You will easily observe that, though water occupies space, it has no
definite shape, but fits itself exactly to the figure of the vessel
which holds it. If the tumbler is cylindrical, the contour of the
surface of the water will be circular when the tumbler is held
vertically, and will change, without the least break or interruption, to
more and more of an oval when the tumbler is inclined, and whatever the
shape of the vessel into which you pour it, the sides of the water
always exactly fit against the sides of the vessel. If you put your
finger into the water you can move it in all directions with scarcely
any feeling of obstacle. If you pull your finger out there is no hole
left, the water on all sides rushing together to fill up the space that
was occupied by the finger. You cannot take up a handful of water, for
it runs away between your fingers, and you cannot raise it into a
permanent heap. All this shows that the parts of water move upon one
another with great ease. The same fact is illustrated if the tumbler is
inclined so that the level of the surface rises above the edge of the
tumbler on one side, and the water is therefore to some extent
unsupported by the tumbler at this point. The water then =flows= over in
a stream and falls to the ground, where it spreads out and runs to the
lowest accessible place, or gradually soaks up into crevices.
Nevertheless, although the parts of the water thus loosely slip and
slide upon one another, yet they hold together to a certain extent. If
the surface of the water is just touched with the finger, a little of it
will adhere; and if the finger is then slowly and carefully raised, the
adjacent water will be raised up into a slender column which acquires a
noticeable length before it breaks. So, in the early morning, after
heavy dew, you may see the water upon cabbage-leaves and blades of grass
in spherical drops, the parts of which similarly hold together.
Material substances, the parts of which are so movable that they fit
themselves exactly to the sides of any vessel which contains them, and
which flow when they are not supported, are called =fluids=, and fluids
the parts of which do not fly off from one another, but hold together as
those of water do, are called =liquids=.
Water therefore is a liquid.