CHAPTER VI.

HEAT--SOLAR AND TERRESTRIAL.

Solar and Terrestrial Heat--Position of the Earth in the Solar

System--Heat and Light associated in the Sunbeam--Transparency

of Bodies to Heat--Heating Powers of the Coloured Rays of

the Spectrum--Undulatory Theory--Conducting Property of the

Earth’s Crust--Convection--Radiation--Action of the Atmosphere

on Heat Rays--Peculiar Heat Rays--Absorption and Radiation

of Heat by dissimilar Bodies--Changes in the Constitution of

Solar Beam--Differences between Transmitted and Reflected

Solar Heat--Phenomena of Dew--Action of Solar Heat on the

Ocean--Circulation of Heat by the Atmosphere and the Ocean--Heat

of the Earth--Mean Temperature--Central Heat--Constant Radiation

of Heat Rays from all Bodies--Thermography--Action of Heat on

Molecular Arrangements--Sources of Terrestrial Heat--Latent

Heat of Bodies--Animal Heat--Eremacausis--Spheroidal State

Cold--Condensation--Freezing--Theories of Heat--Natural

Phenomena--and Philosophical Conclusion.

We receive heat from the sun, associated with light; and we have the

power of developing this important principle by physical, mechanical,

and chemical excitation, from every kind of matter. Our convictions

are, that the calorific element, whether derived from a solar or a

terrestrial source, presents no essential difference in its physical

characters; but as there are some remarkable peculiarities in the

phenomena, as they arise from either one or the other source, it will

assist our comprehension of this great principle, if we consider it

under the two heads.

Untutored man finds health and gladness in the warmth and light of the

sun; he rears a rugged altar, and bows his soul in prayer, to the

principle of fire, which in his ignorance he regards as the giver and

the supporter of life. The philosopher finds life and organization

dependent upon the powers combined in the sunbeam; and, examining

the phenomena of this wonderful band of forces, he is compelled to

acknowledge that the flame upon the altar--on the Persian hills,--was

indeed a dim shadow of the infinite wisdom which abides behind the veil.

The present condition of our earth is directly dependent upon the

amount of heat we receive from the sun. It has frequently been said,

that if it were possible to move this planet so much nearer that orb

that the quantity of heat would be increased, the circumstances of

life would necessarily be so far changed, that all the present races

of animals must perish; and that the same result would happen from

any alteration which threw us yet further from our central luminary,

when, owing to the extremity of cold and the wretchedness of gloom, all

living creatures would equally fail to support their organization.

To move the earth nearer to, or more distant from the sun, is an

impossibility; but it has been argued that those planets which are near

to the sun must possess a temperature which would melt our solid rocks,

and vaporize the ocean,--while Uranus and Neptune must, from their

distance from the source of heat, have so small an amount, that water

must become solid as the rock, and such an atmosphere as that of the

earth exist as a dense liquid.

It will be shown that according to the physical condition of the

material substances, so are their powers regulated of absorbing and

retaining the heat which falls as a radiant power upon their surfaces.

Heat rays, in passing through the attenuated medium of planetary space,

lose none of their power--this we know from the fact that even the less

dense upper region of the earth’s atmosphere takes from the solar rays

but an exceedingly small quantity of heat. Therefore, whether a solar

heat ray traverses through one million, or one hundred million miles of

space, it still retains its power equally of imparting warmth to the

solid matter by which it is intercepted. There is no law of variation

as the inverse square of the distance of those radiating powers.

Consequently, there is no reason why the physical conditions, alike

of the nearest and the most remote planetary bodies, should not be so

adjusted that they all enjoy that life promoting temperature which

belongs to the earth.

All the objects around us are adapted to the circumstances of the

earth’s position in relation to the sun, to which we are bound by the

principle of gravitation; opposed to that centrifugal force which tends

constantly to drive the moving planetary mass off from the centre of

power. The balance maintains its perfect equilibrium, although we have

one power constantly drawing the earth towards the sun, and the other

as constantly exerting itself to move it off into space at a tangent

to the orbit in which the planet moves. In our examination it will be

found that one common system of harmony runs through all the cosmical

phenomena, by which everything is produced that is so beautiful and

joyous in this world.

Heat, and the other elementary radiant principles, are often combined

as the common cause of effects evident to our senses. The warmth of the

solar rays, and their luminous influence, are not, however, commonly

associated in the mind as the results of a single cause. It is only

when we come to examine the physical phenomena connected with these

radiations that we discover the complexity of the inquiry. Yet it is

out of these very subtle researches that we draw the most refined

truths. The high inferences to which the analysis of the subtile

agencies of creation leads us, render science, pursued in the spirit of

truth, a great system of religious instruction.

Although we do not fear that heat and light can be confounded in the

mind, so different are their phenomena,--we have heat rays, as from

dark hot iron, which give no light, while in the full flood of the

lunar rays the heat is scarcely appreciable by the most delicate

instruments;--yet it is important to show how far these two principles

have--been separated from each other. Transparent bodies have varied

powers of calorific transparency, or transcalescence: some obstructing

the heat radiated from bodies of the highest temperatures almost

entirely even in the thinnest layers; whilst others will allow the

warmth of the hand to pass through a thickness of several inches.

Liquid chloride of sulphur, which is of a deep red colour, will allow

63 out of 100 rays of heat to pass, and a solution of carmine in

ammonia, or glass stained with oxides of gold, or copper, rather a

greater number; yet these transparent media obstruct a large quantity

of light. Colourless media obstructing scarcely any light, will, on

the contrary, prevent the passage of calorific rays. Out of every

hundred rays, oil of turpentine will only transmit 31, sulphuric ether

21, sulphuric acid 17, and distilled water only 11. Pure flint glass,

however, is permeated by 67 per cent. of the thermic rays, and crown

glass by 49 per cent. The body possessing the most perfect transparency

to the rays of heat is diaphanous salt-rock, which transmits 92, while

alum, equally translucent, admits the passage of only 12 per cent.[43]

Black mica, obsidian, and black glass, are nearly opaque to light, but

they allow 90 per cent. of radiant heat to pass through them; whereas

a pale green glass, coloured by oxide of copper,[44] covered with a

layer of water, or a very thin plate of alum, will, although perfectly

transparent to light, almost entirely obstruct the permeation of heat

rays.

We thus arrive at the fact that heat and light may be separated from

each other; and if we examine the solar beam by that analysis which the

prism affords, we shall find that there is no correspondence between

intense light and ardent heat. By careful observation, it has been

proved, when we have a temperature of 62° F. in the yellow ray, which

ray has the greatest illuminating power; that below the red ray, out of

the point of visible light, the temperature is found to be 79°, while

at the other end of the spectrum, in the blue ray, it is 56°, and at

the end of the violet ray no thermic action can be detected.[45]

From the circumstance, that as we, by artificial means, raise the

temperature of any body, and produce intense heat, so after a

certain point of thermic elevation has been obtained, we occasion a

manifestation of _light_.[46] It has been concluded, somewhat hastily,

that heat and light differ from each other only in the rapidity of the

undulations of an hypothetical ether.

It must be admitted that the mathematical demonstrations of many of the

phenomena of calorific and luminous power are sufficiently striking to

convince us that a wave-movement is common to both heat and light. The

undulatory theory, however, requires the admission of so many premises

of which we have no proof; its postulates are, indeed, in many cases

so gratuitous, that notwithstanding the array of talent which stands

forward in its support, we must not allow ourselves to be deceived by

the deductions of its advocates, or dazzled by the brilliancy of their

displays of learning.

Radiant heat appears to move in waves; but that calorific effects in

material bodies are established by any system of undulation, is a

deduction without a proof; and the thermic phenomena of matter are as

easily explained by the hypothesis of a diffusive subtile fluid.

We have not, however, to prove the correctness of either of the

opposing views; indeed, it is acknowledged that many phenomena require

for their explanation conditions which are not indicated by either

theory.

The earth receives its heat from the sun; a portion of it is

_conducted_ from particle to particle into the interior of the rocky

crust. Another portion produces warmth in the atmosphere around us,

by _convection_, or the circulation of particles; those warmed by

contact with the surface becoming lighter, and ascending to give place

to the colder and heavier ones. A third portion is radiated off into

space, according to laws which have not been sufficiently investigated,

but which are dependent upon the colour, chemical composition, and

mechanical structure of the surface.

It cannot but be instructive to contemplate the indications which we

have of the dependence of all that is beautiful on earth, on the heat

and light radiations which we receive from the sun. Let us endeavour

to realise some of the effects which arise from even the temporary

deprivation of solar heat.

It is winter, the vegetable world appears chilled to its centre. The

trees, except a few of the hardy evergreens, are bare of leaves, and

stretching forth their branches into the cold air, they realise the

condition of vegetable skeletons. The lowly plants of the hedge-row,

and the grasses of the field, show that their vital power is subdued

to that minimum degree of action which is but a few slight removes

from death. The life of the running stream is suspended, it is cased

in the “thick-ribbed ice,” and the waters beneath no longer send forth

their joyous music to the genial breeze. Even within the temperate

limits of our own land, the aspect of winter convinces the ordinary

observer, that the loss of heat has been followed by diminished

activity in the powers of life; and the philosopher discovers that the

lessened energies of solar light, and the weaker action of the radiant

heat, have aided in producing that repose which is a little more than

sleep--a little less than death.

It is night, and winter: the earth is parting with its heat,--with

the absence of light, there is a still greater loss of vigour, a yet

further diminution of the powers of life. Even the animal races,

sustained by vital influences of a more exalted kind, sink under the

temporary deprivation of the solar rays to a monotonous, a melancholy

repose. All animals undergo different degrees of hybernation, and

each in his winter retreat supports vitality by preying upon himself.

The world is hung in mourning black; there is no play of colours to

harmonize the human spirit by sending their ethereal pulsations to the

human eye, and it is only the consciousness that when the night is at

the darkest, the day is nearest, that even man’s soul is sustained

against the depressing influences of the absence of the sun.

The conditions which we must observe at our own doors cannot fail to

convey as a conviction to the least imaginative mind, that a slightly

prolonged continuance of darkness, with its consequent increase of

coldness, would be fatal to the existence of the organic world.

The sun has entered Aries: it is spring. The length of the day and

night are equal, the powers of light and darkness are now exactly

balanced against each other, and light, like the Archangel, triumphs

over the sombre spirit. The organic world awakes. Chemical action

commences in the seed, the vital spark is kindled in the embryo, and

under the impulsive force of some solar radiations the plant struggles

into light and life. The same invigorating force impels the circulation

of the sap through the capillary tubes of the forest tree, until the

topmost branch trembles with the new flow of life. The buds burst forth

into leaf, and a fresh and lively covering spreads over those branches

which, in their nakedness, could scarcely be distinguished from the

dead.

The animal races are no less sensible of the new influence which is

diffused around. The birds float joyously upon the breeze, and give

to heaven their trilling songs of praise. The beasts come forth from

the clefts of the rocks and the tangled shelters of the forests, and

gambol in the full luxury of their renewed vigour. Man, even man, the

inhabitant of cities, trained and tempered to an artificial state,

awakes of a spring morning with a fuller consciousness of mind, and a

deeper and more pleased sense of his intelligence, than when the fogs

and gloom of winter hung like the charmed robe upon the limbs of the

giant. Now, the dormant poetry of man seeks expression. As the morning

sun is said to have awakened the musical undulations of the Memnonian

statue, so the sun of the vernal morning produces in the mind of the

most earthly, faint pulsations of that heaven-born music, which neither

sin nor sorrow can entirely destroy. The psychologist, in studying the

peculiar phenomena of the human mind, must associate himself with the

natural philosopher, and learn to appreciate the influence of physical

causes in determining effects which our elder philosophers and the

poets of every age have attributed to spiritual agencies.

