Chapter VI: Heat--Solar and Terrestrial (1)

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]