able mainly on this basis. Thus, wheat and clovei- may grow side by side; the ash of the wheat will contain ()7.5% of silica, while that of the clover cojitains only 2.5%. This selective absorption must, however, be in part referred to the power pos.sessed by protoplasm of regulat- ing the admission of solutes.
(2) Water. Entrance of water into the plant to supply losses by evaporation or consumption depends upon similar factors. In a living mature cell, the protoplasm usually lies in a thin layer close to the cell-wall and envelops a water-filled space, the vacuole. (See Growth.) Many substances are constructed by plants which cannot ordinarily pass through the protoplasm, and remain dissolved in the water of the vacuole or cell-sap. These substances exert upon the surrounding layer of protoplasm a definite pres- sure. If tney were in gaseous form this would be their gas pressure. As the}' are dissolved, it is called their osmotic pressure. The osmotic pressure of solutes in the water outside the plant is usually less than that of solutes in the cell-sap. As the solvent moves towaril the region of higher osmotic pressure, i.e., from a place where there is a greater number of water molecules in unit-space, to a place where there are fewer, water usually enters the plant. But if at any time the conditions are reversed, the solutes outside the plant having higher osmotic pressure than those inside, water will leave the plant. This happens in nature sometimes, and it is this condition that makes possible the destruction of weeds by common salt. Gases are absorbed in the same manner as solids ; the a])parent difl'erence in their absorption by land plants is due tti the fact that they mostly become dissolved (and so fitted for absorption) only when they come into contact with the water saturating the cell-wall. This condition among the larger land plants exists only in the walls of cells bordering intercellular spaces. ( See Ai:R.Tiox.) While land plants absorb gases chiefly from the atmosphere, doubtless some ab- sorb them by the roots, notably the oxygen re- quired for their own respiration.
ABSORPTION, Klectrical. A phenomenon
observed in electrical condensers (q.v. ), in which
the dielectric or insulating material between the
conductors is non-homogeneous, e.g., a piece of
glass. It is noted that if such a condenser is
charged, then discharged and allowed to stand
for a short time, there will appear another charge.
If this is discharged, another charge will soon
appear. These secondary charges are said to be
due to electrical absorption. See Electricity.
ABSORPTION OF Gases. The phenom-
enon of the taking up or absorbing of gases by
liquids and solids. Tiie number of cubic centi-
meters of a gas which can be absorbed by one
cubic centimeter of a given liquid at 1.5° C.
is called the "absorption coeflicient" of the liquid
for the gas. The absoiption coefficient of water
for ammonia is T.'iO: for carbon dioxide, 1.0;
for chlorine, 2.4. The mass of the gas absorbed
varies directly as the jjressure: so, if a gas is
forced into a liquid under high pressure, and if
the pressure is afterward released, the gas will
be evolved. This is what ha])pens in the case
of beer and aerated waters. The .ibsor))tion of
gases by solids is called occlusion. The most
conspicuous illustration of this is the power of
palladium to occlude nine hundred times its own
volume of hydrogen.
ABSORPTION OF Waves. Waves of any
kind in any medium carry energy with them;
and, if the energy decreases, the medium is said
to absorb it or to exhibit "absorption." Thus,
if white light falls upon red glass, i.e., if ether-
waves which affect the normal luiman eye with
the sensation "white" are incident upon glass
which appears red to the same eye, all the waves
except those which produce the sensation red
are absorbed by tk2 glass, while the others are
transmitted. ISodies differ greatly in the qual-
ity and quantity of their absorptive power ; bvit
it is a general law that the absorptive power of
a body equals its emissive power under the same
conditions. (See Kahiation.) Absorption is
due to the presence in the pure medium carrying
the waves of some portions of matter whose own
natural period of vibration is the same as that
of the period of the waves; and. therefore, these
portions of matter are set in vibration by "reso-
nance" ( q.v. ) . Thus, if a person sings a pure
note near a piano it may be observed that the
particular string of the piano which of itself
gives the same note is set in vibration by the
air-waves sent out by the singer.
If air-waves of any length fall upon a soft
bod}', such as a cushion or a curtain, there is
absorption, as is shown bj' the fact that the re-
flected waves are nuich less intense than the
incident waves. The energy thus absorbed is
not spent in emitting other waves, but is dis-
sipated throughout the body producing heat
effects. Similarly, if ether-waves fall upon an
absorbing body, the energy absorbed is dissi-
pated in general throughout the smallest par-
ticles of the body producing heat effects. See,
however. Fluorescence.
AB'STINENCE. See Past.
AB'STINENCE SOCFETIES. Associations
to iiromotc total abstinence from alcoholic
liquors as beverages. See Temperance.
ABSTRACTION (Lat. a 6s, away -f trahere,
to draw). In logic, the jirocess by which the
mind separates out marks or characteristics
which are similar in various objects, and disre-
gards the marks or characteristics by which the
objects differ. It also occurs where characteris-
tics of particular objects, or classes of objects,
are replaced by a more general characteristic.
An instance of the first kind is the formation of
the class "biped" by the inclusion of all two-
legged animals. An instance of the second type
is the substitution of the general mark "repro-
duction" for the more special marks, "vivipa-
rous," "oviparous," "fissipaious," etc. The re-
sult of this process is also called an abstraction,
or, if it appears as a word, a concept. The psy-
chology of abstraction consists in describing the
way in which the attention, in passing from one
object to another, fastens upon an element com-
mon to all and dissociates it from its context.
Abstraction is carried out in a state of active
attention (see Attention ), as when the phi-
lologist searches out common or allied roots in
different languages, or when the geologist iden-
tifies strata in different localities and forms the
abstraction of a single ei)och in which they were
laid. The process is, however, facilitated by
the sheer decay of mental complexes; a decay
which obliterates small differences and reduces
mere similarity to indistinguishableness. It
this comes about that we form sketchy, "ab-
stract" images — as of "pen," "house," or "book"
