Chapter VI: Heat--Solar and Terrestrial (2)

Heat is diffused through all bodies in nature, and, as we shall presently see, may be developed in many different ways. We may, therefore, infer, that in converting a sphere of ice into water, and that again into steam, we have done nothing more than interpenetrate the mass with a larger quantity of heat, by which its atoms are more widely separated, and that thus its molecules become free to move about each other. Hence, from a solid state, the water becomes fluid; and then, if the expansive force is continued, an invisible vapour. If these limits are passed by the powers of any greatly increased thermic action, the natural consequence, it must be seen, will be the separation of the atoms from each other, to such an extent that the molecule is destroyed, and chemical decomposition takes place.

By the agency of the electricity of the voltaic battery, we are enabled to produce the most intense heat with which we are acquainted, and by a peculiarly ingenious arrangement Mr. Grove has succeeded in resolving water by the mere action of heat into its constituent elements--oxygen and hydrogen gases. That this decomposition is not due to the voltaic current, but to the heat produced by it, was subsequently proved by employing platina heated by the oxy-hydrogen flame.[69]

This interesting question has been examined with great care by Dr. Robinson of Armagh, who has shown that, as the temperature of water is increased, the affinity of its elements is lessened, until at a certain point it is eventually destroyed. This new and startling fact appears scarcely consistent with our knowledge that a body heated so as to be luminous has the power of causing the combination of the elements of water with explosive violence.[70] But as this acute experimental philosopher somewhat boldly but still most reasonably inquires: “Is it not probable that, if not light, some other actinic power (like that which accompanies light in the spectrum, and is revealed to us by its chemical effects in the processes of photography) is evolved by the heat, and, though invisible, determines, in conjunction with the affinity, that atomic change which transforms the three volumes of oxygen and hydrogen into two of steam?”[71]

This speculation explains, in a very satisfactory manner, some results which were obtained by Count Rumford, in 1798. In a series of experiments instituted for the purpose of examining “those chemical properties of light which have been attributed to it,” he has shown that many cases of chemical decomposition occur in perfect darkness, under the influence of heat, which are precisely similar to those produced by exposure to the sun’s rays.[72]

It must, however, be remembered, that both solar light and heat are sometimes found in direct antagonism to actinic power, and that the most decided chemical changes are produced by those rays in which neither heat nor light can be detected. The most remarkable phenomena of this class will be explained under the head of actinism.

One of the most curious relations which as yet have been discovered between light and heat is, that, the temperature at which all bodies become incandescent, excepting such as are phosphorescent, is uniform. The point on the thermometer (Fahrenheit’s scale) when the eye by perfect repose is enabled to detect the first luminous influence, may probably be regarded as, or very near, 1000°. Daniel has fixed this point at 980°, Wedgwood at 947°, and Draper at 977°.[73] Dr. Robinson and Dr. Draper, by independent observations, have both arrived at the conclusion, that the first gleam of light which appears from heated platina is not red, but of a lavender gray, the same in character of colour as that detected by Sir John Herschel among the most refrangible rays of the solar spectrum.[74]

It must be admitted, that the question of the identity, or otherwise, of light and radiant heat, is beset with difficulties. Many of their phenomena are very similar--many of their modes of action are alike: they are often found as allied agencies; but they as frequently exhibit extreme diversity of action, and they may be separated from each other.

We have now examined the physical conditions and properties of this most important element, and we must proceed to learn something of the means by which it may be developed, independently of its solar source.

This extraordinary principle exists in a latent state in all bodies, and may be pressed out of them. The blacksmith hammers a nail until it becomes red hot, and from it he lights the match with which he kindles the fire of his forge. The iron has by this process become more dense, and percussion will not again produce incandescence until the bar has been exposed in fire to a red heat. The only inference we can draw from this result is, that by hammering the particles have been driven closer together, and the heat driven out; now further hammering will not force the atoms nearer, and consequently no additional quantity of heat can be developed; the iron is made hot in a fire, it absorbs heat, the particles are restored to their former state, and we can now again by hammering develope both heat and light. The Indian produces a spark by the attrition of two pieces of wood. By friction, two pieces of ice may be made to melt each other; and could we, by mechanical pressure, force water into a solid state, an immense quantity of heat would be set free. By the condensation of hydrogen and oxygen gases, pulverulent platinum will become glowing red-hot, and, with certain precautions, even the compact metal, platinum, itself; the heat being derived from the gases, the union of which it has effected. A body passing from the solid to the fluid state absorbs heat from all surrounding substances, and hence a degree of cold is produced. The heat which is thus removed is not destroyed--it is held combined with the fluid; it exists in a latent state. Fluids, in passing into a gaseous form, also rob all surrounding bodies of an amount of heat necessary to maintain the aëriform condition. From the air or from the fluid, this heat may, as we have shown above, be again extracted. Locked in a pint measure of air, there exists sufficient heat to raise several square inches of metal to glowing redness. By the compression of atmospheric air this may be shown, and with a small condensing syringe a sufficient quantity of heat may be set free to fire the _Boletus igniarius_, which, impregnated with nitre, is known as _amadou_. We are acquainted with various sources from which heat may be developed for artificial purposes: the flint-and-steel is an example of the production of heat by mechanical force, and the modern lucifer-match, of the combined action of friction and chemical affinity. These of themselves would admit of a lengthened discourse; but it is necessary that we carefully examine some of the less familiar phenomena of heat under the influences of changes of chemical condition.

