Johann Heinrich Jacob Müller.

Principles of physics and meteorology online

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whilst, however, an alum plate will transmit almost all the rays of
heat that had previously passed through a plate of citric acid.
This phenomenon has the greatest analogy with the transmission
of light through a colored medium; rays of light that have
passed through green glass are, it is well known, easily transmit-
ted through other green glasses, which are absorbed when suffered
to fall upon red glass ; the differences between rays of heat are,
therefore, quite analogous to the differences of color in light.

Similar resemblances have been observed in relation to the
capacity of emission and absorption of bodies.

Rays of heat are refrangible, like rays of light, as may best be
seen by means of a prism of rock salt. Phenomena of polariza-
tion have also been shown in rays of heat.

Distribution of Heat by Conductors. Heat may pass from one
body to another, not only by radiation, but by immediate contact,
and may then be transmitted through the whole mass; there is,
however, a great inequality in different bodies in relation to the
facility with which this is effected ; in many, heat is very easily
transmitted, whilst in others it passes with much less facility from
one particle to another. A match that is burning at one end may
be held between the fingers at the other extremity without any
elevation of temperature being even felt in the wood ; the high
temperature of the burning end is not speedily transmitted to the
rest of the mass of wood, because wood is a bad conductor of
heat. An equally long metallic wire made glowing hot at one
extremity cannot be grasped at the other end without burning the
hand; heat, consequently, distributes itself from the glowing part
to the whole of the rod, metal being a good conductor.


We may make use of Ingenhousz's apparatus (Fig. 514) to show
Fig 514 the inequality of the capacity of different

bodies to transmit heat. Many rods made
of the substances to be compared are in-
serted into the lateral wall of a box of
tin plate, the rods being all of equal
diameter, and all covered with a layer of wax; on pouring boiling
water or hot oil into the box, the heat will penetrate more or less
into the rods and fuse the wax coating. If we assume that one
rod is of copper, another of iron, a third of lead, a fourth of glass,
and the last of wood, the wax coating of copper will be perfectly
fused before the coatings over the other rods are much melted,
showing that copper is the best conductor of these five bodies.
The fusion of the wax is more rapid over the iron than the lead,
and when all the wax has melted off the copper rod, fusion has
only progressed to a very small extent upon the glass rod, while
scarcely a trace of fusion is perceptible on the wooden rod ; which
proves that wood is the worst conductor of heat among these five

Of all bodies, metals are the best conductors of heat ; and
ashes, silk, hair, straw, wood, &c., and porous bodies especially,
are the w r orst.

In practical life we are constantly making numerous applica-
tions of the good or bad capacity of different bodies for conducting
heat. Thus, objects that we wish to protect from the cold, we
surround with bad conductors of heat: twisting straw round trees
and shrubs in winter to save them from the effect of the frost; on
the same principle our clothes keep us warm, owing to their being
made of bad conductors of heat. We can bring a liquid to a state
of boiling much more rapidly in a copper vessel than in one made
of porcelain, and having equally thick walls.

Capacity of Liquids and Gases for conducting Heat. Heat is
distributed through liquids principally by currents, which arise
from the heated particles rising more rapidly to the surface, owing
to their inconsiderable density. These currents may be made
apparent by throwing shavings into water enclosed in a glass ves-
sel, and then heating it slowly from below (Fig. 515), when we
shall see the current rise in the middle, and be directed upwards,
and turn downwards on either side. On heating a liquid from
above, so that the hydrostatic equilibrium is not disturbed, the


Fig. 515.

heat can only be transmitted in the same manner through the

mass of the liquid, as is the case with

solid bodies ; that is to say, by the heat

being conducted from one layer to the

other. In such cases, heat is only

slowly diffused through the mass of the

liquid ; liquids, consequently, are bad

conductors of heat.

In order to convince one's self of the
bad capacity of liquids for conducting
heat, one need only plunge the bulb of
a thermometer into cold water, and then
pour hot oil upon the water. The up-
permost layers of water will scarcely
manifest any elevation of temperature.

