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210
The Fairy-Land of Science
·
Arabella B. Buckley
67
Let
us
try
to
understand
how
these
two
invisible
workers,
the
sun-waves
and
the
air,
deal
with
the
drops
of
water.
I
have
here
a
kettle
(Fig.
18,
p.
76)
boiling
over
a
spirit-lamp,
and
I
want
you
to
follow
minutely
what
is
going
on
in
it.
First,
in
the
flame
of
the
lamp,
atoms
of
the
spirit
drawn
up
from
below
are
clashing
with
the
oxygen-atoms
in
the
air.
This,
as
you
know,
causes
heat-waves
and
light-waves
to
move
rapidly
all
round
the
lamp.
The
light-waves
cannot
pass
through
the
kettle,
but
the
heat-waves
can,
and
as
they
enter
the
water
inside
they
agitate
it
violently.
Quicker,
and
still
more
quickly,
the
particles
of
water
near
the
bottom
of
the
kettle
move
to
and
fro
and
are
shaken
apart;
and
as
they
become
light
they
rise
through
the
colder
water
letting
another
layer
come
down
to
be
heated
in
its
turn.
The
motion
grows
more
and
more
violent,
making
the
water
hotter
and
hotter,
till
at
last
the
particles
of
which
it
is
composed
fly
asunder,
and
escape
as
invisible
vapour.
If
this
kettle
were
transparent
you
would
not
see
any
steam
above
the
water,
because
it
is
in
the
form
of
an
invisible
gas.
But
as
the
steam
comes
out
of
the
mouth
of
the
kettle
you
see
a
cloud.
Why
is
this?
Because
the
vapour
is
chilled
by
coming
out
into
the
cold
air,
and
its
particles
are
drawn
together
again
into
tiny,
tiny
drops
of
water,
to
which
Dr.
Tyndall
has
given
the
suggestive
name
of
water-dust.
If
you
hold
a
plate
over
the
steam
you
can
catch
these
tiny
drops,
though
they
will
run
into
one
another
almost
as
you
are
catching
them.
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