If spirits of wine and water are mixed together, a considerable degree of heat is given out, and by mixing sulphuric acid and water, an infinitely larger quantity. If sulphuric acid (oil of vitriol) and spirit of wine, or nitric acid (aquafortis) and spirits of turpentine, at common temperatures, be suddenly mixed, so much heat is set free as to ignite the spirit. In each of these instances there is a condensation of the fluid. In nearly all cases of solution, cold is produced by the absorption of the heat necessary to sustain the salt in a liquid form; but when potash dissolves in water, heat is given out, which is a fact we cannot yet explain. If potassium is placed on water, it seizes the oxygen of the water and sets fire to the hydrogen gas liberated by the heat produced in the change of form. Antimony and many other metals thrown into chlorine gas ignite and burn with brilliancy: the same phenomenon takes place in the vapours of iodine or bromine. Many chemical combinations, as the chloride of potassium and sulphur explode with a blow; whilst the slightest friction occasions the detonation of the fulminating salts of silver, mercury, and gold. Compounds of nitrogen and chlorine, or iodine, are still more delicately combined--the former exploding with fearful violence on the contact of any oleaginous body, and the latter by the smallest elevation of temperature: both of them destroying the vessels in which they may be contained.

Gun cotton presents some peculiar phenomena which may merit brief attention. This peculiar compound is prepared by the action of nitric acid on cotton fibre. The general appearance of the cotton is not altered, but a remarkable physical change has taken place. It is now soluble in ether, and forms a gelatinous compound:--it explodes violently at a temperature which is insufficient for the combustion of gunpowder. Indeed, from, as it would appear, slight electrical disturbances taking place in the gun cotton itself, it not unfrequently explodes spontaneously. These fearful disturbances of the forces which hold bodies in combination are explained with difficulty. May it not be, that an enormous quantity of the calorific and chemical principles is held in a state of extreme tension around the particles of the compound, and that the equilibrium being destroyed, the whole is developed in destructive rapidity?

The fact of great heat being evolved during the conversion of a body from a solid to a gaseous state, as in the explosion of gunpowder or gun cotton, which is a striking exception to the law of latent heat, as it prevails in most cases, admits of no more satisfactory explanation.

As mechanical force produces calorific excitation, so we find that every movement of sap in vegetables, and of the blood and fluids in the animal economy, causes a sensible increase of heat. The chemical processes constantly going on in plants and animals are another source of heat, in addition to which nervous energy and muscular movement must be regarded as producing the caloric which is essential to the health and life of the latter. Digestion has been considered as a process of combustion; and the action between the elements of food, and the oxygen conveyed by the circulation of the blood to every part of the body, regarded as the source of animal heat; and, without doubt, it is one great source, although it can scarcely be regarded as the only one.[75]

The _vis vitæ_, or vital power, influences the delicate and beautiful system of nerves; and as life (an essence of the rarest and most subtile order, a diffusive influence) runs through them, from the brain to the extremities of the members of the body, it sets those tender threads in rapid vibration, and heat is developed. By this action, the circulation of the blood is effected; the muscle is maintained in an elastic condition, ready to perform the tasks of the will; and through these agencies is the warm and fluid blood fitted to receive its chemical restoratives in the lungs, and the stomach to support changes for which it is designed--chemical also--by which more heat is liberated. Was digestion--_Eremacausis_, as the slow combustion produced by combination with oxygen is called--the only source of animal heat, why should the injury of one filmy nerve place a member of the body for ever in the condition of stony coldness? Or why, chemical action being most actively continued after a violent death, by the action of the gastric juices upon the animal tissues, should not animal heat be maintained for a much longer period than it is found to be after respiration has ceased?[76]

In studying the influences of caloric upon the conditions of matter, we must regard the effects of extreme heat, and also of the greatest degrees of cold which have been obtained.

There are a set of experiments by the Baron Cagniard de la Tour, which appear to have a very important bearing on some conditions that may be supposed to prevail in nature, particularly if we adopt the view of a constantly increasing temperature towards the centre of our earth. If water, alcohol, or ether, is put into a strong glass tube of small bore, the ends hermetically sealed, and the whole exposed to a strong heat, the fluid disappears, being converted into a transparent gas; but, upon cooling, it is again condensed, without loss, into its original fluid state.[77] In this experiment, fluid bodies have been converted into elastic transparent gases with but small change of volume, under the pressure of their own atmospheres. We can readily conceive a similar result occurring upon a far more extensive scale. In volcanic districts, at great depths, and consequently under the pressure of the superincumbent mass, the siliceous rocks, or even metals, may, from the action of intense heat, be brought into a fluid, or even a gaseous condition, without any change of volume, since the elastic force of heat is opposed by the rigid resistance of the pressure of the surrounding rocks. Some beautiful experiments by Mr. Hopkins, of Cambridge, have proved that the temperature necessary to melt a body must be considerably elevated as the mechanical pressure to which it is subjected is increased.

