PROPERTIES OF WOOD.
There are many properties of wood,--some predominant in one species,
some in another,--that make it suitable for a great variety of uses.
Sometimes it is a combination of properties that gives value to a
wood. Among these properties are hygroscopicity, shrinkage, weight,
strength, cleavability, elasticity, hardness, and toughness.
THE HYGROSCOPICITY[1] OF WOOD.
It is evident that water plays a large part in the economy of the
tree. It occurs in wood in three different ways: In the sap which
fills or partly fills the cavities of the wood cells, in the cell
walls which it saturates, and in the live protoplasm, of which it
constitutes 90 per cent. The younger the wood, the more water it
contains, hence the sap-wood contains much more than the heart-wood,
at times even twice as much.
In fresh sap-wood, 60 per cent. of the water is in the cell
cavities, 35 per cent. in the cell walls, and only 5 per cent. in the
protoplasm. There is so much water in green wood that a sappy pole
will soon sink when set afloat. The reason why there is much less
water in heart-wood is because its cells are dead and inactive, and
hence without sap and without protoplasm. There is only what saturates
the cell walls. Even so, there is considerable water in heart-wood.[2]
The lighter kinds have the most water in the sap-wood, thus sycamore
has more than hickory.
Curiously enough, a tree contains about as much water in winter as
in summer. The water is held there, it is supposed, by capillary
attraction, since the cells are inactive, so that at all times the
water in wood keeps the cell walls distended.
THE SHRINKAGE OF WOOD.
When a tree is cut down, its water at once begins to evaporate. This
process is called "seasoning."[A] In drying, the free water within the
cells keeps the cell walls saturated; but when all the free water has
been removed, the cell walls begin to yield up their moisture. Water
will not flow out of wood unless it is forced out by heat, as when
green wood is put on a fire. Ordinarily it evaporates slowly.
[Footnote A: See _Handwork in Wood_, Chapter III.]
The water evaporates faster from some kinds of wood than from other
kinds, _e.g._, from white pine than from oak, from small pieces than
from large, and from end grain than from a longitudinal section; and
it also evaporates faster in high than in low temperatures.
Evaporation affects wood in three respects, weight, strength, and
size. The weight is reduced, the strength is increased, and shrinkage
takes place. The reduction in weight and increase in strength,
important as they are, are of less importance than the shrinkage,
which often involves warping and other distortions. The water in wood
affects its size by keeping the cell walls distended.
If all the cells of a piece of wood were the same size, and had
walls the same thickness, and all ran in the same direction, then the
shrinkage would be uniform. But, as we have seen, the structure of
wood is not homogeneous. Some cellular elements are large, some small,
some have thick walls, some thin walls, some run longitudinally and
some (the pith rays) run radially. The effects will be various in
differently shaped pieces of wood but they can easily be accounted for
if one bears in mind these three facts: (1) that the shrinkage is in
the cell wall, and therefore (2) that the thick-walled cells shrink
more than thin-walled cells and (3) that the cells do not shrink much,
if any, lengthwise.
(1) The shrinkage of wood takes place in the walls of the cells that
compose it, that is, the cell walls become thinner, as indicated by
the dotted lines in Fig. 35, which is a cross-section of a single
cell. The diameter of the whole cell becomes less, and the opening, or
lumen, of the cell becomes larger.
[Illustration: Fig. 35. How Cell Walls Shrink.]
(2) Thick-walled cells shrink more than thin-walled cells, that is,
summer cells more than spring cells. This is due to the fact that they
contain more shrinkable substance. The thicker the wall, the more the
shrinkage.
Consider the effects of these changes; ordinarily a log when drying
begins to "check" at the end. This is to be explained thus:
Inasmuch as evaporation takes place faster from a cross than from a
longitudinal section, because at the cross-section all the cells are
cut open, it is to be expected that the end of a piece of timber, Fig.
36, A, will shrink first. This would tend to make the end fibers bend
toward the center of the piece as in B, Fig. 36. But the fibers are
stiff and resist this bending with the result that the end splits or
"checks" as in C, Fig. 36. But later, as the rest of the timber dries
out and shrinks, it becomes of equal thickness again and the "checks"
tend to close.
[Illustration: Fig. 36. The Shrinkage and Checking at the End of a
Beam.]
(3) For some reason, which has not been discovered, the cells or
fibers of wood do not shrink in length to any appreciable extent. This
is as true of the cells of pith rays, which run radially in the log,
as of the ordinary cells, which run longitudinally in it.
