Harrisburg telegraph. [volume] (Harrisburg, Pa.) 1879-1948, 1919-11-29
Scientific Discussions
by Garrett P. Seryiss

I Why do raindrops fall individu-
ally. not touching each other while
/ in descentI When we observe
; light rain we easily see each drop
as distinct as though coming through
monster sieve held by Il master
hand. And as much we can say for
n heavy rain. What causes that in
( dividualism of the raindrops?-i.
w"
/ To answer that questions we must
begin by considering the way in
which raindrops are formed. When
the air is supersaturated with mois-
ture the latter condenses around
extremely small cores, such as float
ing particles of miscoscopic dust. or
electric nuclei, and thus creates
minute globules of water so light
that they continue to float in the
air. and so numerous that they form
clouds. Although, seen as n whole
from below, they seem to form n
uniform mass. vet the spaces be-
tween the cloud particles are really
large compared with the diameters
of the particles.
However. partly perhaps through
electric attraction, and partly
through being brought into contact
by motions of the air, the particles
enlarge until they become so heavy
that the gravitational attraction of
/ the earth overcomes the buoyant
force OF the air. and then they des-
cend as raindrops.
When they have attained the size
of raindropS, which are but a small
fraction of an inch in diameter
one-tenth inch perhaps for the larg-
est - the retention OF their indi-
Eviduality is probably due in part to
electric repulsion, all having acquir-
ed a similar charge but mainly to
the fact that if they grow too large
by combination they are broken up
by the force of gravity overcoming
their cohesion.
One OF the curious effects of the
cohesion attraction of the mole
cules of a liquid is the production
of surface tension" which acts
as if an elastic film. like an in
visible and immeasurably thin
rubber membrane. We're spread over
the surface of the liquid.
Surface tension is due to the un-
balanced attraction of the liquid
upon those of its molecules which
lie so near the surface that n part
of their sphere of attraction is above
it. In consequence of surface ten
sion a small quantity of water sus-
pended in the air takes the form of
a spherical drop. But if the drop is
too large and heavy the force of
gravity overcomes the surface ten
sion. the intensity of which varies
for different liquids, and the drop is
broken up, or its formation is pre-
vented
The cohesion and surface tension
of mercury, the only metal that re-
mains in a liquid state at ordinary
temperatures, are much greater than
those of water, and you can see the
effect of the difference by sprinkling
water and mercury separately upon
! f1attened, convex drops. while the
mercury take the shape of little
/ spheres. running freely about. Part
| of this difference is due to the great
er adhesion of the water than of
the mercury to the glass.
! Whether there is any fundamen-
! tai difference between the force of
sravitation and that which pro-
iduces cohesive attraction between
1.-THE particles of a substance is an
unsettled question, but both forces
when acting without outside inter
! ference caused a liquid mass to
! take a spherical form, because that
/ is the geometrical figure which
gives the minimum surface in pro-
portion to volume.
So we find that the heavenly
: bodies. including the earth, are more
. or less perfectly spherical, having
/ been once in Il. liquid or vaporous
state. In gaseous nebulae we appar-
entity see spherical suns forming.
which might be fancifully called
''nebula drops being created by the
! condensation of cloudlike matter.
Molecular attraction, the force to
/ which the formation of liquid drops
and the existence of surface tension
are due, also holds the molecules
! of solids together. and in this case it
produces the property that we call
tenacity. A heavy solid will hold
together as long as some force like
gravity does not overmatch it's te-
naCity.
In calculating the relative effect.
! iveness of gravitation and cohesion
| in holding the earth together, p. c..
TAFT found that gravitation is $4,000
! times as effeCtual as the cohesive
! force of sandstone and loo times as
! effectual as that of steel. But if the
/ earth were reduced to a sandstone
/ sphere about twenty-five miles in
radius, or 10 a steel sphere about
/ 400 miles in radius. gravitation and
cohesion would be about equally
effective in holding it together. The
I popularity of cohesion as compared
with gravitation, as Professor Bark
er remarks, is that cohesion is ex-
! erted only through immeasurably
/ 81.