Directly connected with the results of Cagniard de la Tour are a yet more remarkable set of phenomena, which have been investigated by M. Boutigny,[78] and generally known as the “spheroidal condition” of bodies. If water is projected upon very hot metal it instantly assumes a spheroidal form--an internal motion of its particles may be observed--it revolves with rapidity, and evaporates very slowly. If a silver or platinum capsule, when brought to a bright red heat, is filled with cold water, the whole mass assumes the spheroidal state, the temperature of the fluid remaining considerably below the boiling point, so long as the red heat is maintained. If we allow the vessel to cool below redness in the dark, the water then bursts into active ebullition, and is dissipated into vapour with almost explosive violence. An equal quantity of water being projected into two similar vessels, over the fire, one cold and the other red hot, it will be found that the water in the cold vessel will boil and evaporate long before that in the one which is red hot.

Another form of this experiment is exceedingly instructive. If a mass of white hot metal is suddenly plunged into a vessel of cold water, the incandescence is not quenched, the metal shines with a bright white light, and the water is seen to circulate around, but at some distance from the glowing mass, being actually repelled by calorific agency. At length, when the metal cools, the water comes in contact with it, and boils with energy.

A result similar to this was observed by Perkins, but its correctness most unjustly doubted. Having made an iron shell containing water, and carefully plugged up, white hot, it was found that the steam never exerted sufficient force to burst the vessel, as it was expected it would do. He caused a hole to be drilled into the bottom of the white-hot shell, and he was surprised to find that no water flowed through the orifice, until the iron was considerably cooled, when it issued forth with violence in the form of steam. Here we have the _Cagniard de la Tour state_ first induced, and the calorific repulsion of the spheroidal state supervenes. If water is poured upon an iron sieve, the wires of which are made red hot, it will not percolate; but on cooling, it runs through rapidly. M. Boutigny, pursuing this curious inquiry, has recently proved that the moisture upon the skin is sufficient to protect it from disorganization, if the arm is plunged into baths of melted metal. The resistance of the surfaces is so great, that little elevation of temperature is experienced.[79] Professor Plücker, of Bonn, has stated that by washing the arm with ether previously to plunging it into melted metal, the sensation produced, while in the molten mass, is that of freezing coldness.

We have now seen that heat at different degrees of intensity appears to produce chemical composition--that it decomposes combined elements--that it alters the conditions of bodies, and actually maintains so powerfully a repellent force, that fluids cannot touch the heated body. More than this, it exerts a most powerful antagonistic influence over all chemical relations. If, to give one example, the volatile element iodine is put into a glowing hot capsule, it resolves itself immediately into a spheroid. Potash rapidly combines with iodine; but if a piece of this alkali is thrown upon it in the capsule, it also takes the spheroidal form, and both bodies revolve independently of each other, their chemical affinities being entirely suspended;--but allow the capsule to cool, and they combine immediately. Science teaches us that a temperature so exalted as not to burn organic bodies may be produced, and points to us this remarkable fact, that the destructive limits of heat are measured between certain degrees--beyond which a fire, by reason of its intensity, ceases to develope heat. What is the radiant force into which this principle changes?

The experiments of Cagniard de la Tour and of Boutigny (d’Evreux), connect themselves, in a striking manner, with those of Mr. Grove and Dr. Robinson; and they teach us that but a very slight alteration in the proportions of the calorific principle given to this planet would completely change the character of every material substance of which it is composed, unless there was an alteration in the physical condition of the elements themselves.

Supposing the ordeal of fiery purification to take place upon this earth, these experiments appear to indicate the mighty changes which would thence result. There would be no annihilation, but everything would be transformed from the centre of the globe to the verge of its atmosphere--old things would pass away, all things become new, and the beautiful mythos of the phœnix be realized in the fresh creation.

The deductions to be drawn from the results obtained by abstracting heat from bodies are equally instructive. By taking advantage of the cooling produced by the rapid solution of salts of several kinds in water, an intense degree of coldness may be produced.[80] Indeed, the absorption of heat by liquefaction may be shown by the use of metallic bodies alone. If lead, tin, and bismuth, are melted together, and reduced to a coarse powder by being poured into water, and the alloy then dissolved in a large quantity of quicksilver, the thermometer will sink nearly 50 degrees. An intense amount of cold will result from the mixture of muriate of lime and snow, by which a temperature of 50° below the zero of Fahrenheit, or 82° below the freezing point of water, is produced. By such a freezing mixture as this, mercury will be rendered solid. A degree of cold, however, far exceeding it, has lately been obtained by the use of solid carbonic acid and ether.[81] Solid carbonic acid is itself procured from the gas liquefied by pressure; which liquid, when allowed to escape into the air, evaporates so rapidly that a large quantity of it is congealed by being robbed of its combined heat by the vaporizing portion. When this solid acid is united with ether, a bath is formed in which the carbonic acid will remain solid for twenty or thirty minutes. By a mixture of this kind, placed under the receiver of an air-pump, a good exhaustion being sustained, a degree of cold 166° below zero is secured. By this intense cold, many of the bodies which have hitherto been known to us only in the gaseous state have been condensed into liquids and solids. Olefiant gas, a compound of hydrogen and carbon, was brought into a liquid form. Hydriodic and hydrobromic acids could be condensed into either a liquid or a solid form. Phosphuretted hydrogen, a gas which inflames spontaneously when brought into contact with the air or with oxygen, became a transparent liquid at this great reduction of temperature. Sulphurous acid may be condensed, by pressure and a reduction of temperature, into a liquid which boils at 14° Fahrenheit, but by the carbonic acid bath it is converted into a solid body, transparent and without colour. Sulphuretted hydrogen gas solidifies at 122° below zero, and forms a white substance resembling a mass of crystals of sea-salt.

