2 BULLETIN 1136, IT, S. DEPARTMENT OF AGRICULTURE. 
pends upon the quantity present and the purpose for which the 
wood is to be used. Rarely, except for test pieces, is it necessary 
or desirable to remove all the moisture, producing an oven-dry or 
bone-dry condition. 
Moisture in wood is commonly called sap. There is no uni- 
versally accepted definition of this word " sap," and its use causes 
much confusion. The moisture, or sap, in both sapwood and 
heartwood consists almost entirely of water. It does contain, 
however, small percentages of organic and mineral matter. In 
the sapwood these substances are principally sugars of various 
kinds, and in the heartwood they include tannins, coloring matter, 
and various other chemicals. For the purposes of this bulletin, 
sap will be considered to be water only. 
TVater occurs in wood in two distinct forms, spoken of as " free " 
water and " imbibed " water. The free water exists in the cell 
cavities and the imbibed water in the cell walls. Imagine each 
cell of the wood to be a small bucket of some porous or absorbent 
material. If this bucket is filled with water, a certain amount 
will be absorbed by the sides and bottom, in addition to the " pail- 
ful" inside the bucket. This pailful is free water, that absorbed 
by the walls is imbibed water, and the sum of the two represents 
all the water the bucket, or the cell, can hold. A portion or all 
of the free water can be removed from the cell without changing 
the amount of imbibed water in the walls; but when the bucket 
is empty further drying removes water from the walls themselves 
and they begin to dry out. This point at which the bucket becomes 
empty is called the "fiber-saturation point." It has a very im- 
portant bearing upon the process of drying and will be discussed 
more fully later. 
In most living trees there is some free water in both heartwood 
and sapwood. The amount varies considerably depending on a num- 
ber of factors. Thus, sapwood almost always contains more mois- 
ture than heartwood. The butt may contain much more than the top, 
as is evidenced by the sinker stock of redwood and sugar pine. The 
season of year in which the trees are felled may have some influ- 
ence upon the moisture in the sapwood, but this influence is not very 
important. There are a number of instances on record in which 
there was more moisture present in the sapwood in winter than in 
slimmer. The common conception is that the reverse is true. 
Species and locality of growth have an important bearing upon 
the amount of moisture in the living tree. Species growing in 
swampy regions are apt to contain much more moisture and to be 
harder to dry than similar upland species. The oaks are an excellent 
illustration of this fact. On the other hand, certain species contain 
comparatively large amounts of water, even though growing under 
reasonably dry conditions. All of these variations must be taken 
into consideration in the drafting of drying schedules and in the 
actual drying operation. 
MOISTURE DETERMINATION. 
To dry stock successfully and to know when it has reached the 
proper dryness, it is essential that the operator be able to determine 
the amount of moisture in wood at anv time. There are several 