Summer, with its increased heat and light, reigns over the land. The

work of life is now at its maximum, and every energy is quickened

throughout the organic creation. The laws of nature are arranged

on the principle of antagonistic forces, the constant struggle to

maintain them in equilibrium constituting the sensible phenomena of

existence. Heat and light, with chemical power and electricity, have

been quickening the unknown principle of life, until it has become

exhausted in the production of new parts--in the strange phenomenon of

growth--the formation of organized matter from the inorganic stores of

creation.

The autumn, with its tempered sunlight, comes, but in the solar

radiance we discover new powers, and under the influence of these the

flower and the fruit have birth. The store of a new life is centered in

the seed, and though the leaf falls, and the flower fades, a new set

of organisms are produced, by which the continuance of the species is

secured.

Let any man examine himself as the seasons change, and he will soon be

convinced that every alternation of light and darkness, of heat and

its absence, produces new sets of influences equally on the mind and

on the body, showing the entire dependence of the animal and vegetable

kingdoms upon those causes which appear to flow from the centre of our

planetary system.

The phenomena which connect themselves with the changes of the seasons

cannot fail to convince the most superficial thinker that there is an

intimate connection between the sun and the earth which deserves our

close attention.

Indeed, if we examine the most ancient of histories, we find one great

fact at the base of all their philosophies. Moses connects darkness

with a void and formless earth, and light with the creation of harmony

and life. Menis sings of a fearful world by “many formed darkness

encircled,” and links the idea of a “life-breathing divinity” with the

awakening of light upon created things. The Egyptian Isis, the Grecian

Apollo, who,

The Lord of boundless light

Ascending calm o’er the empyrean sails,

And with ten-thousand beams his awful beauty veils,

the fire-worshipper of the Persian hills and the sun-god of the

Peruvian mountains, exhibit, through time and space, the full

consciousness of man to the influences of solar light and heat upon the

organic creations of which he is himself the chief exemplar.

The investigations of modern philosophers have extended these

influences to the inorganic masses which constitute the Planet

EARTH:--and we now know that the physical forces, ever active in

determining the chemical condition and the electrical relations of

matter, are directly influenced by the solar radiations.

Few things within the range of our inquiry are more striking than

the phenomena of calorific radiation and absorption. They display so

perfectly the most refined system of order, and exhibit so strikingly

the admirable adaptation of every formation to its particular

conditions, and for its part in the great economy of being, that they

claim most strongly the study of all who would seek to discover a

poetry in the inferences of science.

Owing to the nature of our atmosphere, we are protected from the

influence of the full flood of solar heat. The absorption of caloric by

the air has been calculated at about one-fifth of the whole in passing

through a column of 6,000 feet. This estimate is, of course, made near

the earth’s surface; but we are enabled, knowing the increasing rarity

of the upper regions of our gaseous envelope in which the absorption

is constantly diminishing, to prove, that about one-third of the solar

heat is lost by vertical transmission through the whole extent of our

atmosphere.[47]

Experience has proved that the conditions of the sun’s rays are not

always the same; and there are few persons who have not observed that

a more than usual scorching influence prevails under some atmospheric

circumstances. This is also evidenced in the effects produced on the

foliage of trees, which, though often attributed to electricity, is

evidently due to heat. An examination of the solar radiations, as

exhibited in the prismatic spectrum, has proved the existence of a

class of heat rays, which manifest themselves by a very peculiar

deoxidizing power quite independent of their caloric properties,

to which the name of _parathermic rays_ has been given.[48] We are

protected from the severe effects of these rays by the ordinary state

of the medium through which the solar heat passes. Our atmosphere is a

mixture of gases and aqueous vapour; and it has been found, as already

stated, that even a thin film of water, however transparent, prevents

the passage of many calorific radiations, and the rays retarded are,

for the most part, of that class which have this peculiar scorching

power. The air is, in this way, the great equaliser of the solar

heat, rendering the earth agreeable to all animals, who, but for this

peculiar absorbent medium, would have to endure, even in our temperate

clime, the burning rays of a more than African sun.

The surface of the earth during the sunshine--and, though in a less

degree, even when the sun is obscured by clouds--is constantly

receiving heat; but the rate of its absorption varies. Benjamin

Franklin showed, by a set of simple but most conclusive experiments,

that a piece of black cloth was warmed much sooner than cloth of a

lighter colour;[49] and we know, from observations of a similar class,

that the bare brown soil receives heat more readily than the bright

green grassy carpet of the earth. Consequently, during the winter

season, relatively to the quantity poured from its source, more heat

penetrates the uncovered soil, than during the spring or summer.

There is a constant tendency to an equilibrium; and, during the night,

the surface is robbed of more heat, by the colder air, than by day; as,

when the earth is not receiving heat, it is constantly radiating it

back into space. Even in these processes of convection and radiation, a

similar law prevails to that which is discovered in examining into the

rate of calorific absorption.

Every tree spreading its green leaves to the sunshine, or exposing

its brown branches to the air--every flower which lends its beauty to

the earth--possesses different absorbing and radiating powers. The

chalice-like cup of the pure white lily floating on the lake--the

variegated tulip--the brilliant anemony--the delicate rose--and the

intensely coloured peony or dahlia--have each powers peculiar to

themselves for drinking in the warming life-stream of the sun, and

for radiating it back again to the thirsting atmosphere. These are

no conceits of a scientific dreamer; they are the truths of direct

induction; and, by experiments of a simple character, they may be put

to a searching test.[50]

A thermometric examination of the various coloured flowers, by

enclosing a delicate thermometer amongst their leaves, will readily

establish the correctness of the one; and by a discovery of recent

date, connected with calorific radiation, which must be particularly

described presently, we can, with equal ease and certainty, test the

truth of the other;[51] the absorption and radiation of heat being

directly regulated by the colours of the surfaces upon which the sun

rays fall.

It follows, as a natural consequence of the position of the sun, as

it regards any particular spot on the earth at a given time, that the

amount of heat is constantly varying during the year. This variation

regulates the seasons.

When it is remembered that the earth is, in the winter, nearly three

millions of miles nearer the sun than in the summer, some explanation

is required to account for our suffering more cold when nearer the

source of heat, than when at the remotest distance.

The earth in her path around the sun describes an ellipse, the

sun’s place being one of its foci. In obedience to the law, already

described, of the conservation of the axis of rotation, the axis of

the earth constantly points towards the star in the constellation of

the Little Bear. Recollecting this, and also the two facts, that a

dense solid body absorbs heat more readily than a fluid one, and that

radiation from the surface is constantly going on when absorption is

not taking place, let us follow the earth in her orbit.

It is the time of the vernal equinox--we have equal day and

night--therefore the periods of absorption and radiation of heat are

alike. But at this time of the year the southern hemisphere is opposite

to the sun, consequently the degree of absorption by the wide-spread

oceans small.

It is the summer solstice--we have sixteen hours of daylight, when the

absorption of heat is going on--and but eight hours of night, during

which heat is passing off. The northern hemisphere is now presented

to the sun, and as here we have the largest portion of dry land, the

powers of absorption are at their maximum.

The autumnal equinox has arrived, with its equal day and night, as in

the spring, but now the whole northern hemisphere is opposite the sun;

hence, according to the laws already explained, we see the causes of

the increased heat of the autumnal season.

The winter solstice has come, with its long night and shortened day.

The time during which radiation is going on is nearly twice that in

which absorption takes place, and the earth is in her worst position

for receiving heat, as that half which has the largest surface of water

is towards the sun.

These are the causes which lead to the variations of the seasons, and

through these we learn why we are colder when near the sun than when at

a considerably greater distance.

An analysis of the spectrum shows us that there are some changes

regularly taking place in the state of the solar beam, which cannot

be referred to the mere alteration of position. It may be inferred,

from facts by long-continued observations, that the three classes of

phenomena--light, heat, and chemical power, distinguished by the term

Actinism--which we detect in the sun’s rays, are constantly changing

their relative proportions. In spring, the chemical agency prevails; in

summer, the luminous principle is the most powerful; and in the autumn,

the calorific forces are in a state of the greatest activity.[52] The

importance of these variations, to the great economy of vegetable life,

will be shown when we come to examine the phenomena connected with

organisation.

A remarkable change takes place in the character of heat in being

reflected from material substances. In nature we often see this fact

curiously illustrated. Snow which lies near the trunks of trees or

wooden poles melts much quicker than that which is at a distance from

them, the sun shining equally on both--the liquefaction commencing on

the side facing the sun, and gradually extending. We see, therefore,

that the direct rays of solar heat produce less effect upon the snow

than those which are radiated from coloured surfaces. By numerous

experiments, it has been shown that these secondary radiations are

more abundantly absorbed by snow or white bodies than the direct solar

rays themselves. Here is one of the many very curious evidences, which

science lays open to us, of the intimate connection between the most

ethereal and the grosser forms of matter. Heat, by touching the earth,

becomes more earth-like. The subtile principle which, like the spirit

of superstition, has the power of passing, unfelt, through the crystal

mass, is robbed of its might by embracing the things of earth; and

although it still retains the evidences of its refined origin, its

movements are shackled as by a clog of clay, and its wings are heavy

with the dust of this rolling ball. It has, however, acquired new

properties, which fit it for the requirements of creation, and by which

its great tasks are facilitated. Matter and heat unite in a common

bond, and, harmoniously pursuing the necessities of some universal law,

the result is the extension of beautiful forms in every kingdom of

nature.

An easy experiment pleasingly illustrates this remarkable change. If a

blackened card is placed upon snow or ice in the sunshine, the frozen

mass underneath it will be gradually thawed, and the card sink into

it, while that by which it is surrounded, although exposed to the full

power of solar heat, is but little disturbed. If, however, we reflect

the sun’s rays from a metal surface, an exactly contrary result takes

place; the uncovered parts are the first to melt, and the blackened

card stands high above the surrounding portion.

The evidences of science all indicate the sun as the source, not only

of that heat which we receive directly through our atmosphere, but

even of that which has been stored by our planet, and which we can,

by several methods, develope. We have not to inquire if the earth was

ever an intensely heated sphere;--this concerns not our question; as

we should, even were this admitted, still have to speculate on the

origin--the primitive source of this caloric.

Before, however, we proceed to the examination of the phenomena of

terrestrial heat, a few of the great results of the laws of radiation

and convection claim our attention.

Nearly all the heat which the sun pours upon the ocean is employed in

converting its water into vapour at the very surface, or is radiated

back from it, to perform the important office of producing those

disturbing influences in the atmosphere, which are essential to the

preservation of the healthful condition of the great aërial envelope in

which we live.

Currents of air are generally due to the unequal degrees in which the

atmosphere is warmed. Heat, by expanding, increases the elasticity,

and lessens the density, of a given mass. Consequently, the air heated

by the high temperature of the tropics, ascends charged with aqueous

vapours, whilst the colder air of the temperate and the frigid zones

flows towards the equator to supply its place. These great currents of

the atmosphere are, independent of the minor disturbances produced by

local causes, in constant flow, and by them a uniformity of temperature

is produced, which could not in any other way be accomplished. By these

currents, too, the equalisation of the constituents of the “breath

of life” is effected, and the purer oxygen of the “land of the sunny

south” is diffused in healthful gales over the colder climes of the

north. The waters, too, evaporated from the great central Atlantic

Ocean, or the far Pacific, are thus carried over the wide-spread

continents, and poured in fertilising showers upon distant lands.

How magnificent are the operations of nature! The air is not much

warmed by the radiations of caloric passing from the sun to the earth;

but the surface soil is heated by its power of absorbing these rays.