A combination of the two gases, chlorine and oxygen, becomes solid at -75°, and the protoxide of nitrogen at -150°. Cyanogen, a compound of carbon and nitrogen--the base of prussic acid--is solidified at 30° below the zero of our thermometric scale. The well-known pungent compound, ammonia, so exceedingly volatile at common temperatures, is converted into a crystalline, translucent, white substance at the temperature of -103°. The difficulties which necessarily attend the exposure of a body to extreme cold and great pressure at the same time, appear to be the only obstacle to the condensation of oxygen, hydrogen, and nitrogen gases. A sufficient amount of condensation was, however, effected by Dr. Faraday, to lead him to the conclusion, arrived at also by other evidences, that hydrogen, the lightest of the ponderable bodies, partakes of the nature of a metal.[82]

During the solidification of water by freezing, some remarkable facts may be noticed.

Water, in cooling, gradually condenses in volume until it arrives at 40° Fahr., which appears to be the point of greatest density. From this temperature to that of 32°, the point at which it begins to solidify, its volume remains unchanged,[82] as crystallisation (freezing) begins, the bulk increases, the mass becomes specifically lighter, and it swims on the surface of the fluid. From 40° to 32° the particles of water must be taking up that new position which is essential to the formation of the solid--ice; and while this is taking place, every substance held in solution by the water is rejected.

If we mix with water the deepest colouring matter--the strongest acid or the most acrid poison--they are each and all rejected during the process of freezing, and if the water has been kept in a state of agitation during the process--so that the liberated particles may not be mechanically entangled--the ice will be transparent, colourless, tasteless, and inert--the substances rejected being gathered together in the centre of the frozen mass in a state of intense concentration. In like manner, even the atmospheric air, which is always held in solution, is rejected, and hence the reason why all the ice which forms upon still ponds is full of air-bubbles, while the ice which is produced in agitated water is perfectly free from them. This in itself is a remarkable condition, the entire bearing of which is not clearly understood; but a still more singular fact has been discovered in intimate connection with the rejection of all matter from a freezing solution. Water, which in this way is freed entirely of air, will not boil at 212° F., the ordinary boiling point of water.

If a mass of ice formed in the manner described is placed in a vessel, and being just covered with a film of oil, to prevent the absorption of air, is melted over a lamp or fire, and the heat continued, it will, so far from being converted into steam at 212°, continue to increase in temperature up to 270° or more, and then burst into ebullition with such explosive violence as to rend the vessel in which it is confined.

From this experiment we learn that did water exist in any other condition than that in which we find it--even with the apparently simple difference of containing no air--it would not be safe to employ it in any culinary or manufacturing operation, since its use would be followed by explosions as dangerous as those of gunpowder.

Such researches as these prove to us the admirable adaptation of all things to their especial ends--the beautiful adjustment of the balance of forces throughout creation.

The refinements of Grecian philosophy saw, without the aids of inductive science, that the outward vesture of nature covered a host of mysterious agencies to which its characteristics were directly due. In their dream of the four elements, fire, the external and visible form of heat, was regarded as the cause of vitality, and the disposer of every organised and unorganised condition of matter. Their idealisations have assumed another form, but the researches of modern science have only established their universality and truth.

The great agents at work in nature--the mighty spirits bound to never-ending tasks, which they pursue with unremitting toil, are of so refined a character, that they will probably remain for ever unknown to us. The arch-evocator, with the wand of induction, calls; but the only answer to his evocation is the manifestation of power in startling effects. Science pursues her inquiries with zeal and care: she tries and tortures nature to compel her to reveal her secrets. Bounds are, however, set to the powers of mortal search: we may not yet have reached the limits within which we are free to exercise our mental strength; but, those limits reached, we shall find an infinite region beyond us, into which even conjecture wanders eyeless and aimless, as the blind Cyclops groping in his melancholy cave.[83]

All we know of heat is, that striking effects are produced which we measure by sensation, and by instruments upon which we have observed that given results will be produced under certain conditions: of anything approaching to the cause of these we are totally ignorant. The wonder-working mover of some of the grandest phenomena in nature--giving health to the organic world, and form to the inorganic mass--producing genial gales and dire tornadoes--earthquake strugglings and volcanic eruptions--ministering to our comforts in the homely fire, and to advancement in civilisation in the mighty furnace, and the ingenious engine which drains our mines, or traverses our country with bird-like speed,--will, in all probability, remain for ever unknown to man. The immortal Newton, many of whose guesses have a prophetic value, thus expresses himself:--“Heat consists in a minute vibratory motion in the particles of bodies, and this motion is communicated through an apparent vacuum by the undulations of a very subtile elastic medium, which is also concerned in the phenomena of light.”

Our experimental labours and our mathematical investigations have considerably advanced our knowledge since the time of Newton; yet still each theory of heat strangely resembles the mystic lamp which the Rosicrucian regarded as a type of eternal life--a dim and flickering symbol, in the tongue-like flame of which imagination, like a child, can conjure many shapes.

Modern theory regards heat as a manifestation of motion, and experiment proves that a body falling through a certain space generates a definite quantity of heat, while observation shows that the waters at the base of the Falls of Niagara possess a temperature 1° higher than when they first glide over the edge of the precipice.

This increase of temperature is due to the mechanical force due to the fall, and is no more an evidence of the conversion of motion into heat, than is the old experiment of rubbing a button until it becomes hot. At all events, the fact that a given amount of mechanical force always produces an equivalent of heat is as applicable to the idea of a “subtile elastic medium” which is diffused through all matter, as to the, at present, favourite hypothesis.

