These, however, may be only the exceptions that prove the rule. It is
recognized as a fundamental law that heat expands and cold contracts;
that the atom when in a state of intense motion (which is the condition
producing the effect that we call "heat") requires more room than when
its motions are of a less amplitude. In other words, an increase in the
amplitude of atomic motion is heating, while a decrease is cooling. It
follows from the above statement that the colder a body becomes the
smaller will be its dimensions. There are two or three, and perhaps
more, exceptions to this rule, and the most notable one is that of
water. Water follows the same law that all other substances do under the
action of heat and cold, within certain limits only. If we take water,
say, at 50 degrees Fahrenheit and subject it to cold it will gradually
contract in bulk until it reaches 39 degrees Fahrenheit. At this point,
very curiously, contraction ceases, and here we find the maximum density
of water. If the temperature is still lowered we find the bulk is
gradually increasing instead of diminishing (as is the rule with other
fluids), and when it reaches the freezing point there is a sudden and
marked expansion, so much so that a cubic foot of ice, which is
solidified water, will not weigh as much as a cubic foot of water before
it freezes--hence it floats.

Let us try an experiment. Take a small glass flask, terminating in a
long neck, say of four to six inches, and of small diameter. Suppose the
water in the glass to be at 50 degrees Fahrenheit. Fill the flask with
water until it stands halfway up the neck at 50 degrees temperature. Now
immerse the flask gradually in hot water, and observe the effect. For a
moment the water will lower in the neck of the tube, but this is due to
the fact that the glass expands before the heat is communicated to the
water and enlarges its capacity. But immediately the water will begin to
rise as the heat is communicated to it, and will continue to expand up
to the boiling point. Now take the flask out of the hot water and
gradually introduce it into a freezing mixture made of broken ice and
salt. Immediately the water will begin to fall in the tube, showing that
it is contracting under the cold, and it will continue to contract until
it reaches a temperature of 39 degrees Fahrenheit, when it will come to
a standstill and then proceed to expand as the temperature of the water
lowers. When it reaches th