MECHANICAL PROPERTIES OF WOODS GROWN IN UNITED STATES. 21 
after which the deflections increase more rapidly than the increase 
in load. 
A timber stressed beyond the elastic limit will not resume its 
original form immediately upon the removal of the load. 
ELASTICITY. 
Elasticity is the property (possessed by most materials) of chang- 
ing form with the application of force and recovering at once upon 
release from the force. 
In any elastic material the amount of compression or deformation 
is proportional to the force applied. 
Air and other gases under compression are elastic. The most 
commonly recognized elastic material is rubber. Timber is elastic 
within comparatively narrow limits. 
The term "very elastic" as applied to wood is indefinite, because it 
may mean that the force required to produce a given deformation is 
great and the recovery sudden as in an ivory ball (see " Modulus of 
elasticity"); or that the amount of distortion to the elastic limit is 
great, as in a rubber ball, or that the wood possesses high elastic 
resilience, a combination of the two properties. (See " Elastic resili- 
ence" or "Work to the elastic limit.") 
FIBER SATURATION POINT. 
(Seep. 11.) 
Green wood usually contains water within the cell walls and "free" 
water in the pores. In drying, the water in the pores is the first to 
be evaporated. The fiber saturation point is that point at which no 
water exists in the pores of the timber but at which the cell walls are 
still saturated with moisture. The fiber saturation point varies with 
the species. The ordinary proportion of moisture — based on the dry 
weight of the wood — at the fiber saturation point is from 20 to 30 
per cent. 
FIBER STRESS AT ELASTIC LIMIT. 
(See pp. 13, 15, and 16.) 
Fiber stress at elastic limit is the stress obtained in a timber by 
loading it to its elastic limit. It is the greatest stress the timber 
will take under a given loading and immediately return to its former 
position. 
FLEXIBILITY. 
Flexibility is that quality which renders a material capable of 
being bent without breaking. Thus, green timber is more flexible 
than dry. 
GREEN. 
Green is the condition of timber as taken from the living tree. 
Immediately upon being sawed from the tree lumber begins to 
lose moisture and otherwise change its condition. The rapidity of 
these changes is determined by the species, humidity, and circu- 
lation of air, heat, etc. 