So far has this view been strained, that the temperature of the planets has been referred to their motions, and speculation has aided the mathematician in determining the cessation of planetary motion, by the conversion of it into heat. It is true that other theorists have supposed points in space upon which this heat might be concentrated and reflected back again to produce motion.

There may be much of the poetic element in such speculations, but it is of that order which belongs rather to the romantic than to the real.

A speculation which has more of truth, and which is, indeed, demonstrable, cannot fail to impress every mind with its beauty, and probable correctness.

In the growth of a tree, its wood and all its products are the result of certain external forces effecting chemical changes. Carbonic acid is decomposed, the carbon is retained, and oxygen given off, and assimilations of a complex character are in constant progress to produce the various compounds of oxygen, hydrogen, nitrogen, and carbon.

Every condition of organised forms is due to the external excitation of light and heat, and in the chemical changes which take place, an equivalent of these principles, or powers--it signifies but little according to which view we may regard them--is absorbed, and retained as essential to the condition of the matter formed. Let us confine our attention to wood--although the position applies equally to every organic product. A cubic foot of wood is formed by the decomposition of a certain quantity of carbonic acid, by the vital function of the plant, excited by the solar rays, which are involved in the mass which nature by “her wondrous alchemy” has made. Eventually this cubic foot of wood is subjected to a process of chemical change--combustion; by the application of a single spark,--and in the disintegration of the wood, its carbon combining with oxygen to form carbonic acid, its hydrogen to form water, which is returned to the air, a large amount of light and heat is produced. This is exactly equivalent to the amount which was engaged in its formation. Indeed, the sunshine which fell upon the leaves of the forest tree, of which the log formed a part, has been hoarded up, and we again develope it in its original state of heat and light.

The vast coal beds of England were formed by the rapid growth and quick decay of a peculiar class of plants under the influence of a tropical sun. They have been buried myriads of ages, under hundreds of feet of sandy rock. By the industry of the miner the coal is brought again to the surface, and we develope from it those powers by which it was formed.

In the fire which gives comfort to our homes--in the furnace which generates force for the purposes of manufacture, or to propel the railway engine and its ponderous train--in the gas with which we illumine our streets and gladden during the long winter nights our apartments, we are developing that heat and light which fell upon the earth with all its quickening influences millions of ages before yet the Creator had called into existence the monarch Man, for whose necessities these wondrous formations were designed.

FOOTNOTES:

[43] The following table of the rays penetrating coloured glass has been given by Melloni, in his memoir _On the Free Transmission of Radiant Heat through Different Bodies_:--

Deep violet 53
Yellowish red (flaked) 53
Purple red (flaked) 51
Vivid red 47
Pale violet 45
Orange red 44
Clear blue 42
Deep yellow 40
Bright yellow 34
Golden yellow 33
Deep blue 33
Apple green 26
Mineral green 23
Very deep blue 19

Translated in the Scientific Memoirs, vol. i. p. 30.

[44] “The physical characters of this species of glass, which acts so differently from the other species of coloured glass in all the phenomena of calorific absorption, are, 1st, its intercepting almost totally the rays which pass through alum; 2nd, its entirely absorbing the red rays of the solar spectrum. I have already stated that their colouration is produced almost entirely by the oxide of copper.

“Thus, the colouring matters of the coloured glasses, while they so powerfully affect the relations of quantity which the different rays of ordinary light bear to each other, exercise no elective action on the concomitant calorific rays. This curious phenomenon is the more remarkable as the colouring matters absorb almost always a very considerable portion of the heat _naturally transmitted by the glass_. The following are, in fact, the calorific transmissions of the seven coloured glasses referred to; the transmission of the common glass being represented by 100; red glass, 82·5; orange, 72·5; yellow, 55; bluish-green, 57·5; blue, 52·5; indigo, 30; violet, 85. The quantity of heat absorbed through the action of the colouring substances is, therefore, 17·5 in the red glass, 27·5 in the orange, 45 in the yellow, 42·5 in the green, 47·5 in the blue, 70 in the indigo, and 15 in the violet. Now, as these absorptions extinguish a proportional part of each of the rays which constitute the calorific stream transmitted by common glass, they may be compared, as we said before, with the absorbent action exercised on light by matters more or less deeply brown or dark, when they are immersed in water, or some other colourless liquid which dissolves, but does not affect them chemically.”--_Annales de Chimie et de Physique_, tom. xl. p. 382.

Guided by these principles, the author selected the glass employed in glazing the Royal Palm-House, at Kew Botanical Gardens, where it was desired to obstruct the passage of those rays which have a particular scorching influence. Of this glass a description was given at the meeting of the British Association at Oxford, which appears in the Transactions for that year. The result has been all that could be desired--not a single instance of scorching having occurred during the three years which have elapsed.

[45] In the _Philosophical Transactions_, vol. xc., the following papers, by Sir William Herschel, may be consulted:--

_Investigation of the powers of the prismatic colours to heat and illuminate objects; with remarks that prove the different refrangibility of radiant heat. To which is added, an inquiry into the method of viewing the sun advantageously, with telescopes of large apertures and high magnifying powers_, p. 255. _Experiments on the refrangibility of the invisible rays of the sun_, p. 284. _Experiments on the solar and on the terrestrial rays that occasion heat; with a comparative view of the laws to which light and heat, or rather the rays which occasion them, are subject; in order to determine whether they are the same or different_, pp. 293, 437.