In addition to "checking" at the end, logs ordinarily show the effect
of shrinkage by splitting open radially, as in Fig. 37. This is to
be explained by two factors, (1) the disposition of the pith (or
medullary) rays, and (2) the arrangement of the wood in annual rings.
[Illustration: Fig. 37. The Shrinkage and Splitting of a Log.]
(1) The cells of the pith rays, as we have seen in Chapter I, run at
right angles to the direction of the mass of wood fibers, and since
they shrink according to the same laws that other cells do, viz., by
the cell wall becoming thinner but not shorter, the strain of their
shrinkage is contrary to that of the main cells. The pith rays, which
consist of a number of cells one above the other, tend to shrink
parallel to the length of the wood, and whatever little longitudinal
shrinkage there is in a board is probably due mostly to the
shrinkage of the pith rays. But because the cells of pith rays do not
appreciably shrink in their length, this fact tends to prevent the
main body of wood from shrinking radially, and the result is that wood
shrinks less radially than tangentially. Tangentially is the only way
left for it to shrink. The pith rays may be compared to the ribs of
a folding fan, which keep the radius of unaltered length while
permitting comparative freedom for circumferential contraction.
(2) It is evident that since summer wood shrinks more than spring
wood, this fact will interfere with the even shrinkage of the log.
Consider first the tangential shrinkage. If a section of a single
annual ring of green wood of the shape A B C D, in Fig. 38, is dried
and the mass shrinks according to the thickness of the cell walls,
it will assume the shape A' B' C' D'. When a number of rings together
shrink, the tangential shrinkage of the summer wood tends to contract
the adjoining rings of spring wood more than they would naturally
shrink of themselves. Since there is more of the summer-wood
substance, the spring-wood must yield, and the log shrinks
circumferentially. The radial shrinkage of the summer-wood, however,
is constantly interrupted by the alternate rows of spring-wood, so
that there would not be so much radial as circumferential shrinkage.
As a matter of fact, the tangential or circumferential shrinkage is
twice as great as the radial shrinkage.
[Illustration: Fig. 38. Diagram to Show the Greater Shrinkage of
Summer Cells, A, B, than of Spring Cells, C, D.]
Putting these two factors together, namely, the lengthwise resistance
of the pith rays to the radial shrinkage of the mass of other fibers,
and second, the continuous bands of summer wood, comparatively free to
shrink circumferentially, and the inevitable happens; the log splits.
If the bark is left on and evaporation hindered, the splits will not
open so wide.
There is still another effect of shrinkage. If, immediately after
felling, a log is sawn in two lengthwise, the radial splitting may be
largely avoided, but the flat sides will tend to become convex, as in
Fig. 39. This is explained by the fact that circumferential shrinkage
is greater than radial shrinkage.
[Illustration: Fig. 39. Shrinkage of a Halved Log.]
If a log is "quartered,"[A] the quarters split still less, as the
inevitable shrinkage takes place more easily. The quarters then
tend to assume the shape shown in Fig. 40, C. If a log is sawed into
timber, it checks from the center of the faces toward the pith, Fig.
40, D. Sometimes the whole amount of shrinkage may be collected in one
large split. When a log is slash-sawed, Fig. 40, I, each board tends
to warp so that the concave side is away from the center of the tree.
If one plank includes the pith, Fig. 40, E and H, that board will
become thinner at its edges than at its center, _i.e._, convex on
both faces. Other forms assumed by wood in shrinking are shown in Fig.
40. In the cases A-F the explanation is the same; the circumferential
shrinkage is more than the radial. In J and K the shapes are accounted
for by the fact that wood shrinks very little longitudinally.
[Footnote A: See _Handwork in Wood_, p. 42.]
[Illustration: Fig. 40. Shapes Assumed by Wood in Shrinking.]
Warping is uneven shrinkage, one side of the board contracting
more than the other. Whenever a slash board warps under ordinary
conditions, the convex side is the one which was toward the center of
the tree. However, a board may be made to warp artificially the other
way by applying heat to the side of the board toward the center of the
tree, and by keeping the other side moist. The board will warp only
sidewise; lengthwise it remains straight unless the treatment is very
severe. This shows again that water distends the cells laterally but
not longitudinally.