The temperature of the air next the earth is raised, and we thus have

the circulation of those beneficial currents which are so remarkably

regular in the Trade Winds. The air heated within the tropics would

ascend directly to the poles, were the earth at rest, but being in

motion, those great aërial currents--the Trade Winds--are produced,

and the periodical monsoons are due to the same cause. A similar

circulation, quite independent of the ordinary tidal movement,

takes place also in the earth-girdling ocean. The water, warmed, by

convection, from the hot surface of the tropical lands, sets across

the Atlantic from the Gulf of Mexico; and being under the influence of

the two forces--gravity and motion--it illustrates the parallelogram

of forces, and flowing along the diagonal, reaches our own shores: the

genial influences of the gulf stream produce that tempered climate

which distinguishes our insular home. Here we have two immense

influences produced by one agency, rendering those parts of the earth

habitable and fertile, which but for these great results would sorrow

in the cheerless aspect of an eternal winter.

The beautiful phenomenon of the formation of dew is also distinctly

connected with the peculiar properties which we have been studying.

When from the bright blue vault of heaven, the sparkling constellations

shower their mild light over the earth, the flowers of the garden

and the leaves of the forest become moist with a fluid of the most

translucid nature. Well might the ancients imagine that the dews were

actually shed from the stars; and the alchemists and physicians of the

middle ages conceive that this pure distillation of the night possessed

subtile and penetrating powers beyond most other things; and the

ladies of those olden times endeavour to preserve their charms in the

perfection of their youthful beauty through the influences of washes

procured from so pure a source.[53]

Science has removed the veil of mystery with which superstition had

invested the formation of dew; and, in showing to us that it is

a condensation of vapour upon bodies according to a fixed law of

radiation, it has also developed so many remarkable facts connected

with the characters of material creations, that a much higher order of

poetry is opened to the mind than that which, though beautiful, sprang

merely from the imagination.

Upon the radiation of heat depends the formation of dew, and bodies

must become colder than the atmosphere before it will be deposited

upon them. At whatever temperature the air may be, it is charged to

saturation with watery vapour, the quantity varying uniformly with

the temperature. Supposing the temperature of the air to be 70° F.,

and that a bottle of water at 60° is placed in it, the air around

the bottle will be cooled, and will deposit on the glass exactly

that quantity of moisture which is due to the difference between the

temperature of the two bodies. Different substances, independent of

colour, have the property of parting with heat from their surfaces at

different rates. Rough and porous surfaces radiate heat more rapidly

than smooth ones, and are consequently reduced in temperature; and, if

exposed, are covered with dew sooner than such as are smooth and dense.

The grass parterre glistens with dew, whilst the hard and stony walk is

unmoistened.[54]

Colourless glass is very readily suffused with dampness, but polished

metals are not so, even when dews are heavily condensed on other

bodies. To comprehend fully the phenomena of the formation of dew,

we must remember that the entire surface of the earth is constantly

radiating heat into space; and that, as by night no absorption is

taking place, it naturally cools.[55] As the substances spread over

the earth become colder than the air, they acquire the power of

condensing the vapour with which the atmosphere is always charged. The

bodies which cover this globe are very differently constituted; they

possess dissimilar radiating powers, and consequently present, when

examined by delicate thermometers, varying degrees of temperature. By

the researches of Dr. Wells,[56] which may be adduced as an example of

the best class of inductive experiments, we learn that the following

differences in sensible heat were observed at seven o’clock in the

evening:--

The air four feet above the grass 60-3/4

Wool on a raised board 54-1/2

Swandown on ditto 53

The surface of the raised board 57

Grass plat 51

Dew is most abundantly deposited on clear, calm nights, during which

the radiation from the surface of the earth is uninterrupted. The

increased cold of such nights over those obscured by clouds is well

known. The clouds, it has been proved, act in the same way as the

screens used by gardeners to protect their young plants from the

frosts of the early spring, which obstruct the radiation, and, in all

probability, reflect a small quantity of heat back to the earth.

It is not improbable that the observed increase in grass crops, when

they have been strewn with branches of trees or any slight shades, may

be due to a similar cause.[57]

There are many remarkable results dependent entirely on the colours

of bodies, which are not explicable upon the idea of difference in

mechanical arrangement. We know that different colours are regulated

by the powers which structures have of absorbing and reflecting light;

consequently a blue surface must have a different order of molecular

arrangement from a red one. But there are some physical peculiarities

which also influence heat radiation, quite independently of this

_surface_ condition. If we take pieces of red, black, green, and yellow

glass, and expose them when the dew is condensing, we shall find that

moisture will show itself first on the yellow, then on the green glass,

and last of all upon the black or red glasses. The same thing takes

place if we expose coloured fluids in white glass bottles or troughs,

in which case the surfaces are all alike. If against a sheet of

glass, upon which moisture has been slightly frozen, we place glasses

similarly coloured to those already described, it will be found that

the earliest heat-rays will so warm the red and the black glasses, that

the ice will be melted opposite to them, long before any change will be

seen upon the frozen film covered by the other colours.

The order in which heat permeates coloured media, it has already been

shown, very nearly agrees with their powers of radiation.

These most curious results have engaged the attention of Melloni,

to whose investigations we owe so much; and from the peculiar order

of radiations, which present phenomena of an analogous character to

those of the coloured rays of light, obtained by him from dissimilarly

coloured bodies, he has been led to imagine the existence of a

“heat-colouration.” That is, the heat-rays are supposed to possess

properties like luminous colour although invisible; and, consequently,

that a blue surface has a strong affinity for the blue heat-rays,

a red surface for the red ones, and so on through the scale. The

ingenuity of this hypothesis has procured it much attention; but now,

when the Newtonian hypothesis of the refrangibility of light is nearly

overturned, we must not, upon mere analogy, rush to the conclusion

that the rays of heat have different orders of refrangibility, which

Melloni’s hypothesis requires.[58]

Can anything be more calculated to impress the mind with the

consciousness of the high perfection of natural phenomena, than the

fact, that the colour of a body should powerfully influence the

transmission of a principle which is diffused through all nature,

and also determine the rate with which it is to pass off from its

surface. Some recent experiments have brought us acquainted with other

facts connected with these heat-radiations, and the power of heat,

as influenced by the calorific rays, to produce molecular changes in

bodies, which bear most importantly on our subject.

If we throw upon a plate of polished metal a prismatic spectrum

(deprived, as nearly as possible, of its chemical power, by being

passed through a deep yellow solution--which possesses this property

in a very remarkable manner, as will be explained when we come to the

examination of the chemical action of the sun’s rays)--it will be

found, if we afterwards expose the plate to the action of vapour, very

slowly raised from mercury, that the space occupied by the red rays,

and those which lie without the spectrum below it, will condense the

vapour thickly, while the portion corresponding with the other rays

will be left untouched. This affords us evidence of the power of solar

heat to produce, very readily, a change in the molecular structure of

solid bodies. If we allow the sun’s rays to permeate coloured glasses,

and then fall upon a polished metallic surface, the result, on exposing

the plate to vapourisation, will be similar to that just described.

Under yellow and green glasses no vapour will be condensed; but on the

space on which the rays permeating a red glass, or even a blackened

one, fall, a very copious deposit of vapour will mark with distinctness

the spaces these glasses covered. More remarkable still, if these or

any other coloured bodies are placed in a box, and a polished metal

plate is suspended a few lines above them, the whole being kept _in

perfect darkness_ for a few hours, precisely the same effect takes

place as when the arrangement is exposed to the full rays of the sun.

Here we have evidence of the radiating heat of bodies, producing even

in darkness the same phenomena as the transmitted heat-rays of the sun.

We must, however, return to the examination of some of these and other

analogous influences under the head of actino-chemistry.

From these curious discoveries of inductive research we learn some high

truths. Associated with light--obeying many of the same laws--moving

in a similar manner--we receive a power which is essential to the

constitution of our planet. This power is often manifested in such

intimate combination with the luminous principle of the solar rays,

that it has been suspected to be but another form of the same agency.

While, however, we are enabled to show the phenomena of one without

producing those which distinguish the other, we are constrained to

regard heat as something dissimilar to light. It is true that we

appear to be tending towards some point of proof on this problem; but

we are not in a position to declare them to be forms of one common

power, or “particular solutions of one great physical equation.”[59]

In many instances it would certainly appear that one of these forces

was directly necessary to the production of the other; but we have also

numerous examples in which they do not stand in any such correlation.

We learn, from the scientific facts which we have been discussing, a

few of the secrets of natural magic. In their relations to heat, every

flower, which adds to the adornment of the wilds of nature or the

carefully-tended garden of the florist, possesses a power peculiar to

itself;

“Naiad-like lily of the vale,”

and,

“---- The pied wind-flowers, and the tulip tall,

And narcissi, the fairest among them all,”

are, by their different colours, prevented from ever having the same

temperatures under the same sunshine.

Every plant bears within itself the measure of the heat which is

necessary for its well-being, and is endued with functions which mutely

determine the relative amount of dew which shall wet its coloured

leaves. Some of the terrestrial phenomena of this remarkable principle

will still further illustrate the title of this volume.

To commence with the most familiar illustrations, let us consider the

consequences of change of temperature. However slight the additional

heat may be to which a body is subjected, it expands under its

influence; consequently, every atom which goes to form the mass of

the earth moves under the excitation, and the first heat ray of the

morning which touches the earth’s surface, sets up a vibration which

is continued as a tremor to its very centre. The differences between

the temperature of day and night are considerable; therefore all bodies

expand under the influence of the higher, and contract under that of

the lower temperature. During the day, any cloud obscuring the sun

produces, in every solid, fluid, or aëriform body, within the range

of solar influence, a check: the particles which had been expanding

under the force of heat suddenly contract. Thus there must of necessity

be, during the hours of sunshine, a tendency in all bodies to dilate,

and during the hours of night they must be resuming their original

conditions.

Not only do dissimilar bodies radiate heat in different degrees, but

they conduct it also with constantly varying rates. Heat passes along

silver or copper with readiness, compared to its progress through

platinum. It is conducted by glass but slowly, and still more slowly

by wood and charcoal. We receive some important intimations of the

molecular structure of matter, from those experiments which prove that

heat is conducted more readily along some lines than others. In some

planes, wood and other substances are better conductors than in others.

The metallic oxides or earths are bad conductors of heat, by which

provision the caloric absorbed by the sun’s rays is not carried away

from the surface of this planet so rapidly as it would have been had

it been of metal, but is retained in the superficial crust to produce

the due temperature for healthful germination and vegetable growth. The

wool and hair of animals are still inferior conductors, and thus, under

changes of climate and of seasons, the beasts of the field are secured

against those violent transitions from heat to cold which would be

fatal to them. Hair is a better conductor than wool: hence, by nature’s

alchemy, hair changed into wool in the animals of some countries on the

approach of winter, and feathers into down.

