STORED ENERGY IN WATER.
In our last chapter we traced the upward movement in the mercury of the
thermometer from 10 degrees below the freezing point up to the boiling
point of water. We found that the thermometer was arrested at 32 degrees
and remained stationary at that point until all the ice was melted,
notwithstanding the fact that heat was being constantly applied. After
the ice is all melted the mercury moves upward until it reaches the
boiling point of water, where the movement is again arrested, and
although the heat is being continuously applied, it remains stationary
until all the water is evaporated. If we push the process still further,
with a sufficient application of energy we can separate the vapor
molecules into their original elements, oxygen and hydrogen.
Let us go back now to the freezing point of water and see what is
becoming of the heat that is consumed in melting the cake of ice, and
still does not produce any effect upon the mercury in the thermometer.
Sensible heat, as before stated, is a movement of the atoms of matter,
and temperature, as it affects the thermometer, is a measure of the
intensity of motion exhibited by these atoms.
In the experiment of the block of ice that in the beginning is 10
degrees below the freezing point, as shown by the thermometer, the
molecules have a definite intensity of motion. The intensity of this
motion increases when heat is applied until it reaches 32 degrees, when
it remains stationary until all of the ice is melted. At this point
there is a rearrangement of the molecules of water as it assumes the
liquid state. To perform this rearrangement requires a certain amount of
work done, which is analogous to the winding up of a weight to a certain
distance. There has been energy used in winding up the weight, but that
energy now is not destroyed, nor still in the form of heat, but is in
the potential state--ready to do some other kind of work. So, the heat
that has been applied to the melting ice has been utilized during the
process of its liquefaction in rearranging the water molecules and
putting them in a state of strain, so to speak, like the weight that is
wound up to a certain height. There is a certain amount of potential
energy that is stored in the molecules of water that will be given up
and become active energy in the form of heat, if the water is again
frozen. To melt a cubic foot of ice requires as much heat as it would
to raise a cubic foot of water 144 degrees Fahrenheit. But, as we have
seen, while all of this energy is absorbed as heat, it is not lost as
energy. It ceases to be kinetic or active and becomes potential energy.
This (let us repeat) has been called latent heat. The term grew out of
the old idea that heat was a fluid and that when it became latent it hid
itself away somewhere in the interatomic spaces of matter and ceased to
be longer sensible heat. It came into existence in the same manner and
occupies the same place in the science of heat that the word "current"
does in the science of electricity: both of them are misnomers.
When the ice is all melted potential energy is no longer stored, but is
manifested in the sensible heating of water, the degree of which is
measurable by the thermometer, until it reaches the boiling point, where
it is again arrested. All of the surplus heat above that temperature is
consumed in rending the liquid water into moisture globules that float
away into the air, each one of them charged with a store of potential
energy. Let us follow this vapor spherule as it floats into the upper
regions of the atmosphere. Myriads of its fellows travel with it until
it reaches a point where condensation takes place, when it collapses and
unites with other vapor particles to form water again. In doing this the
heat that was expended upon it to disengage it (whether the heat was
artificial or that of the sun's rays) now reappears either as sensible
heat or as electricity, or both. And this is what is meant in
meteorology by latent heat becoming sensible heat at the time of
condensation; in fact, it is stored or "potential" energy becoming
active or kinetic, and assumes the form of heat or electricity, as
before stated. We have thus reviewed the matter of the foregoing chapter
in order to follow the course of the stored energy from the melting of
the ice to the vapor, and back again to water: to doubly impress the
fact that the energy used was not consumed, but still exists and is
ready for further work.
During the progress of a hailstorm, it has been stated, one of the
factors that is active to produce this phenomenon is the intense
ascensional force that is given to the moisture-laden air, caused by
intense heat at the surface of the earth. This condition forces the
moisture vapor to higher regions of the atmosphere than is the case with
the ordinary thunderstorm. Another factor that is undoubtedly active in
producing hail under these circumstances is that when condensation takes
place in the higher regions, and is therefore more energetic on account
of the intenser cold, the potential energy that is set free by the
moisture spherules takes, in a larger degree, the form of electricity
rather than heat, as is the case under more ordinary circumstances.
While in the end this electrical energy becomes active heat, it does not
for the time being, and thus favors the ready congelation of the
condensed moisture into hailstones. Hailstorms are always attended by
incessant thunder and lightning, and this fact favors the theory
advanced above.
It will be easily seen from a study of the foregoing what a wonderful
factor evaporation (which is a product of the sun's rays) is, in the
play of celestial dynamics. It ascends from the surface of the earth or
ocean laden with a stored energy, the power of which no man can compute,
and beside which gravitation is a mere point. In the upper regions of
atmosphere this potential force under certain conditions is released and
becomes an active factor, not only in the formation of cloud and the
precipitation of rain, hail, and snow, but it disturbs the equilibrium
of the air and sets that in motion.
Certain physicists deny that evaporation has anything to do with
atmospheric electricity. They tell us that it is caused by the arrest of
the energy of the sunbeam by the clouds and vapor in the upper
atmosphere. We admit that a part of the energy is so arrested, and is
stored, for the time, in moisture globules by a process of cloud
evaporation to transparent vapor again. Yet this does not hinder the
same process from going on at the surface of the earth wherever there is
water or moisture. But they tell us that the electroscope does not show
any signs of electrification in the evaporated moisture. Of course it
does not. The electroscope is not made to detect the presence of energy
except when set free as electricity.
