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upper part and a crack is produced.

1534. Wine-coolers. When wine-coolers have a double
casing, the external space is filled with a non-conductor.

1535. A heated globe cools inwards. When a solid body, a
globe for example, is heated at the surface, the heat passes
gradually from the surface to the centre. The temperature of
the superficial stratum is greater, and the temperature of the
centre less than those of the intermediate parts, and the tem-
perature of the successive strata are gradually less proceeding
from the surface to the centre.



CONDUCTION. 113

But if the globe be previously heated, so as to have an uniform
temperature from the centre to the surface, and be allowed to
cool gradually, the superficial stratum will first fall some
degrees below the stratum within it. This latter will fall
below the next stratum proceeding inwards ; and in the same
way each successive stratum proceeding from the surface to the
centre will attain a temperature a little lower than the stratum
under it, the temperatures augmenting from the surface to the
centre.

After an interval, of greater or less duration according to the
magnitude of the globe, the conductability and specific heat of
the matter of which it is composed, the temperature to which it
had been raised, and the temperature of the medium, it will be
reduced to an uniform temperature, which will be that of the
surrounding medium.

1536. Example of fluid metal cast in spherical mould. If a
mass of fluid metal be cast in a spherical mould, the surface only
will be solidified in the first instance. It will become a sphe-
rical shell, filled with liquid metal. As the cooling proceeds, the
shell will thicken, and after an interval of time, the length of
which will depend on the circumstances above mentioned, the
ball will become solid to its very centre, the last portion soli-
dified being that part of the metal which is at and immediately
around the centre.

It is evident that the superficial stratum will first cease to be
incandescent ; and in the same way each successive stratum pro-
ceeding from the surface to the centre will cease to be incan-
descent before the stratum within it.

If in the process of cooling, and after the globe ceases to be
red hot, it were cut through the centre, it would be found that
the central parts would be still incandescent ; and if its magni-
tude were sufficiently considerable, it would be found that even
after the superficial stratum had been reduced to a moderate
temperature, strata nearer the centre would be red hot, and the
central part still fluid.

1537. Cooling process may be indefinitely protracted. The
interval which must elapse before the thermal equilibrium would
be established, might be hours, days, weeks, months, years, or
even a long succession of ages, according to the magnitude and
physical qualities of the material composing the globe.

1538. Example of the castings of the hydraulic press, which



114 HEAT.

raised the Britannia Bridge The cylinder of the hydraulic
press by which the tubes of the Britannia bridge were elevated
was formed of a mass of fluid iron weighing 22 tons. This
enormous casting, after being left in the mould for three days
and nights, was still red hot at the surface. After standing to
cool in the open air for ten days, it was still so hot that it could
only be approached by men well inured to heat.

1539. Example of streams of volcanic lava. The torrents
of liquid lava which flow from volcanos become solid on their
external surface only to a certain thickness. The lava in the
interior of this shell still continues fluid. The stream of lava
thus forms a vast tube, within which that portion of the lava
still liquid flows for a long period of time. Months and even
years sometimes elapse, before the thermal equilibrium of these
volcanic masses is established.

1540. Example of the earth itself. The globe of the earth
itself presents a stupendous example of the play of these prin-
ciples. The vicissitudes of temperature incidental to the surface
extend to an inconsiderable depth. At the depth of an hundred
feet in our climates, they are completely effaced. At this depth,
the thermometer no longer varies with the seasons. In the
rigour of winter, and the ardour of summer, it stands at the
same point. This stratum, which is called the first stratum
of invariable temperature, is found to be at Paris at the depth
of 88-6 feet. The thermometer in the vaults under the Ob-
servatory at that depth has continued without variation at 1 1 - 82
cent. = 53-50 Fahr. for nearly two centuries.

1541. Temperature increases with the depth. At greater
depths the temperature increases, but is always invariable for
the same depth. An increase of temperature takes place in
descending, at the rate of one degree for every 54-^- feet of
depth. Thus the water which issues from the artesian wells
at Grenelle near Paris, and which rises from a depth of 1800
feet, has a constant temperature of 27'7 cent. = 82 Fahr.

