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 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 grass60–3/4

Wool on a raised board54–1/2

Swandown on ditto53

The surface of the raised board57

Grass plat51

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;

and,

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.

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