In connection with this inquiry, Sir William Herschel remarks, that since a _red glass_ stops no less than 692 out of 1,000 such rays as are of the refrangibility of red light, we have a direct and simple proof, in the case of the red glass, that the rays of light are transmitted, while those of heat are stopped, and that thus they have nothing in common but a certain equal degree of refrangibility, which by the power of the glass must occasion them to be thrown together into the place which is pointed out to us by the visibility of the rays of light.

On the same subject, a Memoir, by Sir Henry Englefield, in the Journal of the Royal Institution for 1802, p. 202, may be consulted; and _Researches on Light_, by the Author.

[46] Dr. Draper, _On the production of light by heat_, in the Phil. Mag. for 1847.

Sir Isaac Newton fixed the temperature at which bodies become self-luminous at 635°; Sir Humphry Davy at 812°; Mr. Wedgewood at 947°; and Mr. Daniell at 980°; whilst Dr. Draper from his experiments gives 977°; and Dr. Robinson 865°.

In a review of the above paper by Melloni, entitled _Researches on the Radiations of Incandescent Bodies, and on the Elementary Colours of the Solar Spectrum_, translated for Silliman’s Journal for August, 1847, he remarks:--

“I say that they conduct, as do others heretofore known on light and radiant heat, to a perfect analogy between the general laws which govern these two great agents of nature. I will add that I regard the theory of their identity as the only one admissible by the rules of philosophy; and that I consider myself obliged to adopt it, until it shall have been proved to me that there is a necessity of having recourse to two different principles, for the explanation of a series of phenomena which at present appear to belong to a solitary agent.”

Reference should also be made to a paper by Dr. Robinson, _On the effects of Heat in lessening the Affinities of the Elements of Water_, in the Transactions of the Royal Irish Academy, 1848, where he says that “when a platinum wire is traversed by a current gradually increased till it produces ignition, the first gleam that appears is not red, but of a colour which, when I first saw it, I compared to the ‘lavender ray’ discovered by Sir John Herschel beyond the violet, though I was surprised at seeing the tint of that most refrangible ray preceding the ray which is least so. It is quite conspicuous at about 865°; and as the mode in which it makes its appearance presents nothing abrupt or discontinuous, it seems likely that it is merely a transition from invisible rays excited at a lower temperature to ordinary light.”--p. 310.

[47] In the _Bakerian Lecture_ for 1842, _On the transparency of the Atmosphere, and the law of extinction of the solar rays in passing through it_, by James D. Forbes, Esq., F.R.S., &c., will be found a most complete investigation of this subject.

The experiments were, for the most part, made in Switzerland with Sir John Herschel’s actinometer, and they prove satisfactorily,--“That the absorption of the solar rays by the strata of air to which we have immediate access, is considerable in amount for even moderate thicknesses.”

[48] After referring to several curious and instructive experiments, in which peculiar chemical changes are produced under the influence of the solar rays by their HEAT, Sir John Herschel says:--

“These rays are distinguished from those of Light by being invisible; they are also distinguished from the pure calorific rays beyond the spectrum, by their possessing properties (_of a peculiar character, referred to in former papers_) either exclusively of the calorific rays, or in a much higher degree. They may perhaps not improperly be regarded as bearing the same relation to the calorific spectrum which the photographic rays do to the luminous ones. If the restriction to these rays of the term _thermic_, as distinct from _calorific_, be not (as I think, in fact, it is not) a sufficient distinction, I would propose the term _parathermic rays_ to designate them. These are the rays which I conceive to be active in producing those singular molecular affections which determine the precipitation of vapours in the experiments of Messrs. Draper, Moser, and Hunt, and which will probably lead to important discoveries as to the intimate nature of those forces resident on the surfaces of bodies, to which M. Dutrochet has given the name of epipolic forces.”--_On certain improvements in Photographic Processes, described in a former communication_ (Phil. Trans, vol. cxxxiii.); and _On the Parathermic Rays of the Solar Spectrum_, Phil. Trans, vol. cxxxiv.

The experiments of Mrs. Somerville, _On the Action of the Rays of the Spectrum on Vegetable Juices_ (Phil. Transactions, vol. cxxxvii.), appear to connect themselves with this particular class of rays in a curious manner.

[49] Experiments on the influence of heat on differently-coloured bodies were first made by Dr. Hooke; and it was not until long after that Franklin made his ingenious experiments. Davy exposed to sunshine six equal pieces of copper, painted white, yellow, red, green, blue, and black, in such a manner that one side only was illuminated. To the dark side he attached a bit of cerate, ascertained by experiment to melt at 700. The cerate attached to the black became fluid first, the blue next, then the green and red, and lastly the yellow and white.--Beddoes’s _Contributions to Physical Knowledge_, and collected works of Sir Humphry Davy, vol. ii. p. 27.

[50] By reference to the Treatise on Heat, in the _Encyclopædia Metropolitana_, numerous suggestive experiments will be found, all bearing on this subject. Peschel’s _Elements of Physics_ may also be consulted with advantage. The fact is, however, simply proved, as stated in the text, by placing the bulbs of delicate thermometers, so as to be completely involved in the petals of flowers exposed to sunshine, shading the upper portion of the stem of the instrument.