It is apparent that the earth is a globe undergoing the
gradual process of cooling, and that each stratum proceeding
inwards towards the centre augments in temperature. It follows,
therefore, that a part at least of the superficial heat of the earth
proceeds from within. It is certain, nevertheless, by taking into
account all the conditions of the question, that the cooling goes
on so slowly, as to have no sensible influence on the tempera-



RADIATION. 115

ture at the surface, which is therefore governed by the solar
heat, and the heat of the medium or space in which our globe,
in common with the other planets, moves. It has been com-
puted that the quantity of central heat which reaches the
surface in a year would not suffice to dissolve a cake of ice a
quarter of an inch think.

1542. The earth was formerly in a state of fusion, and it
is still cooling. The globe of the earth therefore manifesting
the effects of a mass which, having been at some antecedent
period at an elevated temperature, is undergoing the process of
gradually cooling from the surface inwards, it is probable that
its central parts may be still in a state of incandescence or
fusion.



CHAP. X.

RADIATION.

1543. Heat radiates like light Heat, like light, is propagated

through space by radiation in straight lines, and its rays, like
those of light, are subject to transmission, reflection, and ab-
sorption by such bodies as they encounter in various degrees.

All that has been explained (896. et seq.} respecting the re-
flection of light from unpolished, perfectly and imperfectly
polished surfaces, its refraction by transparent media, its inter-
ference, inflexion, and polarization, may, with little modification,
be applied to the rays of heat submitted to like conditions.

1544. Thermal analysis of solar light. It has been already
shown that solar light is a compound principle, consisting of
rays differing one from another, not only in their luminous
qualities of colour and brightness, but also in their thermal and
chemical properties (1076. et seq.*).

Let ss,y%r. 449., represent a pencil of solar light transmitted
through a prism ABC, so as to be resolved into a divergent fan
of rays, and to form a spectrum as described in (1053.) et seq.
Let L and i/ be the limits of the luminous spectrum. If the
bulb of a thermometer be placed at L. it will not indicate any
elevation of temperature ; and if it be gradually moved down-



116



HEAT.



wards along the spectrum, it will not begin to be sensibly
affected until it arrives at the boundary of the violet and blue




Fig. 449.

spaces, where it will show an increased temperature. As it is
moved downwards from this point, the temperature will con-
tinue to increase until it is brought to the lower extremity I/ of
the luminous spectrum. If it be then removed below this point,
instead of falling to the temperature of the medium around the
spectrum, as might be expected, and as would in fact happen
if no rays of heat transmitted through the prism passed below
i/, it will descend slowly and gradually, and will in some cases
even show an increased temperature to a certain small distance
below i/. In fine, it will be found that the thermometer will
not fall to the temperature of the surrounding medium until
it arrives at a certain distance H' below i/, the extremity of the
luminous spectrum.

1545. Thermal solar rays differently refrangible. From
this and other similar experiments, it is inferred that thermal
rays which are not luminous, or at least not sensibly so, enter into
the composition of solar light, and that these rays are differently
refrangible, their mean refrangibility being less than the mean
refrangibility of the luminous rays.

It has been explained (1077) that the chemical rays which
enter into the composition of solar light are also differently
refrangible, and have a mean refrangibility greater than that
of the luminous rays.

1546. Physical analysis of solar light Three spectra.
According to this view of the constitution of solar light, the
prism ABC must be regarded as producing three spectra, a
chemical spectrum cc', a luminous spectrum LI/, and a thermal
spectrum HH'. The luminous or chromatic spectrum, the only
one visible, lies between, and is partly overlaid by, the other



RADIATION. 117

two, the chemical spectrum extending a little above, and the
thermal a little below it. If we imagine a screen MN placed
before the prism, composed of a material pervious to the lumi-
nous, but impervious to the chemical and thermal rays, then the
luminous spectrum LI/ alone will remain, and neither a ther-
mometer nor the chloride of silver, nor any other chemical sub-
stance, will be affected when exposed in it. If the screen MN
be pervious only to the thermal rays, then the luminous and
chemical rays will be intercepted, and the thermal spectrum HH'
alone will be manifested. The thermometer exposed in it will
indicate the variations of calorific influence already explained,
showing the greatest thermal intensity at or near that point at
which the red extremity of the luminous spectrum would have
been found, had the luminous rays not been intercepted.