Despretz has determined the capacity
of liquids for conducting heat, by heat-
ing columns of water 1 metre (yard)
in height and from 0,2 to 0,4 metres (6
to 12 inches) in diameter, by continually

pouring hot water over them from above. This process was con-
tinued for about 30 hours, until the temperature of the columns
was settled and stable on all sides. From these experiments it
follows that the capacity of water for conducting heat is about 96
times less than that of copper.

The air and gases especially are likewise very bad conductors
of heat ; but we are unable, owing to the radiation of heat, to
ascertain their capacity for conducting heat, by means of the
thermometer brought into the different layers of the mass of air to
be examined. That gases generally, and the air in particular,
are bad conductors of heat, is, however, proved by this : that
bodies surrounded on all sides by layers of air, can only be cooled
or heated very slowly, if only the intermixture of the layers of air
be prevented. We thus see the utility of double windows and
double doors in keeping a room warm. The bad capacity for
conducting heat which we perceive in porous bodies, as straw,
wool, &c., depends especially upon their innumerable interstices
being filled with air. Bodies, which we say keep us warm, as,
for instance, our clothes, straw, &c., are not warm in themselves,
but owe the property they possess to their bad power of conduct-


ing heat ; if we wrap any of these round ice, they will hinder its
fusion, by protecting it from all external heat.


Fig. 516.


Generation of Heat by Chemical Combinations. Excepting the
sun, chemical combinations furnish us with the most important
sources of heat. Almost every chemical process is accompanied
by a development of heat.

The development of heat induced by combustion, that is, by a
rapid combination of bodies with oxygen, is of the greatest im-

In order to determine the amount of heat developed in combus-
tion, Rumford made use of the apparatus delineated in Fig. 516.

The box JL is filled with water,
through which passes a worm
tube ; the entrance of this tube
is formed by a funnel, below
which are placed the bodies to
be consumed. The experi-
ment is easily made with oil
and alcohol, which are poured
into a little lamp, which must
be weighed at the beginning
and end of the experiment, in
order to ascertain the quantity
of the material consumed.
The flame and the products of
combustion pass through the
tube, and heat the water of the

apparatus. From the elevation of temperature experienced by
the water, together with the whole apparatus, we may estimate

* Thomson's "Heat and Electricity," 2d edition, 8vo., 1840.


the amount of heat engendered by combustion; but here we must
not disregard the heat carried off by the gaseous products of com-
bustion from the tube.

By experiments of this kind, the following results were obtained
as to the amount of heat developed.

By the combustion of 1 grm. The temperature of 1 kilogramme

(15.444 grs.) of (2.679 Ib.) of water may be raised

Hydrogen 65,32

Olefiant gas 20,96

Absolute alcohol .... 12,52

Charcoal 13,12

Wax 18,90

Rapeseed oil .... 16,75

Tallow 15,06

Animal Heat. The temperature of the heat of blood of all ani-
mals is almost always different from that of the medium in which
they live. The animals of the polar regions are always warmer
than the ice on which they live ; but in the countries on the
equator they are cooler than the glowing air which they inhale.
Neither birds nor fish have the same temperature as the air or the
water surrounding them ; the animal body must consequently have
a peculiar heat, which it is constantly able to engender.

The internal heat of the human body appears to be the same
for all organs, and to be equal to that, to which a small thermo-
meter rises, when we place the bulb under the tongue, and close
the mouth, until it has ceased to rise ; this temperature is about
98 F. Age, climate, health, and disease, can but slightly
affect it.

The blood heat is greater in birds than in any other animals,
amounting, on an average, to 107.6; the blood heat of the mam-
malia is very nearly equal to that of man. In birds and the
mammalia, the blood heat is independent of the temperature sur-
rounding it ; but, in other species of animals, as the amphibia,
fishes, &c., the temperature of the body varies but little from the
surrounding medium.