It is therefore evident that the rate at which solar heat is conducted

into the crust of the earth must alter with the condition of the

surface upon which it falls. The conducting power of all the rocks

which have been examined is found to vary in some degree.[60]

It follows, as a natural consequence of the position of the sun to

the earth, that the parts near the equator become more heated than

those remote from it. As this heat is conducted into the interior of

the mass, it has a tendency to move to the colder portions of it, and

thus the heat absorbed at the equator flows towards the poles, and

from these parts is carried off by the atmosphere, or radiated into

space. Owing to this, there is a certain depth beneath the surface of

our globe at which an equal temperature prevails, the depth increasing

as we travel north or south from the equator, and conforming to the

contour of the earth’s surface, the line sinking under the valleys and

rising under the hills.[61]

A question of great interest, in a scientific point of view, is the

temperature of the centre of the earth. We are, of course, without the

means of solving this problem; but we advance a little way onwards

in the inquiry by a careful examination of subterranean temperature

at such depths as the enterprise of man enables us to reach. These

researches show us, that where the mean temperature of the climate

is 50°, the temperature of the rock at 59 fathoms from the surface

is 60°; at 132 fathoms it is 70°; at 239 fathoms it is 80°: being an

increase of 10° at 59 fathoms deep, or 1° in 35·4 feet; of 10° more at

73 fathoms deeper, or 1° in 43·8 feet; and of 10° more at 114 fathoms

still deeper, or 1° in 64·2 feet.[62]

Although this would indicate an increase to a certain depth of about

one degree in every fifty feet, yet it would appear that the rate of

increase diminishes with the depth. It appears therefore probable, that

the heat of the earth, so far as man can examine it, is due to the

absorption of the solar rays by the surface. The evidences of intense

igneous action at a great depth cannot be denied, but the doctrine of

a cooling mass, and of the existence of an incandescent mass, at the

earth’s centre, remains but one of those guesses which active minds

delight in. The mean annual temperature of this planet is subject to

variations, which are probably dependent upon some physical changes in

the sun himself, or in the atmospheric envelope by which that orb is

surrounded. The variations over the earth’s surface are great. At the

equator we may regard the temperature as uniformly existing at 80°,

while at the poles it is below the freezing point of water; and as far

as observations have been made, the subterranean temperatures bear a

close relation to the thermic condition of the climate of the surface.

The circulation of water through faults or fissures in the strata is,

without doubt, one means of carrying heat downwards much quicker than

it would be conducted by the rocks themselves. It is not, however,

found that the quantity of water increases with the depth. In the mines

of Cornwall, unless where the ground is very loose, miners find that,

after about 150 fathoms (900 feet), the quantity of water rapidly

diminishes. That water must ascend from very much greater depths is

certain, from the high temperatures at which many springs flow out at

the surface. In the United Mines in Cornwall, water rises from one part

of the lode at 90°; and one of the levels in these workings is so hot

that, notwithstanding a stream of cold water is purposely brought into

it to reduce the temperature, the miners work nearly naked, and will

bathe in water at 80° to cool themselves. At the bottom of Tresavean

Mine, in the same county, about 320 fathoms from the surface, the

temperature is 100°.

One cause of the great heat of many of our deep mines, which appears

to have been entirely lost sight of, is the chemical action going

on upon large masses of pyritic matter in their vicinity. The heat,

which is so oppressive in the United Mines, is, without doubt, due to

the decomposition of immense quantities of the sulphurets of iron and

copper known to be in this condition at a short distance from these

mineral works.

The heat which man is enabled to measure beneath the earth’s surface,

appears to be alone due to the conducting powers of the rocks

themselves; it has been observed that the line of equal temperature

follows, as nearly as possible, the elevations and depressions which

prevail upon the surface, and the diminishing rate of increase beyond

this line, certainly is such as would arise, was all the heat so

measured, the result of the passage of the heat by conduction through

the crust of rocks.

Whether or not the subterranean bands of equal heat have any strict

relation, upon a large scale, to the isothermic lines which have been

traced around most portions of our globe, is a point which has not yet

been so satisfactorily determined as to admit of any general deductions.

The Oriental story-teller makes the inner world a place of rare

beauty--a cavern temple, bestudded with self-luminous gems, in which

reside the spiritual beings to whom the direction of the inorganic

world is confided.

The Philosopher, in the height of his knowledge, has had dreams as

absurd as this; and amid the romances of science, there are not to be

found any more strange visions than those which relate to the centre

of our globe. At the same time it must be admitted, that many of the

peculiar phenomena which modern geological researches have brought to

light, are best explained on the hypothesis of a cooling sphere, which

necessarily involves the existence of a very high temperature towards

the centre.

We have already noticed some remarkable differences between solar and

terrestrial heat; but a class of observations by Delaroche[63] still

requires our attention. Solar heat passes freely through colourless

glass, whereas the radiations from a bright fire or a mass of

incandescent metal are entirely obstructed by this medium. If we place

a lamp or a ball of glowing hot metal before a metallic reflector, the

focus of accumulated heat is soon discovered; but if a glass mirror

be used, the light is reflected, but not the heat; whereas, with the

solar rays, but little difference is detected, whether vitreous or

metallic reflectors are employed. It is well known that glass lenses

refract both the light and heat of the sun, and they are commonly known

as burning-glasses: the heat accumulated at their focal point being

of the highest intensity. If, instead of the solar beam, we employ,

in our experiments, an intense heat produced by artificial means,

the passage of it is obstructed, and the most delicate thermometers

remain undisturbed in the focus of the lens. Glass exposed in front

of a fire becomes warm, and by conduction the heat passes through it,

and a secondary radiation takes place from the opposite side.[64] It

has been found that glass is transcalescent, or _diathermic_, to some

rays of terrestrial heat, and _adiathemic_, or opaque for heat, to

others[65]--that the capability of permeating glass increases with the

temperature of the ignited body--and that rays which have passed one

screen traverse a second more readily. It would, however, appear that

something more than a mere elevation of temperature is necessary to

give terrestrial heat-radiations the power of passing through glass

screens, or, in other words, to acquire the properties of solar heat.

To give an example. The heat of the oxy-hydrogen flame is most intense,

yet glass obstructs it, although it may be assisted by a parabolic

reflector. If this flame is made to play upon a ball of lime, by which

a most intense light is produced, the heat, which has not been actually

increased, acquires the power of being refracted by a glass lens, and

combustible bodies may be ignited in its focus.

It certainly appears from these results, that the undulatory hypothesis

holds true, so far as the motion of the calorific power is concerned.

At a certain rate the vibrations are thrown back or stopped by the

opposing body, while in a state of higher excitation, moving with

increased rapidity, they permeate the screen.[66] This does not,

indeed, interfere with the refined theory of Prévost,[67] which

supposes a mutual and equal interchange of caloric between all bodies.

The most general effect of heat is the expansion of matter; solids,

liquids, and airs, all expand under its influence. If a bar of metal

is exposed to calorific action, it increases in size, owing to its

particles being separated farther from each other: by continuing this

influence, after a certain time the cohesion of the mass is so reduced

that it melts, or becomes liquid, and, under the force of a still

higher temperature, this molten metal may be dissipated in vapour. It

would appear as if, under the agency of the heat applied to a body, its

atoms expanded, until at last, owing to the tenuity of the outer layer

or envelope of each atom, they were enabled to move freely over each

other, or to interpenetrate without difficulty. That heat does really

occasion a considerable disturbance in the corpuscular arrangement

of bodies, may be proved by a very interesting experiment. A bar of

heated metal is placed to cool, with one end supported upon a wedge

or a ring of a different metal the other resting on the ground. In

cooling, a distinct musical sound is given out, owing to the vibratory

action set up among the particles of matter moving as the temperature

declines.[68]

Heat is diffused through all bodies in nature, and, as we shall

presently see, may be developed in many different ways. We may,

therefore, infer, that in converting a sphere of ice into water, and

that again into steam, we have done nothing more than interpenetrate

the mass with a larger quantity of heat, by which its atoms are more

widely separated, and that thus its molecules become free to move

about each other. Hence, from a solid state, the water becomes fluid;

and then, if the expansive force is continued, an invisible vapour.

If these limits are passed by the powers of any greatly increased

thermic action, the natural consequence, it must be seen, will be the

separation of the atoms from each other, to such an extent that the

molecule is destroyed, and chemical decomposition takes place.

By the agency of the electricity of the voltaic battery, we are enabled

to produce the most intense heat with which we are acquainted, and by a

peculiarly ingenious arrangement Mr. Grove has succeeded in resolving

water by the mere action of heat into its constituent elements--oxygen

and hydrogen gases. That this decomposition is not due to the voltaic

current, but to the heat produced by it, was subsequently proved by

employing platina heated by the oxy-hydrogen flame.[69]

This interesting question has been examined with great care by Dr.

Robinson of Armagh, who has shown that, as the temperature of water is

increased, the affinity of its elements is lessened, until at a certain

point it is eventually destroyed. This new and startling fact appears

scarcely consistent with our knowledge that a body heated so as to

be luminous has the power of causing the combination of the elements

of water with explosive violence.[70] But as this acute experimental

philosopher somewhat boldly but still most reasonably inquires: “Is

it not probable that, if not light, some other actinic power (like

that which accompanies light in the spectrum, and is revealed to us

by its chemical effects in the processes of photography) is evolved

by the heat, and, though invisible, determines, in conjunction with

the affinity, that atomic change which transforms the three volumes of

oxygen and hydrogen into two of steam?”[71]

This speculation explains, in a very satisfactory manner, some

results which were obtained by Count Rumford, in 1798. In a series of

experiments instituted for the purpose of examining “those chemical

properties of light which have been attributed to it,” he has shown

that many cases of chemical decomposition occur in perfect darkness,

under the influence of heat, which are precisely similar to those

produced by exposure to the sun’s rays.[72]

It must, however, be remembered, that both solar light and heat are

sometimes found in direct antagonism to actinic power, and that the

most decided chemical changes are produced by those rays in which

neither heat nor light can be detected. The most remarkable phenomena

of this class will be explained under the head of actinism.

One of the most curious relations which as yet have been discovered

between light and heat is, that, the temperature at which all bodies

become incandescent, excepting such as are phosphorescent, is uniform.

The point on the thermometer (Fahrenheit’s scale) when the eye by

perfect repose is enabled to detect the first luminous influence, may

probably be regarded as, or very near, 1000°. Daniel has fixed this

point at 980°, Wedgwood at 947°, and Draper at 977°.[73] Dr. Robinson

and Dr. Draper, by independent observations, have both arrived at the

conclusion, that the first gleam of light which appears from heated

platina is not red, but of a lavender gray, the same in character of

colour as that detected by Sir John Herschel among the most refrangible

rays of the solar spectrum.[74]

It must be admitted, that the question of the identity, or otherwise,

of light and radiant heat, is beset with difficulties. Many of their

phenomena are very similar--many of their modes of action are alike:

they are often found as allied agencies; but they as frequently exhibit

extreme diversity of action, and they may be separated from each other.

We have now examined the physical conditions and properties of this

most important element, and we must proceed to learn something of the

means by which it may be developed, independently of its solar source.

This extraordinary principle exists in a latent state in all bodies,

and may be pressed out of them. The blacksmith hammers a nail until it

becomes red hot, and from it he lights the match with which he kindles

the fire of his forge. The iron has by this process become more dense,

and percussion will not again produce incandescence until the bar has

been exposed in fire to a red heat. The only inference we can draw

from this result is, that by hammering the particles have been driven

closer together, and the heat driven out; now further hammering will

not force the atoms nearer, and consequently no additional quantity of

heat can be developed; the iron is made hot in a fire, it absorbs heat,

the particles are restored to their former state, and we can now again

by hammering develope both heat and light. The Indian produces a spark

by the attrition of two pieces of wood. By friction, two pieces of ice

may be made to melt each other; and could we, by mechanical pressure,

force water into a solid state, an immense quantity of heat would be

set free. By the condensation of hydrogen and oxygen gases, pulverulent

platinum will become glowing red-hot, and, with certain precautions,

even the compact metal, platinum, itself; the heat being derived from

the gases, the union of which it has effected. A body passing from the

solid to the fluid state absorbs heat from all surrounding substances,

and hence a degree of cold is produced. The heat which is thus removed

is not destroyed--it is held combined with the fluid; it exists in a

latent state. Fluids, in passing into a gaseous form, also rob all

surrounding bodies of an amount of heat necessary to maintain the

aëriform condition. From the air or from the fluid, this heat may,

as we have shown above, be again extracted. Locked in a pint measure

of air, there exists sufficient heat to raise several square inches

of metal to glowing redness. By the compression of atmospheric air

this may be shown, and with a small condensing syringe a sufficient

quantity of heat may be set free to fire the _Boletus igniarius_,

which, impregnated with nitre, is known as _amadou_. We are acquainted

with various sources from which heat may be developed for artificial

purposes: the flint-and-steel is an example of the production of heat

by mechanical force, and the modern lucifer-match, of the combined

action of friction and chemical affinity. These of themselves would

admit of a lengthened discourse; but it is necessary that we carefully

examine some of the less familiar phenomena of heat under the

influences of changes of chemical condition.