The thinning of the cell walls due to evaporation, is thus seen
to have three results, all included in the term "working," viz.:
_shrinkage_, a diminution in size, _splitting_, due to the inability
of parts to cohere under the strains to which they are subjected, and
_warping_, or uneven shrinkage.
In order to neutralize warping as much as possible in broad board
structures, it is common to joint the board with the annual rings
of each alternate board curving in opposite directions, as shown in
_Handwork in Wood_, Fig. 280, _a_, p. 188.
Under warping is included bowing. Bowing, that is, bending in the form
of a bow, is, so to speak, longitudinal warping. It is largely due to
crookedness or irregularity of grain, and is likely to occur in boards
with large pith rays, as oak and sycamore. But even a straight-grained
piece of wood, left standing on end or subjected to heat on one side
and dampness on the other, will bow, as, for instance a board lying on
the damp ground and in the sun.
[Illustration: Fig. 41. _a_, Star Shakes; _b_, Heart Shakes; _c_, Cup
Shakes or Ring Shakes; _d_, Honeycombing.]
Splitting takes various names, according to its form in the tree.
"Check" is a term used for all sorts of cracks, and more particularly
for a longitudinal crack in timber. "Shakes" are splits of various
forms as: _star shakes_, Fig. 41, _a_, splits which radiate from the
pith along the pith rays and widen outward; _heart shakes_, Fig. 41,
_b_, splits crossing the central rings and widening toward the center;
and _cup_ or _ring shakes_, Fig. 41, _c_, splits between the annual
rings. _Honeycombing_, Fig. 41, _d_, is splitting along the pith rays
and is due largely to case hardening.
These are not all due to shrinkage in drying, but may occur in the
growing tree from various harmful causes. See p. 232.
Wood that has once been dried may again be swelled to nearly if not
fully its original size, by being soaked in water or subjected to wet
steam. This fact is taken advantage of in wetting wooden wedges
to split some kinds of soft stone. The processes of shrinking and
swelling can be repeated indefinitely, and no temperature short of
burning, completely prevents wood from shrinking and swelling.
Rapid drying of wood tends to "case harden" it, _i.e._, to dry and
shrink the outer part before the inside has had a chance to do the
same. This results in checking separately both the outside and the
inside, hence special precautions need to be taken in the seasoning of
wood to prevent this. When wood is once thoroly bent out of shape in
shrinking, it is very difficult to straighten it again.
Woods vary considerably in the amounts of their shrinkage. The
conifers with their regular structure shrink less and shrink more
evenly than the broad-leaved woods.[3] Wood, even after it has been
well seasoned, is subject to frequent changes in volume due to the
varying amount of moisture in the atmosphere. This involves constant
care in handling it and wisdom in its use. These matters are
considered in _Handwork in Wood_, Chapter III, on the Seasoning of
Wood.
THE WEIGHT OF WOOD.
Wood substance itself is heavier than water, as can readily be proved
by immersing a very thin cross-section of pine in water. Since the
cells are cut across, the water readily enters the cavities, and
the wood being heavier than the water, sinks. In fact, it is the air
enclosed in the cell cavities that ordinarily keeps wood afloat, just
as it does a corked empty bottle, altho glass is heavier than water.
A longitudinal shaving of pine will float longer than a cross shaving
for the simple reason that it takes longer for the water to penetrate
the cells, and a good sized white pine log would be years in getting
water-soaked enough to sink. As long as a majority of the cells are
filled with air it would float.
In any given piece of wood, then, the weight is determined by
two factors, the amount of wood substance and the amount of water
contained therein. The amount of wood substance is constant, but the
amount of water contained is variable, and hence the weight varies
accordingly. Moreover, considering the wood substance alone, the
weight of wood substance of different kinds of wood is about the same;
namely, 1.6 times as heavy as water, whether it is oak or pine, ebony
or poplar. The reason why a given bulk of some woods is lighter than
an equal bulk of others, is because there are more thin-walled and
air-filled cells in the light woods. Many hard woods, as lignum vitae,
are so heavy that they will not float at all. This is because the wall
of the wood cells is very thick, and the lumina are small.
In order, then, to find out the comparative weights of different
woods, that is, to see how much wood substance there is in a given
volume of any wood, it is necessary to test absolutely dry specimens.
The weight of wood is indicated either as the weight per cubic foot or
as specific gravity.