1547. Relative refrangibility of the constituents of solar light
vary with the refracting medium. If prisms composed of
different materials be used, it will be found that the mean re-
frangibility of the thermal rays will vary according to the
material of the prism and heat ; consequently, the position of the
point of greatest thermal intensity will be subject to a like
variation.

If a hollow prism be filled with water or alcohol, the point of
greatest thermal intensity will be about the middle of the yellow
space of the luminous spectrum. If a prism of sulphuric acid,
or a solution of corrosive sublimate, be used, it will be in the
orange space. With a crown glass prism it will be in the red
space ; and with one of flint glass, a little below that space.

1548. Invisible rays may be luminous, and all rays may be
thermal. In the preceding explanation, the solar light is re-
garded as consisting of three distinct species of rays, the
chemical, the luminous, and the thermal. It is not necessary,
however, for the explanation of these phenomena, to adopt this
hypothesis. The light may be considered as consisting of rays
which, differing in refrangibility, possess the other physical
qualities also in different degrees. So far as the sensibility of
thermometers enables us to detect the thermal property, it ceases
to exist at a certain point, H G, near the boundary of the blue
and violet spaces ; but the diminution of thermal intensity, in
approaching this point, as indicated by the thermometer, is very
gradual; and it cannot be denied, that a thermal influence may
exist above that point, which is, nevertheless, too feeble to affect
the thermoscopic tests which are used. In the same manner it



118 HEAT.

may be maintained, that a chemical influence may exist below
the point c', but too feeble to affect any of the tests which have
been applied to it.

But it may be asked, if all the component rays possess all
the properties in different degrees, how happens it that the
chemical rays above L, and the thermal rays below i/, are not
visible? To this it may be answered, that the presence of the
luminous quality is determined by its effects on the eye ; and the
discovery of its presence must therefore depend on the sen-
sibility of that organ. To pronounce that there are no luminous
rays beyond the limits of the chromatic spectrum, would be
equivalent to declaring the sensibility of the eye to be unlimited.
Now, it is notorious that the sensibility of sight, in different
persons, is different ; and, even in the same individual, varies at
different times. Circumstances render it highly probable that
many inferior animals have a sensation of light, and a perception
of visible objects, where the human eye has none; and it is
therefore consistent with analogy to admit the possibility, if
not the probability, that the invisible thermal rays below i/, and
the invisible chemical rays above L, may be of the same nature
as the other rays of the spectrum, all enjoying the luminous,
thermal, and chemical properties in common, the apparent
absence of these properties in the extreme rays being ascribable
solely to the want of sufficient sensibility in the only tests of
their presence which we possess.

Fortunately, however, the deductions of physical science,
though they may be facilitated by these and other hypotheses,
are not dependent on them, but on observed facts and phe-
nomena, and cannot, consequently, be shaken by the failure of
such theories.

1549. Refraction of invisible thermal rays. If a hole be
made in the screen upon which the prismatic spectrum is thrown,
in the space L' H' below the red extremity of the spectrum
upon which the invisible thermal rays fall, these rays will pass
through it, and may be submitted to all the experiments on
reflection, refraction, inflexion, interference, and polarization,
which have been explained in relation to light. This has been
done, and they have been found to manifest effects similar to
those exhibited by luminous rays.

1550. Heat radiated from each point on the surface of a body.
It has been shown (902. et seq.) that when a body is either
luminous, like the sun, or illuminated, like the moon, each point



RADIATION. 119

upon its surface is an independent centre of radiation or focus,
from which rays of light diverge or radiate in all directions!
It is the same with regard to heat. All bodies, whatever be
the state or condition, contain more or less of this physical
principle ; and rays of heat accordingly issue from every point
upon their surface, as from a focus, and diverge or radiate in
all directions through the surrounding space.