If spirits of wine and water are mixed together, a considerable degree

of heat is given out, and by mixing sulphuric acid and water, an

infinitely larger quantity. If sulphuric acid (oil of vitriol) and

spirit of wine, or nitric acid (aquafortis) and spirits of turpentine,

at common temperatures, be suddenly mixed, so much heat is set free

as to ignite the spirit. In each of these instances there is a

condensation of the fluid. In nearly all cases of solution, cold is

produced by the absorption of the heat necessary to sustain the salt

in a liquid form; but when potash dissolves in water, heat is given

out, which is a fact we cannot yet explain. If potassium is placed on

water, it seizes the oxygen of the water and sets fire to the hydrogen

gas liberated by the heat produced in the change of form. Antimony

and many other metals thrown into chlorine gas ignite and burn with

brilliancy: the same phenomenon takes place in the vapours of iodine or

bromine. Many chemical combinations, as the chloride of potassium and

sulphur explode with a blow; whilst the slightest friction occasions

the detonation of the fulminating salts of silver, mercury, and

gold. Compounds of nitrogen and chlorine, or iodine, are still more

delicately combined--the former exploding with fearful violence on

the contact of any oleaginous body, and the latter by the smallest

elevation of temperature: both of them destroying the vessels in which

they may be contained.

Gun cotton presents some peculiar phenomena which may merit brief

attention. This peculiar compound is prepared by the action of nitric

acid on cotton fibre. The general appearance of the cotton is not

altered, but a remarkable physical change has taken place. It is

now soluble in ether, and forms a gelatinous compound:--it explodes

violently at a temperature which is insufficient for the combustion

of gunpowder. Indeed, from, as it would appear, slight electrical

disturbances taking place in the gun cotton itself, it not unfrequently

explodes spontaneously. These fearful disturbances of the forces

which hold bodies in combination are explained with difficulty. May

it not be, that an enormous quantity of the calorific and chemical

principles is held in a state of extreme tension around the particles

of the compound, and that the equilibrium being destroyed, the whole is

developed in destructive rapidity?

The fact of great heat being evolved during the conversion of a body

from a solid to a gaseous state, as in the explosion of gunpowder or

gun cotton, which is a striking exception to the law of latent heat, as

it prevails in most cases, admits of no more satisfactory explanation.

As mechanical force produces calorific excitation, so we find that

every movement of sap in vegetables, and of the blood and fluids in

the animal economy, causes a sensible increase of heat. The chemical

processes constantly going on in plants and animals are another source

of heat, in addition to which nervous energy and muscular movement must

be regarded as producing the caloric which is essential to the health

and life of the latter. Digestion has been considered as a process

of combustion; and the action between the elements of food, and the

oxygen conveyed by the circulation of the blood to every part of the

body, regarded as the source of animal heat; and, without doubt, it

is one great source, although it can scarcely be regarded as the only

one.[75]

The _vis vitæ_, or vital power, influences the delicate and beautiful

system of nerves; and as life (an essence of the rarest and most

subtile order, a diffusive influence) runs through them, from the brain

to the extremities of the members of the body, it sets those tender

threads in rapid vibration, and heat is developed. By this action,

the circulation of the blood is effected; the muscle is maintained

in an elastic condition, ready to perform the tasks of the will; and

through these agencies is the warm and fluid blood fitted to receive

its chemical restoratives in the lungs, and the stomach to support

changes for which it is designed--chemical also--by which more heat

is liberated. Was digestion--_Eremacausis_, as the slow combustion

produced by combination with oxygen is called--the only source of

animal heat, why should the injury of one filmy nerve place a member of

the body for ever in the condition of stony coldness? Or why, chemical

action being most actively continued after a violent death, by the

action of the gastric juices upon the animal tissues, should not animal

heat be maintained for a much longer period than it is found to be

after respiration has ceased?[76]

In studying the influences of caloric upon the conditions of matter,

we must regard the effects of extreme heat, and also of the greatest

degrees of cold which have been obtained.

There are a set of experiments by the Baron Cagniard de la Tour, which

appear to have a very important bearing on some conditions that may

be supposed to prevail in nature, particularly if we adopt the view of

a constantly increasing temperature towards the centre of our earth.

If water, alcohol, or ether, is put into a strong glass tube of small

bore, the ends hermetically sealed, and the whole exposed to a strong

heat, the fluid disappears, being converted into a transparent gas;

but, upon cooling, it is again condensed, without loss, into its

original fluid state.[77] In this experiment, fluid bodies have been

converted into elastic transparent gases with but small change of

volume, under the pressure of their own atmospheres. We can readily

conceive a similar result occurring upon a far more extensive scale.

In volcanic districts, at great depths, and consequently under the

pressure of the superincumbent mass, the siliceous rocks, or even

metals, may, from the action of intense heat, be brought into a fluid,

or even a gaseous condition, without any change of volume, since

the elastic force of heat is opposed by the rigid resistance of the

pressure of the surrounding rocks. Some beautiful experiments by Mr.

Hopkins, of Cambridge, have proved that the temperature necessary to

melt a body must be considerably elevated as the mechanical pressure to

which it is subjected is increased.

Directly connected with the results of Cagniard de la Tour are a yet

more remarkable set of phenomena, which have been investigated by

M. Boutigny,[78] and generally known as the “spheroidal condition”

of bodies. If water is projected upon very hot metal it instantly

assumes a spheroidal form--an internal motion of its particles may

be observed--it revolves with rapidity, and evaporates very slowly.

If a silver or platinum capsule, when brought to a bright red heat,

is filled with cold water, the whole mass assumes the spheroidal

state, the temperature of the fluid remaining considerably below the

boiling point, so long as the red heat is maintained. If we allow the

vessel to cool below redness in the dark, the water then bursts into

active ebullition, and is dissipated into vapour with almost explosive

violence. An equal quantity of water being projected into two similar

vessels, over the fire, one cold and the other red hot, it will be

found that the water in the cold vessel will boil and evaporate long

before that in the one which is red hot.

Another form of this experiment is exceedingly instructive. If a mass

of white hot metal is suddenly plunged into a vessel of cold water, the

incandescence is not quenched, the metal shines with a bright white

light, and the water is seen to circulate around, but at some distance

from the glowing mass, being actually repelled by calorific agency. At

length, when the metal cools, the water comes in contact with it, and

boils with energy.

A result similar to this was observed by Perkins, but its correctness

most unjustly doubted. Having made an iron shell containing water, and

carefully plugged up, white hot, it was found that the steam never

exerted sufficient force to burst the vessel, as it was expected

it would do. He caused a hole to be drilled into the bottom of the

white-hot shell, and he was surprised to find that no water flowed

through the orifice, until the iron was considerably cooled, when it

issued forth with violence in the form of steam. Here we have the

_Cagniard de la Tour state_ first induced, and the calorific repulsion

of the spheroidal state supervenes. If water is poured upon an iron

sieve, the wires of which are made red hot, it will not percolate;

but on cooling, it runs through rapidly. M. Boutigny, pursuing this

curious inquiry, has recently proved that the moisture upon the skin

is sufficient to protect it from disorganization, if the arm is plunged

into baths of melted metal. The resistance of the surfaces is so great,

that little elevation of temperature is experienced.[79] Professor

Plücker, of Bonn, has stated that by washing the arm with ether

previously to plunging it into melted metal, the sensation produced,

while in the molten mass, is that of freezing coldness.

We have now seen that heat at different degrees of intensity appears

to produce chemical composition--that it decomposes combined

elements--that it alters the conditions of bodies, and actually

maintains so powerfully a repellent force, that fluids cannot touch the

heated body. More than this, it exerts a most powerful antagonistic

influence over all chemical relations. If, to give one example,

the volatile element iodine is put into a glowing hot capsule, it

resolves itself immediately into a spheroid. Potash rapidly combines

with iodine; but if a piece of this alkali is thrown upon it in

the capsule, it also takes the spheroidal form, and both bodies

revolve independently of each other, their chemical affinities being

entirely suspended;--but allow the capsule to cool, and they combine

immediately. Science teaches us that a temperature so exalted as not to

burn organic bodies may be produced, and points to us this remarkable

fact, that the destructive limits of heat are measured between certain

degrees--beyond which a fire, by reason of its intensity, ceases to

develope heat. What is the radiant force into which this principle

changes?

The experiments of Cagniard de la Tour and of Boutigny (d’Evreux),

connect themselves, in a striking manner, with those of Mr. Grove and

Dr. Robinson; and they teach us that but a very slight alteration in

the proportions of the calorific principle given to this planet would

completely change the character of every material substance of which it

is composed, unless there was an alteration in the physical condition

of the elements themselves.

Supposing the ordeal of fiery purification to take place upon this

earth, these experiments appear to indicate the mighty changes which

would thence result. There would be no annihilation, but everything

would be transformed from the centre of the globe to the verge of its

atmosphere--old things would pass away, all things become new, and the

beautiful mythos of the phœnix be realized in the fresh creation.

The deductions to be drawn from the results obtained by abstracting

heat from bodies are equally instructive. By taking advantage of the

cooling produced by the rapid solution of salts of several kinds in

water, an intense degree of coldness may be produced.[80] Indeed, the

absorption of heat by liquefaction may be shown by the use of metallic

bodies alone. If lead, tin, and bismuth, are melted together, and

reduced to a coarse powder by being poured into water, and the alloy

then dissolved in a large quantity of quicksilver, the thermometer will

sink nearly 50 degrees. An intense amount of cold will result from

the mixture of muriate of lime and snow, by which a temperature of

50° below the zero of Fahrenheit, or 82° below the freezing point of

water, is produced. By such a freezing mixture as this, mercury will

be rendered solid. A degree of cold, however, far exceeding it, has

lately been obtained by the use of solid carbonic acid and ether.[81]

Solid carbonic acid is itself procured from the gas liquefied by

pressure; which liquid, when allowed to escape into the air, evaporates

so rapidly that a large quantity of it is congealed by being robbed of

its combined heat by the vaporizing portion. When this solid acid is

united with ether, a bath is formed in which the carbonic acid will

remain solid for twenty or thirty minutes. By a mixture of this kind,

placed under the receiver of an air-pump, a good exhaustion being

sustained, a degree of cold 166° below zero is secured. By this intense

cold, many of the bodies which have hitherto been known to us only in

the gaseous state have been condensed into liquids and solids. Olefiant

gas, a compound of hydrogen and carbon, was brought into a liquid

form. Hydriodic and hydrobromic acids could be condensed into either a

liquid or a solid form. Phosphuretted hydrogen, a gas which inflames

spontaneously when brought into contact with the air or with oxygen,

became a transparent liquid at this great reduction of temperature.

Sulphurous acid may be condensed, by pressure and a reduction of

temperature, into a liquid which boils at 14° Fahrenheit, but by the

carbonic acid bath it is converted into a solid body, transparent and

without colour. Sulphuretted hydrogen gas solidifies at 122° below

zero, and forms a white substance resembling a mass of crystals of

sea-salt.