What, then, is the source of animal heat? The air which we
inhale becomes changed in the same manner as the air that has
served in the combustion of bodies; the oxygen being converted
into carbonic acid, and a regular process of combustion being


thus carried on in the lungs. Since Lavoisier made this disco-
very, the source of animal heat has ceased to be a mystery. Car-
bon is brought into the body with the food, and is then combined
in the lungs with the oxygen of the inhaled air. By the oxidation
of carbon in the animal body, the same amount of heat must, how-
ever, necessarily be engendered as if the carbon had been con-
verted, by rapid combustion, into carbonic acid.

In a cold medium, men and animals constantly lose more heat
than in a warmer atmosphere ; as, however, the blood heat in the
mammalia and in birds is independent of the temperature of the
air, it is evident that more heat must be engendered in the body
if a greater quantity be withdrawn every moment from it, and
more, consequently, when the body is in a colder air, than when
it gives forth but little heat in a warmer medium. In order, how-
ever, to be able to engender more heat in the same periods of
time, more carbon must be introduced into the body, by the oxi-
dation of which substance heat is developed : in the same manner
as we must consume more fuel in a stove during cold weather
than during a less intense degree of cold, in order to maintain a
constant and fixed temperature in the apartment. Thus, too, we
may understand why the inhabitants of northern countries require
to partake of more food, and especially of the kind containing a
greater amount of carbon, than is necessary for those who live in
hotter zones.

Development of Heat by Mechanical Means. We have already
stated that heat is liberated by the compression of air ; and when
this is rapidly effected, a very considerable elevation of tempera-
ture may be brought about, on which depends the pneumatic
tinder-box. Fluids that do not admit of strong compression, show
but an inconsiderable elevation of temperature. Solid bodies are
often very much heated by compression, as we may observe in
the case of hammering metals and striking coins. It has not yet
been determined with certainty, whether the elevation of the tem-
perature of solid bodies, by compression, must likewise be ascribed
to the circumstance, that their heat is smaller with a greater de-
gree of density, and that, consequently, a part of the heat, which
is maintained in them, as specific heat, escapes in a perceptible
form on their being compressed.

The considerable elevations of temperature occasioned by fric-
tion are generally known. The iron tire of a wheel often becomes


so heated that it will hiss on coming into contact with water ; dry
wood may be ignited by friction, and an iron nail may be brought
into a state of white heat on being held against a moving grind-
stone of 7J feet in diameter. At the present time, we are unable
to afford a satisfactory explanation of these phenomena.

Theoretical Views concerning Heat* We have become ac-
quainted with the most important laws of the phenomena of heat,
without having entered upon the question of what heat really is.f
In this respect, therefore, the theory of heat has been treated pre-
cisely in the same manner as the first part of the theory of light,
where the empirical laws of reflection and refraction were deve-
loped, without anything further being said of the nature of light.
We are, however, still deficient in a theory from which the phe-
nomena of heat may be derived (as the phenomena of light from
the wave theory), not only qualitatively, but also quantitatively.

We generally imagine that heat is an imponderable substance,
penetrating bodies: and this idea answers very well for many
phenomena ; as, for instance, the combination of heat, and the
capacity for conducting heat, affording us a good representation
of these phenomena, the expressions being based upon this view.
If, however, the phenomena of the capacity for conducting heat,
of latent heat, and of diffusion of heat, accord tolerably well with
the idea of a substance of heat, it is, on the other hand, very im-
probable that there are such substances, and more likely that
imponderables will all vanish from physics, as has already been
the case with respect to light. In the theory of heat, the most
important step made, is, probably, that which corresponds to the
introduction of the theory of vibration in the case of light.

There are some phenomena which cannot be reconciled with
the views of heat being a substance; for instance, radiation and
the generation of heat by friction.

The laws of the radiation of heat are so similar to those of the
radiation of light, that we are tempted to ascribe the former like-
wise to a vibration of ether. If, however, radiating heat were
:ransmitted by the vibrations of ether, perceptible heat must like-
wise be occasioned by the vibrations of the material parts of

* Graham's "Elements of Chemistry," 2d edition, 8vo. 1847.
t Thomson's "Heat and Electricity," 2d edition, 8vo. 1840.