[51] Moser, _On Vision, and on the Action of Light on Bodies_: and also _On Latent Light_: Scientific Memoirs, vol. iii. Draper, _On certain Spectral Appearances, and on the Discovery of Latent Light_: Phil. Mag., Nov. 1842.

[52] A particular examination of this curious question will be found in the Author’s report _On the Influence of the Solar Rays on the Growth of Plants_: Reports of the British Association for 1847.

[53] Ammianus Marcellinus ascribes the longevity and robust health of mountaineers to their exposure to the dews of night. Dew was employed by the alchemists in their experiments on the solution of gold. The ladies of old collected the “celestial wash,” which they imagined had the virtue of preserving their fine forms, by exposing heaps of wool to the influences of night radiation. It was supposed that the lean features of the grasshopper arose from that insect feeding entirely on dew: “Dumque thymo pascentur apes, dum rore cicadæ,” Virgil, Eclog.

See some curious remarks by Boyle, _On the Power of Dew in Working on Solid Bodies_: Works of the Honourable R. Boyle, vol. v. p. 121. 1744.

[54] See the _Researches on Heat_, by Professor James Forbes, in the Transactions of the Royal Society of Edinburgh; also Melloni’s papers on the same subject in the _Annales de Chimie_, several of which have been translated into the _Scientific Memoirs_, edited by Mr. Richard Taylor.

[55] The phenomena of dew have constantly engaged the attention of man. Aristotle, in his book _De Mundo_, puts forth some just notions on its nature. An opinion has almost always prevailed that dew falls. Gersten appears to have been the first who opposed this motion. He was followed by Musschenbroek, and then by Du Fay. The researches of Leslie were of a far more exact character. Dr. Wilson, in the Transactions of the Royal Society of Edinburgh, 1st vol., published a _Memoir on Hoar Frost_ of much interest; but the questions involved remained unsettled until the researches of Dr. Wells, which were published in his _Essay on Dew_.

[56] By far the most complete set of experiments on the radiation of heat from the surface at night, which have been published since Dr. Wells’s memoir _On Dew_, are those of Mr. Glaisher, of the Royal Observatory at Greenwich. Instruments of the most perfect kind were employed, and the observations made with sedulous care. The results will be found in a memoir _On the Amount of the Radiation of Heat, at night, from the Earth, and from various bodies placed on or near the Surface of the Earth_, by James Glaisher, Esq., Philosophical Trans. for 1847, part 2.

[57] Dr. Wells noticed the practical fact that very light shades protected delicate plants from frost, by preventing radiation. Mr. Goldsworthy Gurney has made a series of interesting experiments, and he imagines that by shading grasslands with boughs of trees, or any light litter, a more abundant crop is produced. The subject has been discussed in the journals of the Royal Agricultural Society. May not the apparent increase be due entirely to the succulent condition in which a plant always grows in the shade?

[58] This paper of Melloni’s will be found in the _Bibliothèque Universelle de Genève_, for 1843. The conclusions are highly ingenious, but they rest entirely on the analogy supposed to be discovered between the relations of heat, like light, to the coloured rays of the spectrum. This, it must be remembered, is not the case, since even Sir William Herschel showed that red light might exist with only a minimum of calorific power, notwithstanding the fact, that the maximum heat-ray of the spectrum coincides with the red rays.

[59] Dr. Robinson, of Armagh, in his Memoir _On the Effects of Heat in lessening the Affinities of the Elements of Water_.--Transactions of the Royal Irish Academy, vol. xxi. part 2.

[60] On this subject consult Robert Were Fox, _On the Temperature of the Mines of Cornwall_.--Cornwall Geological Transactions, vol. ii.; W. J. Henwood, on the same subject, _Ib._ vol. v.; Reports of the British Association, 1840, p. 315; Edinburgh New Philosophical Journal, vol. xxiv. p. 140.

[61] _On the causes of the temperature of Hot and Thermal Springs; and on the bearings of this subject as connected with the general question regarding the internal temperature of the Earth_: by Professor Gustav Bischoff, of Bonn.--Edinburgh New Philosophical Journal, vol. xx. p. 376; vol. xxiii. p. 330. Some interesting information on the temperature of the ground will be found in Erman’s _Travels in Siberia_, translated by W. D. Cooley, vol. i. p. 339; vol. ii. p. 366. _Sur la Profondeur à laquelle se trouve la couche de Température invariable entre les Tropiques_, by Boussingault: Annales de Chimie et de Physique, 1833, p. 225. Reference may also be made to Humboldt’s _Cosmos_, Otto’s translation; and to the excellent article on _Meteorology_, by George Harvey, in the Encyclopædia Metropolitana. These chthonisothermal lines, as they are called, have been traced by Humboldt and others over extensive districts.

[62] These results are obtained from the valuable observations of Robert Were Fox, Esq., made with great care by that gentleman in several of the Cornish mines: _Report on some observations on Subterranean Temperature_.--British Association Reports, vol. ix. p. 309; Philosophical Magazine, 1837, vol. ii. p. 520.

[63] From his experiments, the following conclusions were arrived at by M. Delaroche:--

1. Invisible radiant heat may, in some circumstances, pass directly through glass.

2. The quantity of radiant heat which passes directly through glass is so much greater, relative to the whole heat emitted in the same direction, as the temperature of the source of heat is more elevated.