It is an interesting fact that different parts of the same tree have
different weights, the wood at the base of the tree weighing more than
that higher up, and the wood midway between the pith and bark weighing
more than either the center or the outside.[4]
The weight of wood has a very important bearing upon its use. A
mallet-head, for example, needs weight in a small volume, but it
must also be tough to resist shocks, and elastic so as to impart its
momentum gradually and not all at once, as an iron head does.
Weight is important, too, in objects of wood that are movable. The
lighter the wood the better, if it is strong enough. That is why
spruce is valuable for ladders; it is both light and strong. Chestnut
would be a valuable wood for furniture if it were not weak, especially
in the spring wood.
The weight of wood is one measure of its strength. Heavy wood is
stronger than light wood of the same kind, for the simple reason that
weight and strength are dependent upon the number and compactness of
the fibers.[5]
THE STRENGTH OF WOOD.
Strength is a factor of prime importance in wood. By strength is meant
the ability to resist stresses, either of tension (pulling), or
of compression (pushing), or both together, cross stresses. When a
horizontal timber is subjected to a downward cross stress, the lower
half is under tension, the upper half is under compression and the
line between is called the neutral axis, Fig. 42.
[Illustration: Fig. 42. A Timber Under Cross Stress, Showing Neutral
Axis, and the Lines of Tension and Compression. A knot occurring in
such a timber should be in the upper half, as at A.]
Wood is much stronger than is commonly supposed. A hickory bar will
stand more strain under tension than a wrought iron bar of the same
length and weight, and a block of long-leaf pine a greater compression
endwise than a block of wrought iron of the same height and weight. It
approaches the strength of cast iron under the same conditions.
Strength depends on two factors: the strength of the individual
fibers, and the adhesive power of the fibers to each other. So, when
a piece of wood is pulled apart, some of the fibers break and some are
pulled out from among their neighbors. Under compression, however,
the fibers seem to act quite independently of each other, each bending
over like the strands of a rope when the ends are pushed together. As
a consequence, we find that wood is far stronger under tension than
under compression, varying from two to four times.
Woods do not vary nearly so much under compression as under tension,
the straight-grained conifers, like larch and longleaf pine, being
nearly as strong under compression as the hard woods, like hickory and
elm, which have entangled fibers, whereas the hard woods are nearly
twice as strong as the conifers under tension.
Moisture has more effect on the strength of wood than any other
extrinsic condition. In sound wood under ordinary conditions, it
outweighs all other causes which affect strength. When thoroly
seasoned, wood is two or three times stronger, both under compression
and in bending, than when green or water soaked.[6]
The tension or pulling strength of wood is much affected by the
direction of the grain, a cross-grained piece being only 1/10th to
1/20th as strong as a straight-grained piece. But under compression
there is not much difference; so that if a timber is to be subjected
to cross strain, that is the lower half under tension and the upper
half under compression, a knot or other cross-grained portion should
be in the upper half.
[Illustration: Fig. 43. Shearing Strength is Measured by the Adhesion
of the Portion A, B, C, D or to the Wood on both sides of it.]
Strength also includes the ability to resist shear. This is called
"_shearing strength_." It is a measure of the adhesion of one part of
the wood to an adjoining part. Shearing is what takes place when the
portion of wood beyond a mortise near the end of a timber, A B C D,
Fig. 43, is forced out by the tenon. In this case it would be shearing
along the grain, sometimes called detrusion. The resistance of the
portion A B C D, _i.e._, its power of adhesion to the wood adjacent
to it on both sides, is its shearing strength. If the mortised piece
were forced downward until it broke off the tenon at the shoulder,
that would be shearing across the grain. The shearing resistance
either with or across the grain is small compared with tension and
compression. Green wood shears much more easily than dry, because
moisture softens the wood and this reduces the adhesion of the fibers
to each other.[7]
CLEAVABILITY OF WOOD.
Closely connected with shearing strength is cohesion, a property
usually considered under the name of its opposite, cleavability,
_i.e._, the ease of splitting.
When an ax is stuck into the end of a piece of wood, the wood splits
in advance of the ax edge. See _Handwork in Wood_, Fig. 59, p. 52. The
wood is not cut but pulled across the grain just as truly as if one
edge were held and a weight were attached to the other edge and it
were torn apart by tension. The length of the cleft ahead of the blade
is determined by the elasticity of the wood. The longer the cleft, the
easier to split. Elasticity helps splitting, and shearing strength and
hardness hinder it.