A wound-up spring does not seem to be electrified, but if it is released
the energy stored in it will be transformed into electricity if the
conditions are right. Just so, the energy required to put the moisture
spherule into a state of strain is latent until some power releases it,
when it reappears as active energy of some form.
We have now followed the relation of heat to water from a point 10
degrees below freezing up to where it was forced into its original
gases, oxygen and hydrogen. These gases have stored in them a wonderful
amount of potential energy. When one pound of hydrogen and eight pounds
of oxygen unite to form water the mechanical value of the energy given
up at that time in the form of heat is represented by 47,000,000 pounds
raised to one foot in height. And this is the measure of the energy that
was put into nine pounds of water to force it from a state of vapor into
its constituent gases. After the combination of the gases into a state
of vapor the temperature sinks to that of boiling water. The amount of
energy given up in condensing the nine pounds of vapor into nine pounds
of water is equal to 6,720,000 foot-pounds. If this nine pounds of water
is now cooled from the boiling point to 32 degrees Fahrenheit we come to
the final fall, where the potential energy that is stored in the
operation of melting ice is given up suddenly at the moment of freezing,
which in nine pounds of water is 993,546 foot pounds.
Professor Tyndall, in speaking of the amount of energy that is given up
between the points where the constituent gases unite to form nine pounds
of water and the point where it congeals as ice, says: "Our nine pounds
of water, at its origin and during its progress, falls down three
precipices--the first fall is equivalent in energy to the descent of a
ton weight down a precipice 22,320 feet high-over four miles; the second
fall is equal to that of a ton down a precipice 2900 feet high, and the
third is equal to a fall of a ton down a precipice 433 feet high. I have
seen the wild stone avalanches of the Alps, which smoke and thunder down
the declivities with a vehemence almost sufficient to stun the observer.
I have also seen snowflakes descending so softly as not to hurt the
fragile spangles of which they are composed. Yet to produce from aqueous
vapor a quantity which a child could carry of that tender material
demands an exertion of energy competent to gather up the shattered
blocks of the largest stone avalanche I have ever seen and pitch them to
twice the height from which they fell."
When we contemplate the foregoing facts as related to so small an amount
of water as nine pounds, and multiply this result by the amount of snow-
and rainfall each year and the amount of ice that is congealed and again
liquefied by the power of the sun's rays, we are appalled, and shrink
from the task of attempting to reduce the amount of energy expended in a
single year to measurable units.
Having considered water in its relation to heat in the preceding
chapters, we will now take up the subject of water in its relation to
ice and snowfall and the phenomena exhibited in ice rivers, commonly
called glaciers.
When water is under pressure the freezing point is reduced several
degrees below 32 degrees Fahrenheit. This fact has been determined by
confining water in a close vessel and putting it under pressure and
subjecting it to a freezing mixture, and by this means determining the
freezing point under such conditions. By putting a bullet or something
of that nature into the water that is subjected to pressure one can tell
by shaking it when the freezing point is reached. If water is put under
pressure and cooled to a point below 32 degrees, and yet still remains
in the liquid state, it may be suddenly congealed by taking off the
pressure; this shows that the pressure helps to hold the molecules in
the position necessary for the liquid state, and prevents the
rearrangement of them that takes place at the moment of freezing. When
the water molecules are arranged for the liquid condition they may be
compared to a spring that is wound up and held in position by the heat
energy that is stored in the water. And when this energy is given up to
a certain degree the power that holds the spring wound up is suddenly
released, when it unwinds and occupies a larger space. There is a force
that we may call polar force, which is constantly tending to push the
molecules of water into an arrangement such as we see when
crystallization takes place--as it always does in the act of freezing.
These polar forces cannot act so long as the energy in the form of heat
is sufficient to hold the water in the fluid state. But the moment this
energy, which tends to hold it in the fluid state, falls below that
which tends to rearrange it into the crystalline form, it is overcome by
the superior power of the latter force, and we have the phenomenon of
solidified water.
A very interesting experiment may be performed with a block of ice by
anyone when the ice is near the melting point. If a wire is put around
the ice and a sufficient weight is suspended to it, the pressure of the
wire on the ice will gradually liquefy that portion immediately under
the wire, which allows it to sink into the ice slowly, and as this
process goes on the ice freezes together again behind the wire, so that
in time the wire will pass entirely through the block and leave it still
a solid block, as it was before the experiment began.
This is an interesting fact which it will be well to remember when we
come to explain glacial action, or rather the law that governs glacial
action. If we take two pieces of melting ice and bring them together
they immediately congeal at the point of contact. This phenomenon is
called "regelation." Ice has some of the properties of a viscous
substance. It will yield slowly to pressure, especially when near the
melting point, but if put under a tensional strain it will break, as any
brittle substance will, so that it has the properties of both viscosity
and brittleness. Ordinarily we are in the habit of treating water as a
fluid and ice as a solid, but from what has gone before the reader must
understand that in a certain sense ice should be treated as having
semi-fluidic properties.