. From the inves- 

 tigation of 429 American species, as published in the report of 

 the Tenth Census of the United States, it appears that 242 species, 

 including most of the commercial woods, lie between 0.45 and 

 0.75 in specific gravity. 



TABLE III 



One Hundred and Fifty Trees of the United States Arranged in 

 Order of the Average Specific Gravity of Their Dry Woods 

 (Tenth Census). 



Species Sp. Gr. 



Quercus prinoides 86 



Quercus chrysolepis 85 



Hicoria alba S4 



Ostrya virginiana 83 



Sp. Gr. 



Condalia ferrea 1 . 30 



Guaiacum sanctum 1 . 14 



Quercus virens 95 



Quercus texana 91 



wood to an equal volume of water at its greatest density, which occurs at a 

 temperature of 4° C. (39.2° F.). A cubic foot of pure water at this temperature 

 weighs 62.43 pounds. Dividing the weight in pounds of a cubic foot of 

 wood by 62.43 will give the specific gravity of the wood. 