1551. Why bodies are not therefore indefinitely cooled.

This being the case, it would follow that by such continual and
unlimited radiation, bodies -would gradually lose their heat, and
indefinitely fall in temperature. It must be considered, however,
that such radiation being universal, each body, while it thus
radiates heat, receives upon its surface the rays of heat which
proceed from other bodies around it. So many of these rays
as it absorbs tend to increase its temperature, and to replace
the heat dispersed by its own radiation. There is thus between
body and body a continual interchange of heat by radiation ;
and according as this interchange is equal or unequal, the tem-
perature of the radiating body will rise or fall. If it radiate
more than it absorbs, it will fall ; if less, it will rise. If it absorb
as much exactly as it radiates, its temperature will be maintained
stationary.

1552. Radiation is superficial or nearly so. Radiation takes
place altogether from points either on the surface or at a very
small depth below it. The circumstances which affect it have
been made manifest by a beautiful series of experiments made
by the late Sir John Leslie. The principles on which his mode
of experimenting was founded, are easily explained.

1553. Reflection of heat.~Let a cubical canister of tin,

fig. 450., be placed in the axis of
a parabolic metallic reflector M,
in the focus /of which is placed
the bulb of a sensitive differential
thermometer. If the canister
be placed with one of its sides at
right angles to the axis of the re-
flector, and be filled with boiling
water, the thermometer will in-
stantly show an increase of tem-
perature caused by the heat radiated from the surface of the
canister, and collected into a focus upon the ball by the
reflector.




120 HEAT.

The experiment may be varied by filling the canister with
liquids at all temperatures, with snow and with freezing mix-
tures having various degrees of artificial cold. The surface of
the canister may be varied in material by attaching to it
different substances, such as paper, metallic foil, glass, por-
celain, &c. It may be varied in texture by rendering it rough
or smooth, and in colour by any colouring matter.

In this way the influence of all these physical conditions
upon the radiation from the surface may be, and has been,
ascertained.

The results of such experimental researches have been
briefly as follows :

1554. Rate of radiation proportional to excess of temperature
of radiator above surrounding medium. The rate at which
the radiating body loses or gains temperature, other things being
the same, is proportional to the difference between its own tem-
perature and that of the surrounding medium, where this dif-
ference is not of very extreme amount.

1555. Intensity inversely as square of distance. The in-
tensity of the heat radiated is, like that of light, other things
being the same, inversely as the square of the distance from the
centre of radiation (907).

1556. Influence of surface on radiating power. The ra-
diating power varies with the nature of the surface, and its
degree of polish or roughness.

In general, the more polished a surface is, the less will be its
radiation. Whatever tarnishes or roughens the surface of metal,
increases its radiation.

Metallic are in general less powerful radiators than non-
metallic surfaces.

1557. Rejection of heat. When the rays of heat encounter
any surface, they are more or less reflected from it. Surfaces,
therefore, in relation to heat, are perfect or imperfect, good or
bad reflectors.

In the experiments above described, the reflecting powers
of different surfaces were ascertained by constructing the
concave reflector M of different materials, or by coating its
surface variously, or, in fine, by submitting its surface to any
desired physical conditions- Thus, when a reflector of glass is
substituted for one of metal, the radiating surface of the canister
remaining the same, it is found that the effect on the thermo-



EADIATION. 121

meter is diminished. Glass is therefore a less perfect re-
flector than metal. If the surface of the reflector be coated
with lampblack, no effect whatever is produced on the ther-
mometer. Such a surface does not, therefore, reflect the
thermal rays.

1558. Absorption of heat. To determine the physical con-
ditions which affect the absorbing power of a surface, it is only
necessary, in the experiment above described, to vary the surface
of the ball^of the thermometer, which is placed in the focus
of the reflector, for, as the heat is radiated by c and reflected
by M, it is absorbed by t.

By coating the ball of the thermometer, therefore, with
metallic foil, paper, lampblack, and other substances, and by
rendering it in various degrees rough and smooth, the effects
of these modifications on the thermometer are rendered manifest,
and the comparative absorbing powers are ascertained.

