South Bend news-times. (South Bend, Ind.) 1913-1938, 1917-02-13
Why " aterDrop on
a Hot StoUe Darts
around in a Ball
BY Garrett p. Serviss.

of red-hot metal will take the form
OF bead, at first racing around like
quicksilver and later becoming sta
tionary, and gradually disappear
ing? l have noticed that drops OF
they meet, unite into one large bead,
then become stationary. and at last
vanish in vapor.-F. G. b"
Your question leads into the
"fairyland of science" with which
few are as familiar as they ought
TO be for their own pleasure and
profit. The teachers of science today
seem to have wandered far from the
fiowery path that was pursued by
the greatest of them all who ever
used the English language, Prof
John Tyndall
If there was a John Tyndall in
every school nature would be far
less Of a sealed book than she is
To the majority of educated" peo-
pie He used to answer questions
like yours by means of experiments
and explanations that electrified and
fascinated his auditors. He would
have shown you how a drop of water
thrown upon q hot plate generated
enough vapor at its lower surface 10
lift it out of contact with the plate,
or. as he was accustomed, pictur-
esquely. TO put the statement. the drop
was Sustained by the recoil OF the
molecular projectiles discharged
from it's under surface"
The darting about is due to in
equalities in the forces acting upon
tn drop. Its final disappearance
arises from the gradual evaporation
of the water composing the drop.
But even more interesting than the
conduct of the water in cushioning
itself on its own vapor is the man
ner in which ii assumes a globular
shape, while, as you acutely noticed.
a number of racing drops when
meeting combine to form a single
larger drop.
The explanation of these phe-
nomena takes US into the sub
microscopic world of the mole
cules, where things are so small
that man may never succeed in &C
tually seeing them. although he per-
as IL they were visible.
The attraction of the molecules
for one another is the key of the
secret, together with the fact that
globular or spherical, figure is
the shape which includes the great
est volume within the smallest sur
face so that any collection of MOLE
cules when free to move among
themselves Gas they are in a liq-
uid). and not interfered with, in
evitably take the spherical form,
and if they are but little interfered
with they approximate to that form.
Thus when the water is lifted out
of contact with the hot plate it's
molecules draw one another around
this common center as closely az
possible and form a bead-shaped
drop.
If such a drop encounters an
other the molecules of the two in
termingle, and pull one another
symmetrically around the common
center of the entire mass. and so a
larger drop is formed from the two
smaller ones.
But a drop of water is a far more
wonderful thing than this outline
description of the way il assumes
its round form indicates. 1t has a
virtual. or potential, skin. This is
due TO what is called surface-ten-
sion. one of the most surprising OF
natures commonly unnoticed forces.
The surface~tension arises from the
1 -
and II exists. like an elastic mem
brane, on the surface Of every liquid
whether il is collected into a sphere
or not.
The thickness of the surface film,
or quasi embrance. is equal 10 the
radius of the sphere OF the mole
cular attraction. This radius. or the
distance over which the attraction
of any molecule Of the liquid acts
upon its neighbors. is excessively
small, perhaps not more than a
inch, but that is sufficient to include
a large number of molecules.
Any molecules that lie nearer the
surface of a drop, or a body of
water. than the length Of the radius
of molecular attraction are drawn
inward. or downward, by a greater
sum OF attractions than that which
draws them outward or upward, be-
cause a part of their spheres lies be-
yond the surface of the water,
where there is no attraction. Thus
a residual force acts upon those
molecules. tending 10 hold them IN.
and this results in a tension resem-
bling that OF a stretched membrane,
covering the water surface.
A familiar example of the strain
of surface-te tension is furnished by a
soap bubble. which. il the pipestem
through which il has been filled
with air is left open. will contract,
driving the air out OF the stem,
sometimes with force enough TO
blow out a candle. Different liquids
have different degrees of surface
tension, and when two or more liq-
uids or fluids are in contact a ten
sion is esta blished along their sur
faces of contact which often pre-
sents astonishing phenomena.
II is surface-tensiOn that causes
oil poured upon the sea 10 spread
out into an almost. infinitely thin