A combination of the two gases, chlorine and oxygen, becomes solid at

-75°, and the protoxide of nitrogen at -150°. Cyanogen, a compound

of carbon and nitrogen--the base of prussic acid--is solidified at

30° below the zero of our thermometric scale. The well-known pungent

compound, ammonia, so exceedingly volatile at common temperatures,

is converted into a crystalline, translucent, white substance at the

temperature of -103°. The difficulties which necessarily attend the

exposure of a body to extreme cold and great pressure at the same time,

appear to be the only obstacle to the condensation of oxygen, hydrogen,

and nitrogen gases. A sufficient amount of condensation was, however,

effected by Dr. Faraday, to lead him to the conclusion, arrived at

also by other evidences, that hydrogen, the lightest of the ponderable

bodies, partakes of the nature of a metal.[82]

During the solidification of water by freezing, some remarkable facts

may be noticed.

Water, in cooling, gradually condenses in volume until it arrives at

40° Fahr., which appears to be the point of greatest density. From this

temperature to that of 32°, the point at which it begins to solidify,

its volume remains unchanged,[82] as crystallisation (freezing) begins,

the bulk increases, the mass becomes specifically lighter, and it swims

on the surface of the fluid. From 40° to 32° the particles of water

must be taking up that new position which is essential to the formation

of the solid--ice; and while this is taking place, every substance held

in solution by the water is rejected.

If we mix with water the deepest colouring matter--the strongest acid

or the most acrid poison--they are each and all rejected during the

process of freezing, and if the water has been kept in a state of

agitation during the process--so that the liberated particles may not

be mechanically entangled--the ice will be transparent, colourless,

tasteless, and inert--the substances rejected being gathered together

in the centre of the frozen mass in a state of intense concentration.

In like manner, even the atmospheric air, which is always held in

solution, is rejected, and hence the reason why all the ice which

forms upon still ponds is full of air-bubbles, while the ice which is

produced in agitated water is perfectly free from them. This in itself

is a remarkable condition, the entire bearing of which is not clearly

understood; but a still more singular fact has been discovered in

intimate connection with the rejection of all matter from a freezing

solution. Water, which in this way is freed entirely of air, will not

boil at 212° F., the ordinary boiling point of water.

If a mass of ice formed in the manner described is placed in a vessel,

and being just covered with a film of oil, to prevent the absorption of

air, is melted over a lamp or fire, and the heat continued, it will,

so far from being converted into steam at 212°, continue to increase

in temperature up to 270° or more, and then burst into ebullition with

such explosive violence as to rend the vessel in which it is confined.

From this experiment we learn that did water exist in any other

condition than that in which we find it--even with the apparently

simple difference of containing no air--it would not be safe to employ

it in any culinary or manufacturing operation, since its use would be

followed by explosions as dangerous as those of gunpowder.

Such researches as these prove to us the admirable adaptation of all

things to their especial ends--the beautiful adjustment of the balance

of forces throughout creation.

The refinements of Grecian philosophy saw, without the aids of

inductive science, that the outward vesture of nature covered a host

of mysterious agencies to which its characteristics were directly

due. In their dream of the four elements, fire, the external and

visible form of heat, was regarded as the cause of vitality, and the

disposer of every organised and unorganised condition of matter. Their

idealisations have assumed another form, but the researches of modern

science have only established their universality and truth.

The great agents at work in nature--the mighty spirits bound to

never-ending tasks, which they pursue with unremitting toil, are of so

refined a character, that they will probably remain for ever unknown

to us. The arch-evocator, with the wand of induction, calls; but the

only answer to his evocation is the manifestation of power in startling

effects. Science pursues her inquiries with zeal and care: she tries

and tortures nature to compel her to reveal her secrets. Bounds are,

however, set to the powers of mortal search: we may not yet have

reached the limits within which we are free to exercise our mental

strength; but, those limits reached, we shall find an infinite region

beyond us, into which even conjecture wanders eyeless and aimless, as

the blind Cyclops groping in his melancholy cave.[83]

All we know of heat is, that striking effects are produced which we

measure by sensation, and by instruments upon which we have observed

that given results will be produced under certain conditions: of

anything approaching to the cause of these we are totally ignorant.

The wonder-working mover of some of the grandest phenomena in

nature--giving health to the organic world, and form to the inorganic

mass--producing genial gales and dire tornadoes--earthquake strugglings

and volcanic eruptions--ministering to our comforts in the homely fire,

and to advancement in civilisation in the mighty furnace, and the

ingenious engine which drains our mines, or traverses our country with

bird-like speed,--will, in all probability, remain for ever unknown to

man. The immortal Newton, many of whose guesses have a prophetic value,

thus expresses himself:--“Heat consists in a minute vibratory motion in

the particles of bodies, and this motion is communicated through an

apparent vacuum by the undulations of a very subtile elastic medium,

which is also concerned in the phenomena of light.”

Our experimental labours and our mathematical investigations have

considerably advanced our knowledge since the time of Newton; yet still

each theory of heat strangely resembles the mystic lamp which the

Rosicrucian regarded as a type of eternal life--a dim and flickering

symbol, in the tongue-like flame of which imagination, like a child,

can conjure many shapes.

Modern theory regards heat as a manifestation of motion, and experiment

proves that a body falling through a certain space generates a definite

quantity of heat, while observation shows that the waters at the base

of the Falls of Niagara possess a temperature 1° higher than when they

first glide over the edge of the precipice.

This increase of temperature is due to the mechanical force due to

the fall, and is no more an evidence of the conversion of motion into

heat, than is the old experiment of rubbing a button until it becomes

hot. At all events, the fact that a given amount of mechanical force

always produces an equivalent of heat is as applicable to the idea of

a “subtile elastic medium” which is diffused through all matter, as to

the, at present, favourite hypothesis.

So far has this view been strained, that the temperature of the planets

has been referred to their motions, and speculation has aided the

mathematician in determining the cessation of planetary motion, by

the conversion of it into heat. It is true that other theorists have

supposed points in space upon which this heat might be concentrated and

reflected back again to produce motion.

There may be much of the poetic element in such speculations, but it is

of that order which belongs rather to the romantic than to the real.

A speculation which has more of truth, and which is, indeed,

demonstrable, cannot fail to impress every mind with its beauty, and

probable correctness.

In the growth of a tree, its wood and all its products are the result

of certain external forces effecting chemical changes. Carbonic acid

is decomposed, the carbon is retained, and oxygen given off, and

assimilations of a complex character are in constant progress to

produce the various compounds of oxygen, hydrogen, nitrogen, and carbon.

Every condition of organised forms is due to the external excitation

of light and heat, and in the chemical changes which take place, an

equivalent of these principles, or powers--it signifies but little

according to which view we may regard them--is absorbed, and retained

as essential to the condition of the matter formed. Let us confine

our attention to wood--although the position applies equally to every

organic product. A cubic foot of wood is formed by the decomposition

of a certain quantity of carbonic acid, by the vital function of the

plant, excited by the solar rays, which are involved in the mass which

nature by “her wondrous alchemy” has made. Eventually this cubic foot

of wood is subjected to a process of chemical change--combustion; by

the application of a single spark,--and in the disintegration of the

wood, its carbon combining with oxygen to form carbonic acid, its

hydrogen to form water, which is returned to the air, a large amount of

light and heat is produced. This is exactly equivalent to the amount

which was engaged in its formation. Indeed, the sunshine which fell

upon the leaves of the forest tree, of which the log formed a part, has

been hoarded up, and we again develope it in its original state of heat

and light.

The vast coal beds of England were formed by the rapid growth and quick

decay of a peculiar class of plants under the influence of a tropical

sun. They have been buried myriads of ages, under hundreds of feet of

sandy rock. By the industry of the miner the coal is brought again

to the surface, and we develope from it those powers by which it was

formed.

In the fire which gives comfort to our homes--in the furnace which

generates force for the purposes of manufacture, or to propel the

railway engine and its ponderous train--in the gas with which we

illumine our streets and gladden during the long winter nights our

apartments, we are developing that heat and light which fell upon

the earth with all its quickening influences millions of ages before

yet the Creator had called into existence the monarch Man, for whose

necessities these wondrous formations were designed.

FOOTNOTES:

[43] The following table of the rays penetrating coloured glass has

been given by Melloni, in his memoir _On the Free Transmission of

Radiant Heat through Different Bodies_:--

Deep violet 53

Yellowish red (flaked) 53

Purple red (flaked) 51

Vivid red 47

Pale violet 45

Orange red 44

Clear blue 42

Deep yellow 40

Bright yellow 34

Golden yellow 33

Deep blue 33

Apple green 26

Mineral green 23

Very deep blue 19

Translated in the Scientific Memoirs, vol. i. p. 30.

[44] “The physical characters of this species of glass, which acts

so differently from the other species of coloured glass in all the

phenomena of calorific absorption, are, 1st, its intercepting almost

totally the rays which pass through alum; 2nd, its entirely absorbing

the red rays of the solar spectrum. I have already stated that their

colouration is produced almost entirely by the oxide of copper.

“Thus, the colouring matters of the coloured glasses, while they

so powerfully affect the relations of quantity which the different

rays of ordinary light bear to each other, exercise no elective

action on the concomitant calorific rays. This curious phenomenon is

the more remarkable as the colouring matters absorb almost always

a very considerable portion of the heat _naturally transmitted by

the glass_. The following are, in fact, the calorific transmissions

of the seven coloured glasses referred to; the transmission of the

common glass being represented by 100; red glass, 82·5; orange, 72·5;

yellow, 55; bluish-green, 57·5; blue, 52·5; indigo, 30; violet, 85.

The quantity of heat absorbed through the action of the colouring

substances is, therefore, 17·5 in the red glass, 27·5 in the orange,

45 in the yellow, 42·5 in the green, 47·5 in the blue, 70 in the

indigo, and 15 in the violet. Now, as these absorptions extinguish a

proportional part of each of the rays which constitute the calorific

stream transmitted by common glass, they may be compared, as we said

before, with the absorbent action exercised on light by matters more

or less deeply brown or dark, when they are immersed in water, or some

other colourless liquid which dissolves, but does not affect them

chemically.”--_Annales de Chimie et de Physique_, tom. xl. p. 382.

Guided by these principles, the author selected the glass employed in

glazing the Royal Palm-House, at Kew Botanical Gardens, where it was

desired to obstruct the passage of those rays which have a particular

scorching influence. Of this glass a description was given at the

meeting of the British Association at Oxford, which appears in the

Transactions for that year. The result has been all that could be

desired--not a single instance of scorching having occurred during the

three years which have elapsed.

[45] In the _Philosophical Transactions_, vol. xc., the following

papers, by Sir William Herschel, may be consulted:--

_Investigation of the powers of the prismatic colours to heat

and illuminate objects; with remarks that prove the different

refrangibility of radiant heat. To which is added, an inquiry into the

method of viewing the sun advantageously, with telescopes of large

apertures and high magnifying powers_, p. 255. _Experiments on the

refrangibility of the invisible rays of the sun_, p. 284. _Experiments

on the solar and on the terrestrial rays that occasion heat; with a

comparative view of the laws to which light and heat, or rather the

rays which occasion them, are subject; in order to determine whether

they are the same or different_, pp. 293, 437.

In connection with this inquiry, Sir William Herschel remarks, that

since a _red glass_ stops no less than 692 out of 1,000 such rays as

are of the refrangibility of red light, we have a direct and simple

proof, in the case of the red glass, that the rays of light are

transmitted, while those of heat are stopped, and that thus they have

nothing in common but a certain equal degree of refrangibility, which

by the power of the glass must occasion them to be thrown together into

the place which is pointed out to us by the visibility of the rays of

light.

On the same subject, a Memoir, by Sir Henry Englefield, in the Journal

of the Royal Institution for 1802, p. 202, may be consulted; and

_Researches on Light_, by the Author.