That the phenomena of heat actually arise from such vibrations,
is very probable, although we are not able, even in a satisfactory
degree, to explain all phenomena of heat on this hypothesis ; and
we are still unable to dispense with the idea of a substance of heat
in our representations and descriptions.

In order to explain the phenomena of heat by vibrations, we
must assume, that the temperature of bodies increases with the
amplitude of the oscillations ; and by such means we may also
explain expansion by heat.

The number of the vibrations is increased on the transition from
the solid to the fluid, and from the latter to the gaseous condition.
An increase in the number of the vibrations is, with an equal
amount of motion, alone possible when the amplitude is less ; and
thus we may explain the combination of heat.






THE heating of the earth's surface, and of the atmosphere, by
which alone the vegetable and animal world can thrive, is alone
owing to the rays of the sun, which must thus be regarded as the
source of all life, upon our planet. Where the mid-day sun stands
vertically above the heads of the inhabitants, and its rays strike
the earth's surface at a right angle, a luxuriant vegetation is
developed, if a second condition of its existence, namely, mois-
ture, be not wanting ; but where the solar rays constantly fall too
obliquely to produce any marked effect, nature is chained in
eternal ice, and all animal and vegetable life ceases.

In order to take a general survey of the distribution of heat
on the earth's surface, we must, in the first place, investigate
the consequences produced by the diurnal and annual motion of
the earth.

In consequence of the annual motion of the earth, the sun
continually alters its apparent position in the heavens; the path
which it traverses, during the year, passes through twelve con-
stellations, called the signs of the zodiac.

If we suppose the vault of heaven to be one large concave
sphere, the path of the sun will describe a large circle upon it,

> The want of space prevents this important subject being treated as fully here
as it deserves. The reader is, therefore, referred to the excellent translation of
KAEMTZ'S Complete Course of Meteorology, with Notes by C. V. Walker, illustrated
with 15 plates. London, 1845.


generally known by the name of the elliptic. This line does not
coincide with the celestial equator, intersecting it at an angle of
23 28'.

Twice in the year, namely, on the 21st of March, and on the
21st of September, the sun passes the celestial equator. From
March till September it is on the north, and from September to
March on the south, hemisphere; on the 21st of June it reaches
its most northern, and on the 21st of December its most southern,
point ; being, on the first-named day, at 23 28' north, and the
last-named, at 23 28' south, of the celestial equator.

The direction of our earth's axis coincides with the axis of the
heavens, the plane of the terrestrial equator, with that of the
celestial equator ; if, therefore, the sun stand directly upon the
celestial equator, its rays strike the earth's surface at every place
upon the terrestrial equator perpendicularly at mid-day, whilst
they only glance over the two terrestrial poles, striking the parts
contiguous to them very obliquely.

If we suppose two circles to be drawn upon the earth's surface
parallel with the equator, one 23 28' north, and the other equally
far south of it, the former will be the tropic of Cancer, and the
latter the tropic of Capricorn. All places lying upon these tropics
receive once in the year the sun's rays perpendicularly, this being
on the 21st of June for the tropic of Cancer, and the 21st of
December for the tropic of Capricorn.

The whole terrestrial zone lying between those two tropics is
termed the hot zone, because the rays of the sun falling but very
little obliquely, are able here to produce the most powerful effect.

Heat is tolerably equally distributed throughout the whole year
on the equator, because the sun's rays strike the earth rectangu-
larly twice annually, while they do not fall very obliquely at any
time intervening between these periods.

The more we approach the tropics, the more marked are the
differences of temperature at different periods of the year. In
the tropics, the solar rays only fall once in the year perpendicularly
on the .earth's surface, and once they make an angle of 47 with
the direction of the plumb line, falling, consequently, with very
considerable obliquity ; the temperatures of the hottest and coldest
season, separated by a period of half a year, differ very consider-
ably from each other.


