_The Laws of Heat with respect to Water._
The manner in which heat is transmitted through fluids is
altogether different from the mode in which it passes through
solids; and hence the waters of the earth’s surface produce
peculiar effects upon its condition as to temperature. Moreover,
water is susceptible of evaporation in a degree depending upon the
increase of heat; and in consequence of this property it has most
extensive and important functions to discharge in the economy of
nature. We will consider some of the offices of this fluid.
1. Heat is communicated through water, not by being _conducted_
from one part of the fluid to another, as in solid bodies, but
(at least principally) by being _carried_ with the parts of the
fluid by means of an intestine motion. Water expands and becomes
lighter by heat, and, therefore, if the upper parts be cooled
below the subjacent temperature, this upper portion will become
heavier than that below, bulk for bulk, and will descend through
it, while the lower portion rises to take the upper place. In this
manner the colder parts descend, and the warmer parts ascend by
contrary currents, and by their interchange and mixture, reduce
the whole to a temperature at least as low as that of the surface.
And this equalization of temperature by means of such currents,
is an operation of a much more rapid nature than the slow motion
of conduction by which heat creeps through a solid body. Hence,
alternations of heat and cold, as day and night, summer and
winter, produce in water, inequalities of temperature much smaller
than those which occur in a solid body. The heat communicated is
less, for transparent fluids imbibe heat very slowly; and the cold
impressed on the surface is soon diffused through the mass by
internal circulation.
Hence it follows that the ocean, which covers so large a portion
of the earth, and affects the temperature of the whole surface
by its influence, produces the effect of making the alternations
of heat and cold much less violent than they would be if it were
absent. The different temperatures of its upper and lower parts
produce a current which draws the seas, and by means of the seas,
the air, towards the mean temperature. And this kind of circulation
is produced, not only between the upper and lower parts, but also
between distant tracts of the ocean. The great Gulf Stream which
rushes out of the Gulf of Mexico, and runs across the Atlantic to
the western shores of Europe, carries with it a portion of the
tropical heat into northern regions: and the returning current
which descends along the coast of Africa, tends to cool the parts
nearer the equator. Great as the difference of temperature is in
different climates, it would be still greater if there were not
this equalizing and moderating power exerted constantly over the
whole surface. Without this influence, it is probable that the two
polar portions of the earth, which are locked in perpetual ice and
snow, and almost destitute of life, would be much increased.
We find an illustration of this effect of the ocean on temperature,
in the peculiarities of the climates of maritime tracts and
islands. The climate of such portions of the earth, corrected in
some measure by the temperature of the neighbouring sea, is more
equable than that of places in the same latitudes differently
situated. London is cooler in summer and warmer in winter than
Paris.
2. Water expands by heat and contracts by cold, as has been
already said; and in consequence of this property, the coldest
portions of the fluid generally occupy the lower parts. The
continued progress of cold produces congelation. If, therefore, the
law just mentioned had been strictly true, the lower parts of water
would have been first frozen; and being once frozen, hardly any
heat applied at the surface could have melted them, for the warm
fluid could not have descended through the colder parts. This is
so far the case, that in a vessel containing ice at the bottom and
water at the top, Rumford made the upper fluid boil without thawing
the congealed cake below.
Now, a law of water with respect to heat operating in this manner,
would have been very inconvenient if it had obtained in our lakes
and seas. They would all have had a bed of ice, increasing with
every occasion, till the whole was frozen. We could have had no
bodies of water, except such pools on the surfaces of these icy
reservoirs as the summer sun could thaw, to be again frozen to
the bottom with the first frosty night. The law of the regular
contraction of water by cold till it became ice, would, therefore,
be destructive of all the utility of our seas and lakes. How is
this inconvenience obviated?
It is obviated by a modification of the law which takes place when
the temperature approaches this limit. Water contracts by the
increase of cold, till we come _near_ the freezing temperature;
but then, by a further increase of cold, it contracts no more, but
expands till the point at which it becomes ice. It contracts in
cooling down to 40 degrees of Fahrenheit’s thermometer; in cooling
further it expands, and when cooled to 32 degrees, it freezes.