[46] Dr. Draper, _On the production of light by heat_, in the Phil.

Mag. for 1847.

Sir Isaac Newton fixed the temperature at which bodies become

self-luminous at 635°; Sir Humphry Davy at 812°; Mr. Wedgewood at 947°;

and Mr. Daniell at 980°; whilst Dr. Draper from his experiments gives

977°; and Dr. Robinson 865°.

In a review of the above paper by Melloni, entitled _Researches on the

Radiations of Incandescent Bodies, and on the Elementary Colours of the

Solar Spectrum_, translated for Silliman’s Journal for August, 1847, he

remarks:--

“I say that they conduct, as do others heretofore known on light and

radiant heat, to a perfect analogy between the general laws which

govern these two great agents of nature. I will add that I regard

the theory of their identity as the only one admissible by the rules

of philosophy; and that I consider myself obliged to adopt it, until

it shall have been proved to me that there is a necessity of having

recourse to two different principles, for the explanation of a series

of phenomena which at present appear to belong to a solitary agent.”

Reference should also be made to a paper by Dr. Robinson, _On the

effects of Heat in lessening the Affinities of the Elements of Water_,

in the Transactions of the Royal Irish Academy, 1848, where he says

that “when a platinum wire is traversed by a current gradually

increased till it produces ignition, the first gleam that appears is

not red, but of a colour which, when I first saw it, I compared to

the ‘lavender ray’ discovered by Sir John Herschel beyond the violet,

though I was surprised at seeing the tint of that most refrangible ray

preceding the ray which is least so. It is quite conspicuous at about

865°; and as the mode in which it makes its appearance presents nothing

abrupt or discontinuous, it seems likely that it is merely a transition

from invisible rays excited at a lower temperature to ordinary

light.”--p. 310.

[47] In the _Bakerian Lecture_ for 1842, _On the transparency of the

Atmosphere, and the law of extinction of the solar rays in passing

through it_, by James D. Forbes, Esq., F.R.S., &c., will be found a

most complete investigation of this subject.

The experiments were, for the most part, made in Switzerland with Sir

John Herschel’s actinometer, and they prove satisfactorily,--“That

the absorption of the solar rays by the strata of air to which we

have immediate access, is considerable in amount for even moderate

thicknesses.”

[48] After referring to several curious and instructive experiments, in

which peculiar chemical changes are produced under the influence of the

solar rays by their HEAT, Sir John Herschel says:--

“These rays are distinguished from those of Light by being invisible;

they are also distinguished from the pure calorific rays beyond the

spectrum, by their possessing properties (_of a peculiar character,

referred to in former papers_) either exclusively of the calorific

rays, or in a much higher degree. They may perhaps not improperly be

regarded as bearing the same relation to the calorific spectrum which

the photographic rays do to the luminous ones. If the restriction

to these rays of the term _thermic_, as distinct from _calorific_,

be not (as I think, in fact, it is not) a sufficient distinction, I

would propose the term _parathermic rays_ to designate them. These are

the rays which I conceive to be active in producing those singular

molecular affections which determine the precipitation of vapours in

the experiments of Messrs. Draper, Moser, and Hunt, and which will

probably lead to important discoveries as to the intimate nature of

those forces resident on the surfaces of bodies, to which M. Dutrochet

has given the name of epipolic forces.”--_On certain improvements in

Photographic Processes, described in a former communication_ (Phil.

Trans, vol. cxxxiii.); and _On the Parathermic Rays of the Solar

Spectrum_, Phil. Trans, vol. cxxxiv.

The experiments of Mrs. Somerville, _On the Action of the Rays of the

Spectrum on Vegetable Juices_ (Phil. Transactions, vol. cxxxvii.),

appear to connect themselves with this particular class of rays in a

curious manner.

[49] Experiments on the influence of heat on differently-coloured

bodies were first made by Dr. Hooke; and it was not until long after

that Franklin made his ingenious experiments. Davy exposed to sunshine

six equal pieces of copper, painted white, yellow, red, green, blue,

and black, in such a manner that one side only was illuminated. To

the dark side he attached a bit of cerate, ascertained by experiment

to melt at 700. The cerate attached to the black became fluid first,

the blue next, then the green and red, and lastly the yellow and

white.--Beddoes’s _Contributions to Physical Knowledge_, and collected

works of Sir Humphry Davy, vol. ii. p. 27.

[50] By reference to the Treatise on Heat, in the _Encyclopædia

Metropolitana_, numerous suggestive experiments will be found, all

bearing on this subject. Peschel’s _Elements of Physics_ may also be

consulted with advantage. The fact is, however, simply proved, as

stated in the text, by placing the bulbs of delicate thermometers,

so as to be completely involved in the petals of flowers exposed to

sunshine, shading the upper portion of the stem of the instrument.

[51] Moser, _On Vision, and on the Action of Light on Bodies_: and also

_On Latent Light_: Scientific Memoirs, vol. iii. Draper, _On certain

Spectral Appearances, and on the Discovery of Latent Light_: Phil.

Mag., Nov. 1842.

[52] A particular examination of this curious question will be found in

the Author’s report _On the Influence of the Solar Rays on the Growth

of Plants_: Reports of the British Association for 1847.

[53] Ammianus Marcellinus ascribes the longevity and robust health of

mountaineers to their exposure to the dews of night. Dew was employed

by the alchemists in their experiments on the solution of gold. The

ladies of old collected the “celestial wash,” which they imagined had

the virtue of preserving their fine forms, by exposing heaps of wool

to the influences of night radiation. It was supposed that the lean

features of the grasshopper arose from that insect feeding entirely on

dew: “Dumque thymo pascentur apes, dum rore cicadæ,” Virgil, Eclog.

See some curious remarks by Boyle, _On the Power of Dew in Working on

Solid Bodies_: Works of the Honourable R. Boyle, vol. v. p. 121. 1744.

[54] See the _Researches on Heat_, by Professor James Forbes, in the

Transactions of the Royal Society of Edinburgh; also Melloni’s papers

on the same subject in the _Annales de Chimie_, several of which have

been translated into the _Scientific Memoirs_, edited by Mr. Richard

Taylor.

[55] The phenomena of dew have constantly engaged the attention of

man. Aristotle, in his book _De Mundo_, puts forth some just notions

on its nature. An opinion has almost always prevailed that dew falls.

Gersten appears to have been the first who opposed this motion. He was

followed by Musschenbroek, and then by Du Fay. The researches of Leslie

were of a far more exact character. Dr. Wilson, in the Transactions of

the Royal Society of Edinburgh, 1st vol., published a _Memoir on Hoar

Frost_ of much interest; but the questions involved remained unsettled

until the researches of Dr. Wells, which were published in his _Essay

on Dew_.

[56] By far the most complete set of experiments on the radiation

of heat from the surface at night, which have been published since

Dr. Wells’s memoir _On Dew_, are those of Mr. Glaisher, of the Royal

Observatory at Greenwich. Instruments of the most perfect kind were

employed, and the observations made with sedulous care. The results

will be found in a memoir _On the Amount of the Radiation of Heat, at

night, from the Earth, and from various bodies placed on or near the

Surface of the Earth_, by James Glaisher, Esq., Philosophical Trans.

for 1847, part 2.

[57] Dr. Wells noticed the practical fact that very light shades

protected delicate plants from frost, by preventing radiation. Mr.

Goldsworthy Gurney has made a series of interesting experiments, and

he imagines that by shading grasslands with boughs of trees, or any

light litter, a more abundant crop is produced. The subject has been

discussed in the journals of the Royal Agricultural Society. May not

the apparent increase be due entirely to the succulent condition in

which a plant always grows in the shade?

[58] This paper of Melloni’s will be found in the _Bibliothèque

Universelle de Genève_, for 1843. The conclusions are highly ingenious,

but they rest entirely on the analogy supposed to be discovered

between the relations of heat, like light, to the coloured rays of the

spectrum. This, it must be remembered, is not the case, since even Sir

William Herschel showed that red light might exist with only a minimum

of calorific power, notwithstanding the fact, that the maximum heat-ray

of the spectrum coincides with the red rays.

[59] Dr. Robinson, of Armagh, in his Memoir _On the Effects of Heat in

lessening the Affinities of the Elements of Water_.--Transactions of

the Royal Irish Academy, vol. xxi. part 2.

[60] On this subject consult Robert Were Fox, _On the Temperature of

the Mines of Cornwall_.--Cornwall Geological Transactions, vol. ii.; W.

J. Henwood, on the same subject, _Ib._ vol. v.; Reports of the British

Association, 1840, p. 315; Edinburgh New Philosophical Journal, vol.

xxiv. p. 140.

[61] _On the causes of the temperature of Hot and Thermal Springs;

and on the bearings of this subject as connected with the general

question regarding the internal temperature of the Earth_: by Professor

Gustav Bischoff, of Bonn.--Edinburgh New Philosophical Journal, vol.

xx. p. 376; vol. xxiii. p. 330. Some interesting information on

the temperature of the ground will be found in Erman’s _Travels in

Siberia_, translated by W. D. Cooley, vol. i. p. 339; vol. ii. p.

366. _Sur la Profondeur à laquelle se trouve la couche de Température

invariable entre les Tropiques_, by Boussingault: Annales de Chimie et

de Physique, 1833, p. 225. Reference may also be made to Humboldt’s

_Cosmos_, Otto’s translation; and to the excellent article on

_Meteorology_, by George Harvey, in the Encyclopædia Metropolitana.

These chthonisothermal lines, as they are called, have been traced by

Humboldt and others over extensive districts.

[62] These results are obtained from the valuable observations

of Robert Were Fox, Esq., made with great care by that gentleman

in several of the Cornish mines: _Report on some observations on

Subterranean Temperature_.--British Association Reports, vol. ix. p.

309; Philosophical Magazine, 1837, vol. ii. p. 520.

[63] From his experiments, the following conclusions were arrived at by

M. Delaroche:--

1. Invisible radiant heat may, in some circumstances, pass directly

through glass.

2. The quantity of radiant heat which passes directly through glass

is so much greater, relative to the whole heat emitted in the same

direction, as the temperature of the source of heat is more elevated.

3. The calorific rays which have already passed through a screen of

glass, experience, in passing through a second glass screen of a

similar nature, a much smaller diminution of their intensity than they

did in passing through the first screen.

4. The rays emitted by a hot body differ from each other in their

faculty to pass through glass.

5. A thick glass, though as much or more permeable to light than a thin

glass of worse quality, allows a much smaller quantity of radiant heat

to pass. The difference is so much the less as the temperature of the

radiating source is more elevated.

6. The quantity of heat which a hot body yields in a given time, by

radiation to a cold body situate at a distance, increases, _cæteris

paribus_, in a greater ratio than the excess of temperature of the

first body above the second.--Journal de Physique, vol. lxxv.

[64] Sir David Brewster differs from the conclusions arrived at by

Delaroche. He thus explains his views:--“The inability of radiant

heat to pass through glass, may be considered as a consequence of its

refusing to yield to the refractive force; for we can scarcely conceive

a particle of radiant matter freely permeating a solid body, without

suffering some change in its velocity and direction. The ingenious

experiments of M. Prévost, of Geneva, and the more recent ones of

M. Delaroche, have been considered as establishing the permeability

of glass to radiant heat. M. Prévost employed moveable screens of

glass, and renewed them continually, in order that the result which

he obtained might not be ascribed to the heating of the screen; but

such is the rapidity with which heat is propagated through a thin

plate of glass, that it is extremely difficult, if not impossible, to

observe the state of the thermometer before it has been affected by

the secondary radiation from the screen. The method employed by M.

