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LIFE
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131
oils formed in their bodies.
These oils, being lighter than”
water, tend to diminish the weight of the animals that
contain them—.
Those round globules of oil mentioned On
page 80 as diStinguishing characters in-the drifting eggS of
so many shesprobably tend to help in keeping the egg
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suspended in the water (Plate 29).
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But of all the modes of obtaining buoyancy the indirect
methods of producing the same effect as a reduction in
specic gravity are the most wonderful.
If we drop a stone
into water, and watch it sinking, we shall notice that its
.sinking speed is considerably less than if it were falling
through air only.
The speed is reduced by the frictional
resistance set up by the stone as it moves through the
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water.
_This property
to the movement of a
body is known as “
viscosity.”
Some liquids are naturally
more “ viscous ” than others ; they are said to have a high
viscosity. 'Treacle is a very viscous uid ; many oils also
are
highly viscous.
ThuS the frictional' resistance to &
falling body would be greater in treacle than in sea water.
,
Now, of course, the total frictional resistance experienced
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by a falling body depends on the amount of area exposed
against the uid through which it is
-' We know
that 'a slate takes longer to sirik atwaÿs thanif it is on
edge.
Clearly then, if the frictional ré'sistancecan be made
innitely great compared with the actual Weight of the body
itself, a stage Will be reached at which it would counteract
the force due to gravity and, the body would no longer
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sink, but become suspended in the water.
The structure of many plankton organisms shows an
attempt to increase the frictional resistance, which eVidently
succeeds in keeping the organism almost suspended in the
water.
Although they cannot completely counteract the
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effect of gravity they'can, nevertheless, come very near
it
so that sinking is
slow.
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