Hence, the greatest density of the fluid is at 40 degrees, and
water of this temperature, or near it, will lie at the bottom
with cooler water or with ice floating above it. However much the
surface be cooled, water colder than 40 cannot descend to displace
water warmer than itself. Hence we can never have ice formed at
the bottom of deep water. In approaching the freezing point, the
coldest water will rise to the surface, and the congelation will
take place there; and the ice so formed will remain at the surface,
exposed to the warmth of the sunbeams and the air, and will not
survive any long continuance of such action.
Another peculiarity in the laws which regulate the action of cold
on water is, that in the very act of freezing a further sudden
and considerable expansion takes place. Many persons will have
known instances of vessels burst by the freezing of water in them.
The consequence of this expansion is, that the specific gravity
of ice is less than that of water of any temperature; and it
therefore always floats in the unfrozen fluid. If this expansion of
crystallization did not exist, ice would float in water which was
below forty degrees, but would sink when the fluid was above that
temperature: as the case is, it floats under all circumstances. The
icy remnants of the effects of winter, which the river carries down
its stream, are visible on its surface till they melt away; and
the icebergs which are detached from the shores of the polar seas,
drift along, exposed to the sun and air, as well as to the water in
which they are immersed.
These laws of the effect of temperature on water are truly
remarkable in their adaptation to the beneficial course of things
at the earth’s surface. Water contracts by cold; it thus equalizes
the temperature of various times and places; but if its contraction
were continued all the way to the freezing point, it would bind a
great part of the earth in fetters of ice. The contraction then
is here replaced by expansion, in a manner which but slightly
modifies the former effects, while it completely obviates the bad
consequences. The further expansion which takes place at the point
of freezing, still further facilitates the rapid removal of the
icy chains, in which parts of the earth’s surface are at certain
seasons bound.
We do not know how far these laws of expansion are connected with
and depend on more remote and general properties of this fluid, or
of all fluids. But we have no reason to believe that, by whatever
means they operate, they are not laws selected from among other
laws which might exist, as in fact for other fluids other laws do
exist. And we have all the evidence, which the most remarkable
furtherance of important purposes can give us, that they _are_
selected, and selected with a beneficial design.
3. As water becomes ice by cold, it becomes steam by heat. In
common language, steam is the name given to the vapour of _hot_
water; but in fact a vapour or steam rises from water at all
temperatures, however low, and even from ice. The expansive force
of this vapour increases rapidly as the heat increases; so that
when we reach the heat of boiling water, it operates in a far
more striking manner than when it is colder; but in all cases
the surface of water is covered with an atmosphere of aqueous
vapour, the pressure or _tension_ of which is limited by the
temperature of the water. To each degree of pressure in steam there
is a _constituent temperature_ corresponding. If the surface of
water is not pressed by vapour with the force thus corresponding
to its temperature, an immediate _evaporation_ will supply the
deficiency. We can compare the tension of such vapour with that of
our common atmosphere; the pressure of the latter is measured by
the barometrical column, about thirty inches of mercury; that of
watery vapour is equal to one inch of mercury at the constituent
temperature of 80 degrees, and to one-fifth of an inch, at the
temperature of 32 degrees.
Hence, if that part of the atmosphere which consists of common air
were annihilated, there would still remain an atmosphere of aqueous
vapour, arising from the waters and moist parts of the earth; and
in the existing state of things this vapour rises in the atmosphere
of dry air. Its distribution and effects are materially influenced
by the vehicle in which it is thus carried, as we shall hereafter
notice; but at present we have to observe the exceeding _utility_
of water in this shape. We remark how suitable and indispensable to
the well-being of the creation it is, that the fluid should possess
the property of assuming such a form under such circumstances.
The _moisture_ which floats in the atmosphere is of most essential
use to vegetable life.[7] “The leaves of living plants appear to
act upon this vapour in its elastic form, and to absorb it. Some
vegetables increase in weight from this cause when suspended in the
atmosphere and unconnected with the soil, as the house-leek and the
aloe. In very intense heats, and when the soil is dry, the life
of plants seems to be preserved by the absorbent power of their
leaves.” It follows from what has already been said, that, with an
increasing heat of the atmosphere, an increasing quantity of vapour
will rise into it, if supplied from any quarter. Hence it appears
that aqueous vapour is most abundant in the atmosphere when it is
most needed for the purposes of life; and that when other sources
of moisture are cut off, this is most copious.