Delaroche, of observing the difference of effect, when a blackened

glass screen and a transparent one were made successively to intercept

the radiant heat, is liable to an obvious error. The radiant heat would

find a quicker passage through the transparent screen; and, therefore,

the difference of effect was not due to the transmitted heat, but to

the heat radiated from the anterior surface. The truth contained in M.

Delaroche’s fifth proposition is almost a demonstration of the fallacy

of all those that precede it. He found that ‘a thick plate of glass,

though as much or more permeable to light than a thin glass of worse

quality, allowed a much smaller quantity of radiant heat to pass.’ If

he had employed very thick plates of the purest flint glass, or thick

masses of fluid that have the power of transmitting light copiously,

he would have found that not a single particle of heat was capable of

passing directly through transparent media.”--Sir D. Brewster, _On new

properties of heat as exhibited in its propagation along plates of

glass_. Philosophical Transactions, vol. cvi. p. 107.

[65] _Proposal of a New Nomenclature for the Science of Calorific

Radiations_, by M. Melloni. Bibliothèque Universelle de Genève, No.

70. Scientific Memoirs, vol. iii. part 12. Many of the terms, as

_Diathermasy_, or transparency for heat; _Adiathermasy_, opacity for

heat; _Thermochroic_, coloured for heat, and others, are valuable

suggestions of forms of expression which are required in dealing with

these physical phenomena.

[66] For a careful examination of the several theories of heat consult

Dr. Young’s Course of Lectures on Natural Philosophy, &c., Lecture 52,

_On the Measures and the Nature of Heat_; also Powell’s very excellent

_Reports on Radiant Heat_--Reports of the British Association, 1832,

1840. The transcendental view which the immaterial theory leads to,

cannot be better exemplified than by the following quotation from that

inexplicable dream of a talented man, _Elements of Physiophilosophy_,

by Lorenz Oken, M.D. (translated for the Ray Society, by Alfred Tulk):--

“Heat is not matter itself any more than light is; but it is only the

act of motion in the primary matter. In heat, as well as in light,

there certainly resides a material substratum; yet, this substratum

does not give out heat and light; but the _motion_ only of the

substratum gives out heat, and the _tension_ only of the substratum

light. There is no body of heat; nitrogen is the body of heat, just

as oxygen may be called the body of fire. Heat is real space; into it

all forms have been resolved, as all materiality has been resolved

into gravity, and all activity, all polarity, into light. Heat is the

universal form, consequently the want of form.”

[67] Mémoires de la Société Physique, &c., de Genève, tom. ii. art. 2.

[68] This curious phenomenon was first observed by Mr. Trevelyan, whose

_Notice regarding some Experiments on the Vibration of Heated Metals_

will be found in the Transactions of the Royal Society of Edinburgh,

vol. xii., 1837. In a Memoir in the same volume, entitled _Experimental

Researches regarding certain vibrations which take place between

metallic masses having different temperatures_, Professor Forbes draws

the following conclusions:--

1. “The vibrations never take place between substances of the same

nature.

2. “Both substances must be metallic. (This is now proved not to be

necessary.)

3. “The vibrations take place with an intensity proportional (within

certain limits) to the difference of the conducting powers of the

metals for heat or electricity; the metal having the least conducting

power being necessarily the coldest.

4. “The time of contact of two points of the metals must be longer than

that of the intermediate portions.

5. “The impulse is received by a distinct and separate process at each

contact of the bar and block, and in no case is the metallic connection

of the bearing points in the bar, or those of the block, in any way

essential.

6. “The intensity of the vibration is (under certain exceptions)

proportional to the difference of temperature of the

metals.”--Transactions of the Royal Society of Edinburgh, vol. xii.

[69] The Bakerian Lecture. _On certain Phenomena of Voltaic Ignition,

and the Decomposition of Water into its Constituent Gases by Heat_: by

W. R. Grove, Esq.--Philosophical Transactions, 1847. Part 1.

[70] Davy’s _Researches on Flame_. Works, vol. vi.--Philosophical

Transactions for 1817.

[71] _On the Effect of Heat in lessening the affinities of the Elements

of Water_: by the Rev. Thomas Romney Robinson, D.D.--Transactions of

the Royal Irish Academy, vol. xxi. part 2.

[72] _An Inquiry concerning the Chemical Properties that have been

attributed to Light_: by Benjamin, Count of Rumford.--Philosophical

Transactions, vol. lxxxviii. p. 449.--The results obtained by Count

Rumford were probably due to the non-luminous heat-rays--parathermic

rays--which are known to be given off by boiling water.

[73] For Dr. Drapers paper, see Philosophical Magazine for May, 1847,

vol. xxx. 3rd series.

[74] _On the Action of the Rays of the Solar Spectrum on Vegetable

Colours_: by Sir J. F. W. Herschel, Bart.

The proof of the continuation of the visible prismatic spectrum beyond

the extreme violet may be witnessed in the following manner:--“Paper

stained with tincture of turmeric is of a yellow colour; and, in

consequence, the spectrum thrown in it, if exposed in open daylight,

is considerably affected in its apparent colours, the blue portion

appearing violet, and the violet very pale and faint; but beyond the

region occupied by the violet rays, is distinctly to be seen a faint

prolongation of the spectrum, terminated laterally, like the rest of

it, by straight and sharp outlines, and which, in this case, affects

the eye with the sensation of a pale yellow colour.”--Philosophical

Transactions, p. 133.

[75] The most complete exposition of the theory that animal heat is

derived from chemical action only, will be found in _Animal Chemistry,

or Chemistry in its applications to Physiology and Pathology_, by

Justus Liebig: translated by Dr. Gregory. The conclusions arrived at

by the author, notwithstanding his high--and deservedly high--position

in chemical science, must, however, be received with great caution,

many of them being founded on most incorrect premises, and his

generalizations being of the most hasty and imperfect character. At

page 22 the following passage occurs:--“If we were to go naked, like

certain savage tribes, or if in hunting or fishing we were exposed

to the same degree of cold as the Samoiedes, we should be able, with

ease, to consume ten pounds of flesh, and, perhaps, a dozen of tallow

candles into the bargain, daily, as warmly clad travellers have related

with astonishment of these people. We should then also be able to take

the same quantity of brandy or train-oil without bad effects, because

the carbon and hydrogen of these substances would only suffice to keep

up the equilibrium between the external temperature and that of our

bodies.”

A brief examination will exhibit the error of this. The analysis of

Beef, by D. Lyon Playfair, is as follows:--

Carbon 51·83

Hydrogen 7·57

Nitrogen 15·01

Oxygen 21·37

Ashes 4·23

And the following has been given by Chevreul as the composition of

mutton tallow:--

Carbon 96

Hydrogen 16

Nitrogen 16

Oxygen 48

About three times the quantity of oxygen to the carbon eaten, is

required to convert it into carbonic acid; hence, the Samoiede, eating

more highly carbonized matter, must inspire 288 oz. of oxygen daily,

or nearly eight times as much as the “ordinary adult.” By the lungs

he must take into the body 2,304 cubic feet of air besides what will

be absorbed by the skin. His respirations must be so much quickened,

that at the lowest possible calculation he must have 500 pulsations a

minute. Under such conditions it is quite clear man could not exist.

There is no disputing the fact of the enormous appetites of these

people; but all the food is not removed from the system as carbonic

acid gas.

[76] An interesting paper by Dr. Davy, _On the Temperature of Man_,

will be found in the Philosophical Transactions, vol. cxxxvi. p.

319.--Sir Humphry Davy, in his _Consolations in Travel, or the Last

Days of a Philosopher_, in his fourth dialogue, _The Proteus_, has

several ingenious speculations on this subject.

[77] _Exposé de quelques résultats obtenus par l’action combinée de la

chaleur et de la compression sur certains liquides, tels que l’eau,

l’alcool, l’éther sulfurique, et l’essence de pétrole rectifiée_: par

M. le Baron Cagniard de la Tour.

The three following conclusions are arrived at:--

1. Que l’alcool à 36 degrés, l’essence de pétrole rectifiée à 42

degrés, et l’éther sulfurique soumis à l’action de la chaleur et de la

compression, sont susceptibles de se réduire complètement en vapeur

sous un volume un peu plus que double de celui de chaque liquide.

2. Qu’une augmentation de pression, occasionnée par la présence de

l’air dans plusieurs des experiences qui viennent d’être citées, n’a

point apporté d’obstacle à l’évaporation du liquide dans le même

espace; qu’elle a seulement rendu sa dilatation plus calme et plus

facile à suivre jusqu’au moment où le liquide semble s’évanouir

tout-à-coup.

3. Que l’eau, quoique susceptible sans doute d’être réduite en vapeur

très-comprimée, n’a pu être soumise à des experiences complètes,

faute de moyens suffisans pour assurer l’exacte fermeture de la

marmite de compression, non plus que dans les tubes de verre dont elle

altère la transparence en s’emparant de l’alcali qui entre dans leur

composition.--Annales de Chimie, vol. xxi.

[78] _Sur les phénomènes qui présentent les corps projetés sur des

surfaces chaudes_: par M. Boutigny (d’Evreux).--Annales de Chimie et de

Physique, vol. xi. p. 16. _Congélation du mercure en trois secondes,

en vertu de l’état sphéroïdal dans un creuset incandescent_: by M.

Faraday.--Ibid., vol. xix. p. 383.

_Spheroidal Condition of Bodies_ (Extrait d’une Note de M. Boutigny

d’Evreux).

“Au nombre des propriétés des corps à l’état sphéroïdal, il en est cinq

qui me paraissent caractéristiques et fondamentales, et c’est sur ces

cinq propriétés que je base la définition que je soumets aujourd’hui au

jugement de l’Académie. Ces cinq propriétés sont:--

“1. La forme arrondie que prend la matière sur une surface chauffée à

une certaine température.

“2. Le fait de la distance permanente qui existe entre le corps à

l’état sphéroïdal et le corps sphéroïdalisant.

“3. La propriété de réfléchir le calorique rayonnant.

“4. La suspension de l’action chimique.

“5. La fixité de la température des corps à l’état sphéroïdal.

“Cela posé, voici la définition que je propose: un corps projeté sur

une surface chaude est à l’état sphéroïdal quand il revêt la forme

arrondie et qu’il se maintient sur cette surface au delà du rayon de sa

sphère d’activité physique et chimique; alors il réfléchit le calorique

rayonnant, et ses molécules sont, quant à la chaleur, dans un état

d’équilibre stable; c’est-à-dire, à une température invariable, ou qui

ne varie que dans des limites étroites.”--Comptes Rendus, 6 Mars, 1848.

[79] _Some Facts relative to the Spheroidal State of Bodies, Fire

Ordeal, Incombustible Man, &c._: by P. H. Boutigny (d’Evreux),

Philosophical Magazine, No. 233 (third, series), p. 80; Comptes Rendus,

May 14, 1849.

[80] The theory of freezing mixtures is deduced from the doctrine of

latent caloric. These are mixtures of saline substances which, at the

common temperature, by their mutual chemical action, pass rapidly into

the fluid form, or are capable of being rapidly dissolved in water,

and, by this quick transition to fluidity, absorb caloric, and produce

degrees of cold more or less intense.--Rev. Francis Lunn, _On Heat_:

Encyclopædia Metropolitana.

[81] _Propriétés de l’Acide Carbonique liquide_, par M. Thilorier,

Annales de Chimie, vol. lx. p. 427. _Solidification de l’Acide

Carbonique_: Ibid. p. 432.

[82] _On the Liquefaction and Solidification of Bodies generally

existing as Gases_, by Michael Faraday, D.C.L., F.R.S., &c.;

Philosophical Transactions, vol. cxxxvi, p. 155.

[83] Burns, in one of his most natural and pathetic letters.