WATER AND ICE.
We now have entered upon a subject that is of intense interest, studied
from the standpoint of facts as they exist to-day and of history as we
read it in the rocks and bowlders that we find distributed over the face
of the earth.
The whole northern part of the United States extending to a point south
of Cincinnati was at one time covered with a great ice-sheet, traces of
which are plainly visible to anyone who has made anything of a study of
this subject. The glaciers now to be seen in British Columbia and
Alaska, great as they seem to one viewing them to-day, are by comparison
with what once existed simply microscopic specks of ice. Glaciers, like
rivers, flow by gravity, following the lowest bed and lines of least
resistance; the difference being that in the one case the flow is rapid,
while in the other it is scarcely visible, except by measurement from
day to day. Before entering upon a description of the law that governs
the flow of glaciers, let us stop and give a little study to the
phenomena of water as exhibited when it is at the freezing point. Water
is such a large factor in the make-up of our globe and the air that
surrounds it that it becomes a very interesting and important study to
anyone who wishes to understand the phenomena of nature that are closely
related to it.
As all know, pure water is a compound of two gases, oxygen and hydrogen,
combined in the proportion of two atoms of hydrogen and one of oxygen.
Let us now study this fluid in its relation to heat. The reader is
referred to the chapters on heat in Vol. II., where it is stated that
heat is a mode of motion. It is also stated that heat is a form of
energy, and that energy is indestructible, that an unvarying amount of
it exists in some form or another throughout the universe. It is not
always manifested as heat or electricity, although both of these are
always in evidence as active agents of force. Much of the energy is
simply stored--all the time possessing the ability to do work or to be
converted into any of its known forms, such as heat, light, electricity,
or mechanical motion. A weight that is wound up has required a certain
amount of energy to elevate it to the position that it occupies. While
in its elevated position it possesses energy, although not active.
Energy in this form is called potential (possible) energy, and has the
power to do work if released. Active energy is called kinetic (moving)
energy, and the sum of these two energies is a constant quantity.
We will now study energy as it is related to water in the form of heat.
There is a kind of heat called "latent heat," which is not heat at all,
but stored energy, waiting to be turned into heat, or light, or some
other active form. Properly speaking, heat is a movement of the atoms of
matter, the intensity of which is measurable in degrees, and called its
temperature. To use the term latent heat as meaning concealed heat,
which must reappear as heat, is a misnomer and is very misleading. If it
is proper to call a wound-up spring or weight latent heat then its
present use is a correct one. What was formerly termed latent heat is
simply a form of potential energy. When sensible heat that is
measurable, as temperature, disappears in the performance of some sort
of work, especially in connection with certain phenomena relating to
water, we call it--or rather miscall it--latent heat: but the phrase
would better be "stored energy."
The action of water under heat is very peculiar, and in order to get a
correct understanding of the phenomena exhibited in glacial action we
also need to understand the phenomena of water at the freezing point. As
is well known, fresh water freezes at 32 degrees Fahrenheit, and at the
moment of freezing there is a sudden expansion to such an extent that a
cubic foot of ice will occupy a much larger space than it will in the
form of water; and because it occupies so much larger space it is
lighter than the same bulk of water would be, and therefore it floats in
water.
At the point of freezing, the thermometer if placed on the ice will
register 32 degrees. If the ice is allowed to melt, the water at the
moment of liquefaction would be found to register the same degree of
temperature as the ice when first frozen. And yet there has been a vast
expenditure of energy between the points of liquefaction and
congelation, notwithstanding the temperature of ice may be lowered,
after it is formed, many degrees, which is measurable by the
thermometer. Suppose we take a piece of ice which is 10 degrees below
the freezing point and insert in it a thermometer. If now we apply heat
to this ice the thermometer will gradually rise until it reaches the
melting point at 32 degrees Fahrenheit, where it will stand until all
the ice is melted. The application of heat is going on steadily, but
there are no indications of movement in the mercury until the last trace
of ice with which it is in contact has been liquefied. After the ice is
all melted, if the application of heat to the body of liquefied ice be
continued, the column of mercury will resume its movement upward until
it reaches the boiling point, where it is again arrested. And no matter
how much heat is applied to the boiling water, if in an open vessel, the
thermometer remains the same until all the water is evaporated. Here are
two curious facts, and they are facts that, if we can master them, will
serve as a key to the understanding of much that is mysterious in
nature.
It will be our endeavor to give the reader a mental picture of what is
taking place during the time the ice is melting and the thermometer is
stationary. Do not suppose that you can understand this, even so far as
it is understandable, by a casual reading without thought. No man was
ever yet able to present a picture to the mind of another, however
clearly and simply it may be done, unless that other mind is receptive.
When a photographer trains his camera upon an object, however intense
the light may be and however clean-cut the picture that is thrown upon
the plate in the camera, unless that plate is properly sensitized so
that the picture may be impressed upon it, all of the other conditions
are in vain. The reader is always a part of the book he is reading.