A normal piece of wood splits easily along two surfaces, (1) along any
radial plane, principally because of the presence of the pith rays,
and, in regular grained wood like pine, because the cells are radially
regular; and (2) along the annual rings, because the spring-wood
separates easily from the next ring of summer-wood. Of the two, radial
cleavage is 50 to 100 per cent. easier. Straight-grained wood is
much easier to split than cross-grained wood in which the fibers are
interlaced, and soft wood, provided it is elastic, splits easier than
hard. Woods with sharp contrast between spring and summer wood, like
yellow pine and chestnut, split very easily tangentially.
All these facts are important in relation to the use of nails. For
instance, the reason why yellow pine is hard to nail and bass easy is
because of their difference in cleavability.
ELASTICITY OF WOOD.
Elasticity is the ability of a substance when forced out of
shape,--bent, twisted, compressed or stretched, to regain its former
shape. When the elasticity of wood is spoken of, its ability to spring
back from bending is usually meant. The opposite of elasticity is
brittleness. Hickory is elastic, white pine is brittle.
Stiffness is the ability to resist bending, and hence is the opposite
of pliability or flexibility. A wood may be both stiff and elastic; it
may be even stiff and pliable, as ash, which may be made into splints
for baskets and may also be used for oars. Willow sprouts are flexible
when green, but quite brittle when dry.
Elasticity is of great importance in some uses of wood, as in long
tool handles used in agricultural implements, such as rakes, hoes,
scythes, and in axes, in archery bows, in golf sticks, etc., in all of
which, hickory, our most elastic wood, is used.[8]
HARDNESS OF WOOD.
Hardness is the ability of wood to resist indentations, and depends
primarily upon the thickness of the cell walls and the smallness
of the cell cavities, or, in general, upon the density of the wood
structure. Summer wood, as we have seen, is much harder than spring
wood, hence it is important in using such wood as yellow pine on
floors to use comb-grain boards, so as to present the softer spring
wood in as narrow surfaces as possible. See _Handwork in Wood_, p. 41,
and Fig. 55. In slash-grain boards, broad surfaces of both spring
and summer wood appear. Maple which is uniformly hard makes the best
floors, even better than oak, parts of which are comparatively soft.
The hardness of wood is of much consequence in gluing pieces together.
Soft woods, like pine, can be glued easily, because the fibers can
be forced close together. As a matter of fact, the joint when dry is
stronger than the rest of the board. In gluing hard woods, however, it
is necessary to scratch the surfaces to be glued in order to insure a
strong joint. It is for the same reason that a joint made with liquid
glue is safe on soft wood when it would be weak on hard wood.[9]
TOUGHNESS OF WOOD.
Toughness may be defined as the ability to resist sudden shocks and
blows. This requires a combination of various qualities, strength,
hardness, elasticity and pliability. The tough woods, _par
excellence_, are hickory, rock elm and ash. They can be pounded,
pulled, compressed and sheared. It is because of this quality that
hickory is used for wheel spokes and for handles, elm for hubs, etc.
In the selection of wood for particular purposes, it is sometimes one,
sometimes another, and more often still, a combination of qualities
that makes it fit for use.[10]
It will be remembered that it was knowledge of the special values
of different woods that made "the one horse shay," "The Deacon's
Masterpiece."
"So the Deacon inquired of the village folk
Where he could find the strongest oak,
That couldn't be split nor bent nor broke,--
That was for spokes and floor and sills;
He sent for lancewood to make the thills;
The cross bars were ash, from the straightest trees,
The panels of whitewood, that cuts like cheese,
But lasts like iron for things like these.
The hubs of logs from the "Settler's Ellum,"--
Last of its timber,--they couldn't sell 'em.
Never an ax had seen their chips,
And the wedges flew from between their lips,
Their blunt ends frizzled like celery tips;
Step and prop-iron, bolt and screw,
Spring, tire, axle and linch pin too,
Steel of the finest, bright and blue;
Thorough brace, bison skin, thick and wide;
Boot, top dasher from tough old hide,
Found in the pit when the tanner died.
That was the way to "put her through."
'There!' said the Deacon, 'naow she'll dew!'"
[Footnote 1: Hygroscopicity, "the property possessed by
vegetable tissues of absorbing or discharging moisture and
expanding or shrinking accordingly."--_Century Dictionary._]
[Footnote 2: This is shown by the following table, from
Forestry Bulletin No. 10, p. 31, _Timber_, by Filibert Roth:
POUNDS OF WATER LOST IN DRYING 100 POUNDS OF GREEN WOOD IN THE KILN.