On either side of the hot zone, extending from the tropics to
the polar zones, (the polar zones are those which have 24 hours
exactly for their longest day, and lie exactly 66 32' north and
south of the equator,) are the northern and southern temperate
zones ; the four seasons of the year are most strongly characterized
in these zones ; in general, heat diminishes with the distance from
the equator.

Around the poles, extending to the polar tropics, are the north-
ern and southern frigid zones.

In consequence of the rotation of the earth upon its axis, the
sun appears to participate in the apparent motion of all the pla-
nets ; and another result of this diurnal motion, is evidently the
alternation between day and night. It is only during the former
period that the solar rays warm the earth's surface, which, after
sunset, radiates heat towards the heavens, without the loss of heat
being compensated for; during the night, therefore, the surface
of the earth must be cooled.

Under the equator the day and night are equal throughout the
year, each day and night lasting 12 hours; as soon, however, as
we remove from the equator, the length of the day varies with the
season of the year, the variation becoming more striking as we
approach nearer to the poles. The following table contains the
length of the longest day for different geographical latitudes :

Polar elevation. Length of the longest day.

.... 12 hours.

16 44' . . . . 13

30 48' . . . . 14 "

49 22' . . . .16 "

63 23' . . . .20 "

66 32' . . . .24 "

67 23' . . . . 1 month.

73 39' . . . .3 months.

90 .... 6 u

At the equator, therefore, the variation in the day's length can-
not exercise any influence upon the course of the heat in the dif-
ferent seasons of the year. As the inequality in the length of the
days is not very considerable even under the tropics, the variation
in the length of the day between the tropics cannot very much



increase or diminish the differences of temperature between the
hot and cold seasons of the year ; this is the case, to a very con-
siderable degree, in high latitudes.

In summer, when the sun's rays fall less obliquely, the sun
remains longer above the horizon in high latitudes; this longer
period compensates for what is lost in intensity by the solar rays,
and it thus happens that it may be very hot during the summer
even at places which are far removed from the equator, (at St.
Petersburgh, for instance, the thermometer sometimes rises in a
hot summer to 86 ;) in the winter, on the other hand, when the
more obliquely falling solar rays have only little power of acting,
the day is very short, and the night, during which period the earth
radiates its heat, extremely long ; in consequence of which, the
temperature must fall very low at this season. The difference
between the temperature of summer and winter will, therefore,
generally be greater the further we remove from the equator.

At Bogota, which is 4 35' N. of the equator, the difference of
temperature between the hottest and coldest month amounts only
to 3; in Mexico (19 25' N. lat.) this difference is 14, at Paris
(48 50' N. lat.) 48, and for St. Petersburgh (59 56' N. lat.

From the above indicated considerations, it follows, therefore :

1. That heat must diminish from the equator towards the

2. That, in the vicinity of the equator, heat is distributed tole-
rably equally over the whole year; that, consequently, the charac-
ter of our seasons ceases there to be recognizable.

3. That the seasons always differ more in proportion as we go
further from the equator, and that, at the same time, the difference
between the summer and winter temperature becomes always
more considerable.

4. That, even in the neighborhood of the polar circles, the
summer may be very hot.

This we find fully confirmed by experience, notwithstanding
which, such a consideration can only teach us roughly to know
the distribution of heat upon the earth, it being impossible, from
the geographical latitude of a place, to draw any conclusion, even
remotely certain, as to its climatic relations.

If the whole earth's surface were covered by water, or if it were


all formed of solid plane land, possessing everywhere the same
character, and having an equal capacity at all places for absorb-
ing and again radiating heat, the temperature of a place would
depend only on its geographical latitude, and, consequently, all
places having the same latitude would have a like climate. Now,
however, the action that may be produced by the solar rays is
modified by manifold causes, the climate of one district depending
not only upon the direction of the solar rays, but also upon the
circumstances under which they act, such as the conformation of
the land and the sea, the direction and height of the mountain
range, the direction of the prevailing wind, &c. Hence it follows
that places of the same geographical latitude have frequently a

Online LibraryJohann Heinrich Jacob MüllerPrinciples of physics and meteorology → online text (page 44 of 55)