In this way it has been ascertained that the same physical
conditions which increase the radiation and diminish the reflec-
tion, increase the absorption. The best radiators are the most
powerful absorbers and the most imperfect reflectors.

1559. Tabular statement of radiating and reflecting
poivers. The relative radiating, absorbing, and reflecting
powers of various surfaces have been submitted to a still more
rigorous analysis by M. Melloni, whose researches were greatly
favoured by the fine climate of Naples, where they were prin-
cipally made. The results are given in the following table,
in the first column of which the numbers express the radiating
and absorbing powers, that of a surface covered with the smoke
of a lamp being expressed by 100. The absorbing power of this
surface is complete. The reflecting power is, as will be ob-
served, the complement of the absorbing power.



122



HEAT.



Table showing the absorbing and reflecting Powers of various
Surfaces according to the Experiments of Melloni.





li


-




i!






**


=;







Is


Names.


1?




Kama.


o


J!




If


S*




3j_




Smoke-blackened surface


100





Metallic mirrors a little tar-






Carbonate of lead -


100





nished - - - -


17


83


Writing paper


98
90


2
10


nearly polished
Brass cast, imperfectly po-


14


86


China ink


85


15


lished - ...


11


89


Gumlac -


72




hammered,


9


91


Silver foil on glass
Cast iron polished -


27
25


73
75


highly polished
,, cast, ,,


7

7


93
93


Mercury (nearly) -
Wrought iron polished
Zinc polished
Steel,


23

23
19
17


77
77
81
83


Copper coated on iron -
varnished -
hammered or cast
Gold plating -


7
14
7
5


93
86
93
95


Platinum, thick coat, imper
fectly polish d


24


76


Gold deposited on polished
steel


3


97


plate on coppe
leaves


17
17


83


Silver, hammered and well
polished - - - -


3


97


Tin- - - -


14


86


Silver, cast, and well polished


3


97



1560. Singular anomaly in the reflection from metallic
surfaces. The numbers given in this table, which will be ob-
served to differ considerably from those determined by Leslie
and others, have been obtained by the recent elaborate experi-
mental researches of MM. de la Provostaye and Desains. In
these experiments an anomalous circumstance was observed on
varying the angle of incidence of the thermal rays. It was
found that, in the case of glass, the proportion of rays reflected
increased with the angle of incidence, as happens with luminous
rays, but that with polished metallic surfaces, the same propor-
tion was reflected at all incidences up to 70, and beyond this
limit the proportion reflected, instead of increasing, as would
have been expected, was greatly diminished.

1561. Thermal equilibrium maintained by the interchange of
heat by radiation and absorption. From all that has been
here explained it will be apparent that the state of thermal
equilibrium is maintained among any system of bodies by a
continual interchange of heat by radiation and absorption. The
heat which each body receives from others in its presence, it
partly absorbs and partly reflects. Those rays which it absorbs
tend to raise its temperature ; and this temperature would soon
rise above that which the thermal equilibrium requires, but
that the body radiates heat from all points of its surface ; and



RADIATION. 12,3

the total quantity thus radiated is equal to the total quantity
absorbed. If either of these quantites were permanently greater
or less than the other, the temperature of the body would either
indefinitely rise, or indefinitely fall, according as the heat
absorbed or radiated might be in excess.

If a body, at any given temperature, be placed among other
bodies, it will immediately affect them thermally, just as a
candle brought into a room illuminates all bodies in its pre-
sence, with this difference, however, that if the candle be extin-
guished, no more light is diffused by it ; but no body can be
thermally extinguished. All bodies, however low be their tem-
perature, contain heat, and therefore radiate it.

1562. Erroneous hypothesis of radiation of cold. If a ball
of ice be brought into the presence of a thermometer, the ther-
mometer will fall ; and hence it was erroneously inferred that
the ice emitted rays of cold. The effect, however, is otherwise
explained. The ice and the ball of the thermometer both
radiate heat, and each absorbs more or less of what the other
radiates towards it. But the ice being at a lower temperature
than the thermometer, radiates less than the thermometer, and
therefore the thermometer absorbs less than the ice, and con-
sequently falls.



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