4. _Clouds_ are produced by aqueous vapour when it returns to the
state of water. This process is _condensation_, the reverse of
evaporation. When vapour exists in the atmosphere, if in any manner
the temperature becomes lower than the _constituent temperature_,
requisite for the maintenance of the vapoury state, some of the
steam will be condensed and will become water. It is in this manner
that the curl of steam from the spout of a boiling tea-kettle
becomes visible, being cooled down as it rushes to the air. The
steam condenses into a fine watery powder, which is carried about
by the little aerial currents. Clouds are of the same nature with
such curls, the condensation being generally produced when air,
charged with aqueous vapour, is mixed with a colder current, or has
its temperature diminished in any other manner.
Clouds, while they retain that shape, are of the most essential
use to vegetable and animal life. They moderate the fervour of
the sun, in a manner agreeable, to a greater or less degree, in
all climates, and grateful no less to vegetables than to animals.
Duhamel says that plants grow more during a week of cloudy weather
than a month of dry and hot. It has been observed that vegetables
are far more refreshed by being watered in cloudy than in clear
weather. In the latter case, probably the supply of fluid is too
rapidly carried off by evaporation. Clouds also moderate the
alternations of temperature, by checking the radiation from the
earth. The coldest nights are those which occur under a cloudless
winter sky.
The uses of clouds, therefore, in this stage of their history,
are by no means inconsiderable, and seem to indicate to us that
the laws of their formation were constructed with a view to the
purposes of organized life.
5. Clouds produce _rain_. In the formation of a cloud the
precipitation of moisture probably forms a fine watery _powder_,
which remains suspended in the air in consequence of the minuteness
of its particles: but if from any cause the precipitation is
collected in larger portions, and becomes _drops_, these descend by
their weight and produce a shower.
However rain is formed, it is one of the consequences of the
capacity of evaporation and condensation which belongs to water,
and its uses are the result of the laws of those processes. Its
uses to plants are too obvious and too numerous to be described. It
is evident that on its quantity and distribution depend in a great
measure the prosperity of the vegetable kingdom: and different
climates are fitted for different productions, no less by the
relations of dry weather and showers, than by those of hot and cold.
6. Returning back still further in the changes which cold can
produce on water, we come to _snow_ and _ice_: snow being
apparently frozen vapour, aggregated by a confused action of
crystalline laws; and ice being water in its fluid state,
solidified by the same crystalline forces. The impression of
these agents on the animal feelings is generally unpleasant,
and we are in the habit of considering them as symptoms of the
power of winter to interrupt that state of the elements in which
they are subservient to life. Yet, even in this form, they are
not without their uses.[8] “Snow and ice are bad conductors of
cold; and when the ground is covered with snow, or the surface
of the soil or of water is frozen, the roots or bulbs of plants
beneath are protected by the congealed water from the influence
of the atmosphere, the temperature of which, in northern winters,
is usually very much below the freezing point; and this water
becomes the first nourishment of the plant in early spring. The
expansion of water during its congelation, at which time its volume
increases one-twelfth, and its contraction in bulk during a thaw,
tend to pulverize the soil, to separate its parts from each other,
and to make it more permeable to the influence of the air.” In
consequence of the same slowness in the conduction of heat which
snow thus possesses, the arctic traveller finds his bed of snow
of no intolerable coldness; the Esquimaux is sheltered from the
inclemency of the season in his snow hut, and travels rapidly and
agreeably over the frozen surface of the sea. The uses of those
arrangements, which at first appear productive only of pain and
inconvenience, are well suited to give confidence and hope to our
researches for such usefulness in every part of the creation. They
have thus a peculiar value in adding connexion and universality to
our perception of beneficial design.
7. There is a peculiar circumstance still to be noticed in the
changes from ice to water and from water to steam. These changes
take place at a particular and invariable degree of heat; yet
they do not take place suddenly when we increase the heat to this
degree. This is a very curious arrangement. The temperature _makes
a stand_, as it were, at the point where thaw, and where boiling
take place. It is necessary to apply a considerable quantity of
heat to produce these effects; all which heat disappears, or
becomes _latent_, as it is called. We cannot raise the temperature
of a thawing mass of ice till we have thawed the whole. We cannot
raise the temperature of boiling water, or of steam rising from it,
till we have converted all the water into steam. Any heat that we
apply while these changes are going on is absorbed in producing the
changes.
