Some properties increase directly with increase in specific 
gravity while others increase more rar>idly. Crushing strength parallel 
to groin and shrinkage are examples of properties that vary directly 
with the specific gravity. Modulus of rupture, on the other hand, varies 
from one species to another as the 1-1/4 power of the specific gravity. 
Other properties are related to specific gravity by equations of still 
higher powers; for instance, the exponent of specific gravity for the 
variation in hardness is 2-1/4. It is evident, therefore, that small 
differences in specific gravity may result in large differences in cer- 
tain strength properties. 
Specific gravity affords an index of strength also for dif- 
ferent pieces of the same species. In fact, the relationship is closer 
than that between the averages of different species. Furthermore, the 
curve representing the relationship of pieces within a species is 
usually of a slightly higher power than that representing the average 
values for different species. (See accompanying figures). 
Some species of wood contain relatively large amounts of resins, 
gums, and extractives, which, of course, add to the weight but do not 
contribute to the strength as would a like amount of wood substance. 
Furthermore, the different species of ^ood vary somewhat in the struc- 
tural arrangement of the fibers. For these reasons it is apparent that 
two species which may be identical in specific gravity may exhibit dif- 
ferent average strength characteristics. This fact is illustrated by 
the scattering of points in the accompanying figure showing the relation 
betweem mod\ilus of rupture and specific gravity. Hence the specific 
gravity relationship should be taken as a general trend rather than a 
perfectly uniform law. A departure from the general curve that applies 
to most species usually indicates some exceptional characteristic of a 
species, which may make it particularly desirable for certain use re- 
quirements. (The term extractives is used for the compounds that can 
be removed from the wood of some species by passing cold or hot ^ater, 
alcohol, or other solvent through it when it is in the form of sawdust. 
Extractives may be referred to in terms of the solvent used, such as hot- 
water extractives, for example.) 
Minimum Specific Gravity Requirement 
The minimum strength values that may be expected from random 
stock of any species may be materially raised .by eliminating a rela- 
tively small portion of the material. This is accomplished by fixing a 
minimum specific gravity requirement (table 2) .as one .of the specifica- 
tions for aircraft wood, thus rejecting light-weight stock. The inspec- 
tion can usually be made satisfactorily by visual examination, but in 
certain cases it may be desirable or even necessary to resort to actual 
specific gravity determinations. Such determinations made from time to 
time are 'of value to aircraft inspectors in familiarizing them with the 
relation between appearance and specific gravity. 
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1303 