Sap-wood or Heart-wood
outer part. or interior.
1. Pines, cedars, spruces, and firs 45-65 16-25
2. Cypress, extremely variable 50-65 18-60
3. Poplar, cottonwood, basswood 60-65 40-60
4. Oak, beech, ash, elm, maple, birch, hickory,
chestnut, walnut, and sycamore 40-50 30-40
]
[Footnote 3: The following table from Roth, p. 37, gives the
approximate shrinkage of a board, or set of boards, 100 inches
wide, drying in the open air:
Shrinkage
Inches.
1. All light conifers (soft pine, spruce, cedar, cypress) 3
2. Heavy conifers (hard pine, tamarack, yew, honey locust,
box elder, wood of old oaks) 4
3. Ash, elm, walnut, poplar, maple, beech, sycamore,
cherry, black locust 5
4. Basswood, birch, chestnut, horse chestnut, blue beech,
young locust 6
5. Hickory, young oak, especially red oak Up to 10
The figures are the average of radial and tangential
shrinkages.]
[Footnote 4: How much different woods vary may be seen by the
following table, taken from Filibert Roth, _Timber_, Forest
Service Bulletin No. 10, p. 28:
WEIGHT OF KILN-DRIED WOOD OF DIFFERENT SPECIES.
------------------------------------+---------------------------------
| Approximate.
+-----------+---------------------
| | Weight of
| +---------+-----------
| Specific | 1 cubic | 1,000 feet
| weight. | foot. | of lumber.
------------------------------------+-----------+---------+-----------
| | Pounds | Pounds
(a) Very heavy woods: | | |
Hickory, oak, persimmon, | | |
osage, orange, black | | |
locust, hackberry, blue | | |
beech, best of elm, and ash | 0.70-0.80 | 42-48 | 3,700
(b) Heavy woods: | | |
Ash, elm, cherry, birch, | | |
maple, beech, walnut, sour | | |
gum, coffee tree, honey | | |
locust, best of southern | | |
pine, and tamarack | .60-.70 | 36-42 | 3,200
(c) Woods of medium weight: | | |
Southern pine, pitch pine, | | |
tamarack, Douglas spruce, | | |
western hemlock, sweet gum, | | |
soft maple, sycamore, light | | |
sassafras, mulberry, | | |
grades of birch and cherry | .50-.60 | 30-36 | 2,700
(d) Light woods: | | |
Norway and bull pine, red | | |
cedar, cypress, hemlock, | | |
the heavier spruce and fir, | | |
redwood, basswood, chestnut, | | |
butternut, tulip, catalpa, | | |
buckeye, heavier grades of | | |
poplar | .40-.50 | 24-30 | 2,200
(e) Very light woods: | | |
White pine, spruce, fir, white | | |
cedar, poplar | .30-.40 | 18-24 | 1,800
------------------------------------+-----------+---------+-----------
]
[Footnote 5: For table of weights of different woods see
Sargent, _Jesup Collection,_ pp. 153-157.]
[Footnote 6: See Forestry Bulletin No. 70, pp. 11, 12, and
Forestry Circular No. 108.]
[Footnote 7: For table of strengths of different woods, see
Sargent, _Jesup Collection_, pp. 166 ff.]
[Footnote 8: For table of elasticity of different woods, see
Sargent, _Jesup Collection_, pp. 163 ff.]
[Footnote 9: For table of hardnesses of different woods, see
Sargent, _Jesup Collection_, pp. 173 ff.]
[Footnote 10: For detailed characteristics of different woods
see Chapter III.]
THE PROPERTIES OF WOOD.
REFERENCES[A]
Moisture and Shrinkage.
Roth, _For. Bull._, No. 10, pp. 25-37.
Busbridge, _Sci. Am. Sup._ No. 1500. Oct. 1, '04.
Weight, Strength, Cleavability, Elasticity and Toughness.
Roth, _For. Bull._, 10, p. 37-50.
Boulger, pp. 89-108, 129-140.
Roth, _First Book_, pp. 229-233.
Sargent, _Jesup Collection_, pp. 153-176.
Forest Circulars Nos. 108 and 139.
[Footnote A: For general bibliography, see p. 4.]