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form of that same substance, will not float in the liquid form.
As an example, liquid hydrogen has a density of about 0.071
grams per cubic centimeter, but solid hydrogen has a density of
about 0.086 grams per cubic centimeter. If a cubic centimeter of
solid hydrogen were completely immersed in liquid hydrogen it
would still weigh 0.015 grams and would be pulled downward
by gravity. Sinking slowly (against the resistance of the liquid
hydrogen) but definitely, it would eventually reach the bottom
of the container, or the bottom of the ocean, if there were that
much liquid hydrogen.
(You might suspect that the solid hydrogen would melt on the
way downward, but not if the ocean of liquid hydrogen were at
its freezing point and we'll suppose it is.)
In the same way, solid iron would sink downward through an
ocean of liquid iron, solid mercury through liquid mercury, solid
sodium chloride through liquid sodium chloride, and so on. This
is so general a situation that if you took a thousand solids at
random, you would be very likely to find that in each case the
solid form would sink through the liquid form and you would be
tempted to make that a universal rule.
But you can't, for there are exceptions.
And of these, by far the most important one is water.
At ioo° C. (water's boiling point under ordinary conditions),
water is as un-dense as it can be and still remain liquid. Its density
* Except at the "critical temperature," something which need not concern
us now.
COLD WATER 95
then is about 0.958 grams per cubic centimeter. As the tempera-
ture drops the density rises: 0.965 at 900 C, 0.985 at still lower
temperature, and so on until at 40 C, it is 1.000 grams per cubic
centimeter.
To put it another way, a single gram of water has a volume of
1.043 cubic centimeters at 100° C, but contracts to a volume of
1.000 cubic centimeters at 40 C.
Judging from what is true of other substances, we would have
every right to expect that this increase in density and decrease in
volume would continue as the temperature dropped below 40 C.
It does notl
The temperature of 4° C* represents a point of maximum
density for liquid water. As the temperature drops below that, the
density starts to decrease again (very slightly, to be sure) and by
the time one reaches o° C, the density is 0.9999 grams per cubic
centimeter, so that a gram of water takes up 1.0001 cubic centi-
meters. The difference in density at o° C. as compared with that at
40 C. is trifling, but it is in the "wrong" direction, and that makes
it crucial.
At o° C. water freezes if further heat is withdrawn, and by
everything we leam from other solidifications we would have a
right to expect a sharp increase of density. We would be wrong!
There is a sharp decrease in density.
Whereas water at o° C. has, as I said, a density of 0.9999 grams
per cubic centimeter, it freezes into ice at o° C. with a density of
only about 0.92 grams per cubic centimeter.
If a cubic centimeter of ice is completely immersed in water,
with both at a temperature of o° C, then the weight of the ice is
0.08 grams and there is, so to speak, a negative gravitational ef-
fect upon it. It therefore rises to the surface of the water. The
rise continues till only enough of it is submerged to displace its
own weight (as measured in air) of the denser, liquid water. Since
a cubic centimeter of ice at o° C. weighs 0.92 grams and it takes
only 0.92 cubic centimeters of water at o° C. to weigh 0.92
* 3.980 C, to be more accurate.
96 THE PROBLEM OF OCEANS
grams, it turns out that when the ice is floating, 92 per cent of its
substance is below water and 8 per cent is above.
What we would ordinarily expect, judging from almost all other
solids immersed in their own liquid form, is that 100 per cent of
the ice would be submerged and o per cent exposed. It follows,
then, as I said earlier, that the surprising thing is not that so much
of an iceberg is invisible, but that so much of it (or, indeed, any of
it at all) is exposed.
Well, why is that?
Let's begin with ice. In ordinary ice, each water molecule has
four other molecules surrounding it with great precision of orien-
tation. The hydrogen atom of each water molecule is pointed in
the direction of the oxygen atom of a neighbor and this orienta-
tion is maintained through the small electrostatic attraction in-
volved in the hydrogen bond (as described in the previous
chapter).
The hydrogen bond is weak and does not suffice to draw the
molecules very close together. The molecules remain unusually far
apart, therefore, and if a scale model is built of the molecular
structure of ice, it is seen that there are enough spaces between
the molecules to make up a very finely ordered array of "holes."
Nothing visible, you understand, for the holes are only about an
atom or so in diameter.
Still, this makes ice less dense than it would be if there were
a closer array of molecules.
As the temperature of the ice rises, its molecules vibrate and
move still farther apart, so that its density falls, reaching a
minimum of the aforementioned 0.92 grams per cubic centimeter
at o° C. At that temperature of o° C, however, the molecular
vibration has reached the point where it just balances the attrac-
tive forces between the molecules. If further heat is added, the
molecules can break free and can begin to slip and slide past each
other. In doing so, however, some of them fall into the holes.
As ice melts, then, the tendency to decrease the density
through increased vibrational energy is countered by the dis-
COLD WATER 97
appearance of the holes, and more than countered. For that rea-
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