The consequences of this property of _latent heat_ are very
important. It is on this account that the changes now spoken of
necessarily occupy a considerable time. Each part in succession
must have a proper degree of heat applied to it. If it were
otherwise, thaw and evaporation must be instantaneous: at the
first touch of warmth, all the snow which lies on the roofs of our
houses would descend like a waterspout into the streets: all that
which rests on the ground would rush like an inundation into the
water courses. The hut of the Esquimaux would vanish like a house
in a pantomime: the icy floor of the river would be gone without
giving any warning to the skaiter or the traveller: and when, in
heating our water, we reached the boiling point, the whole fluid
would “flash into steam,” (to use the expression of engineers,)
and dissipate itself in the atmosphere, or settle in dew on the
neighbouring objects.
It is obviously necessary for the purposes of human life, that
these changes should be of a more gradual and manageable kind
than such as we have now described. Yet this gradual progress of
freezing and thawing, of evaporation and condensation, is produced,
so far as we can discover, by a particular contrivance. Like the
freezing of water from the top, or the floating of ice, the
moderation of the rate of these changes seems to be the result of
a _violation_ of a law: that is, the simple rule regarding the
effects of change of temperature, which at first sight appears to
be the law, and which, from its simplicity, would seem to us the
most obvious laws for these as well as other cases, is modified
at certain critical points, _so as to_ produce these advantageous
effects:--why may we not say _in order to_ produce such effects?
8. Another office of water which it discharges by means of its
relations to heat, is that of supplying our _springs_. There can be
no doubt that the old hypotheses which represent springs as drawing
their supplies from large subterranean reservoirs of water, or from
the sea by a process of subterraneous filtration, are erroneous
and untenable. The quantity of evaporation from water and from wet
ground is found to be amply sufficient to supply the requisite
drain. Mr. Dalton calculated[9] that the quantity of rain which
falls in England is thirty-six inches a year. Of this he reckoned
that thirteen inches flow off to the sea by the rivers, and that
the remaining twenty-three inches are raised again from the ground
by evaporation. The thirteen inches of water are of course supplied
by evaporation from the sea, and are carried back to the land
through the atmosphere. Vapour is perpetually rising from the
ocean, and is condensed in the hills and high lands, and through
their pores and crevices descends, till it is deflected, collected,
and conducted out to the bay, by some stratum or channel which is
watertight. The condensation which takes place in the higher parts
of the country, may easily be recognised in the mists and rains
which are the frequent occupants of such regions. The coldness of
the atmosphere and other causes precipitate the moisture in clouds
and showers, and in the former as well as in the latter shape, it
is condensed and absorbed by the cool ground. Thus a perpetual and
compound circulation of the waters is kept up; a narrower circle
between the evaporation and precipitation of the land itself,
the rivers and streams only occasionally and partially forming a
portion of the circuit; and a wider interchange between the sea and
the lands which feed the springs, the water ascending perpetually
by a thousand currents through the air, and descending by the
gradually converging branches of the rivers, till it is again
returned into the great reservoir of the ocean.
In every country, these two portions of the aqueous circulation
have their regular, and nearly constant, proportion. In this
kingdom the relative quantities are, as we have said, twenty-three
and thirteen. A due distribution of these circulating fluids in
each country appears to be necessary to its organic health; to the
habits of vegetables, and of man. We have every reason to believe
that it is kept up from year to year as steadily as the circulation
of the blood in the veins and arteries of man. It is maintained by
a machinery very different, indeed, from that of the human system,
but apparently as well, and, therefore, we may say as clearly, as
that, adapted to its purposes.
By this machinery, we have a connexion established between the
atmospheric changes of remote countries. Rains in England are often
introduced by a south-east wind. “Vapour brought to us by such
a wind, must have been generated in countries to the south and
east of our island. It is, therefore, probably, in the extensive
valleys watered by the Meuse, the Moselle, and the Rhine, if not
from the more distant Elbe, with the Oder and the Weser, that the
water rises, in the midst of sunshine, which is soon afterwards
to form _our_ clouds, and pour down _our_ thunder-showers.”
“Drought and sunshine in one part of Europe may be as necessary
to the production of a wet season in another, as it is on the
great scale of the continents of Africa and South America; where
the plains, during one-half the year, are burnt up, to feed
the springs of the mountain; which in their turn contribute to
inundate the fertile valleys and prepare them for a luxuriant
vegetation.”[10] The properties of water which regard heat make one
vast _watering-engine_ of the atmosphere.