DENSITY OF WOOD SUBSTANCE AND POROSITY OF 
WOOD 
By Frederick Duneap, 
Forest Assistant , Forest-Products Laboratory , Forest Service 
PURPOSE OF THE INVESTIGATION 
The investigation the results of which are presented in this paper was 
made to determine the density of the lignocellulose which makes up the 
walls of the cells of which wood is composed as a basis for calculating 
the porosity of wood. 1 The density of a block of wood differs from that 
of the lignocellulose of which it is built up, because wood is a porous 
structure and is not a homogeneous solid. Lignocellulose has a density 
greater than unity and sinks in water; blocks of wood float, especially 
when dry, because the air they contain buoys them up. 
The volume of a block of wood is thus made up of two parts, a frame¬ 
work of lignocellulose and a large number of small cavities inclosed in 
the former. The cavities may be occupied by a variety of fluids, incrust- 
ing materials, and solid substances. The ratio of the sum of the volumes 
of these cavities to the volume of the entire block is the measure of the 
porosity of the wood. The volume of the entire block is readily ascer¬ 
tained (by submersion, for example), so that the determination of poros¬ 
ity resolves itself into a determination of the volume of the cavities. 
The volume of the cavities may be measured directly by filling them 
with a fluid of known density; actually such measurements have never 
met with entire success. The same result can also be reached indirectly 
by determining the density of the wood substance and then calculating 
its volume in a block of wood; the difference between the gross volume 
of the block and the volume of the solid substance is the volume of the 
cavities. 
RELATION TO TECHNICAL PROBLEMS 
If the heat conductivity of wood is greater than that of water, the 
cavities in wet wood offer an impediment to the flow of heat. If the 
conductivity of water is greater, the flow of heat is accelerated by its 
presence in the cavities. Air in the cavities of dry wood will likewise 
affect heat conductivity. In other words, the conductivity of a block 
of wood is in part a function of its porosity. The true significance of 
heat conductivity when measured on blocks of wood can be properly 
interpreted only when the porosity is known and when the extent to 
1 The research work was carried on at the Forest-Products Laboratory, maintained by the Forest Serv¬ 
ice in cooperation with the University of Wisconsin, at Madison, Wis. 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C. 
(423) 
Vol. II, No. 6 
Sept. 21, 1914 
F-a 