3. The calorific rays which have already passed through a screen of glass, experience, in passing through a second glass screen of a similar nature, a much smaller diminution of their intensity than they did in passing through the first screen.

4. The rays emitted by a hot body differ from each other in their faculty to pass through glass.

5. A thick glass, though as much or more permeable to light than a thin glass of worse quality, allows a much smaller quantity of radiant heat to pass. The difference is so much the less as the temperature of the radiating source is more elevated.

6. The quantity of heat which a hot body yields in a given time, by radiation to a cold body situate at a distance, increases, _cæteris paribus_, in a greater ratio than the excess of temperature of the first body above the second.--Journal de Physique, vol. lxxv.

[64] Sir David Brewster differs from the conclusions arrived at by Delaroche. He thus explains his views:--“The inability of radiant heat to pass through glass, may be considered as a consequence of its refusing to yield to the refractive force; for we can scarcely conceive a particle of radiant matter freely permeating a solid body, without suffering some change in its velocity and direction. The ingenious experiments of M. Prévost, of Geneva, and the more recent ones of M. Delaroche, have been considered as establishing the permeability of glass to radiant heat. M. Prévost employed moveable screens of glass, and renewed them continually, in order that the result which he obtained might not be ascribed to the heating of the screen; but such is the rapidity with which heat is propagated through a thin plate of glass, that it is extremely difficult, if not impossible, to observe the state of the thermometer before it has been affected by the secondary radiation from the screen. The method employed by M. Delaroche, of observing the difference of effect, when a blackened glass screen and a transparent one were made successively to intercept the radiant heat, is liable to an obvious error. The radiant heat would find a quicker passage through the transparent screen; and, therefore, the difference of effect was not due to the transmitted heat, but to the heat radiated from the anterior surface. The truth contained in M. Delaroche’s fifth proposition is almost a demonstration of the fallacy of all those that precede it. He found that ‘a thick plate of glass, though as much or more permeable to light than a thin glass of worse quality, allowed a much smaller quantity of radiant heat to pass.’ If he had employed very thick plates of the purest flint glass, or thick masses of fluid that have the power of transmitting light copiously, he would have found that not a single particle of heat was capable of passing directly through transparent media.”--Sir D. Brewster, _On new properties of heat as exhibited in its propagation along plates of glass_. Philosophical Transactions, vol. cvi. p. 107.

[65] _Proposal of a New Nomenclature for the Science of Calorific Radiations_, by M. Melloni. Bibliothèque Universelle de Genève, No. 70. Scientific Memoirs, vol. iii. part 12. Many of the terms, as _Diathermasy_, or transparency for heat; _Adiathermasy_, opacity for heat; _Thermochroic_, coloured for heat, and others, are valuable suggestions of forms of expression which are required in dealing with these physical phenomena.

[66] For a careful examination of the several theories of heat consult Dr. Young’s Course of Lectures on Natural Philosophy, &c., Lecture 52, _On the Measures and the Nature of Heat_; also Powell’s very excellent _Reports on Radiant Heat_--Reports of the British Association, 1832, 1840. The transcendental view which the immaterial theory leads to, cannot be better exemplified than by the following quotation from that inexplicable dream of a talented man, _Elements of Physiophilosophy_, by Lorenz Oken, M.D. (translated for the Ray Society, by Alfred Tulk):--

“Heat is not matter itself any more than light is; but it is only the act of motion in the primary matter. In heat, as well as in light, there certainly resides a material substratum; yet, this substratum does not give out heat and light; but the _motion_ only of the substratum gives out heat, and the _tension_ only of the substratum light. There is no body of heat; nitrogen is the body of heat, just as oxygen may be called the body of fire. Heat is real space; into it all forms have been resolved, as all materiality has been resolved into gravity, and all activity, all polarity, into light. Heat is the universal form, consequently the want of form.”

[67] Mémoires de la Société Physique, &c., de Genève, tom. ii. art. 2.

[68] This curious phenomenon was first observed by Mr. Trevelyan, whose _Notice regarding some Experiments on the Vibration of Heated Metals_ will be found in the Transactions of the Royal Society of Edinburgh, vol. xii., 1837. In a Memoir in the same volume, entitled _Experimental Researches regarding certain vibrations which take place between metallic masses having different temperatures_, Professor Forbes draws the following conclusions:--

1. “The vibrations never take place between substances of the same nature.

2. “Both substances must be metallic. (This is now proved not to be necessary.)

3. “The vibrations take place with an intensity proportional (within certain limits) to the difference of the conducting powers of the metals for heat or electricity; the metal having the least conducting power being necessarily the coldest.

4. “The time of contact of two points of the metals must be longer than that of the intermediate portions.

5. “The impulse is received by a distinct and separate process at each contact of the bar and block, and in no case is the metallic connection of the bearing points in the bar, or those of the block, in any way essential.

6. “The intensity of the vibration is (under certain exceptions) proportional to the difference of temperature of the metals.”--Transactions of the Royal Society of Edinburgh, vol. xii.

[69] The Bakerian Lecture. _On certain Phenomena of Voltaic Ignition, and the Decomposition of Water into its Constituent Gases by Heat_: by W. R. Grove, Esq.--Philosophical Transactions, 1847. Part 1.

[70] Davy’s _Researches on Flame_. Works, vol. vi.--Philosophical Transactions for 1817.

[71] _On the Effect of Heat in lessening the affinities of the Elements of Water_: by the Rev. Thomas Romney Robinson, D.D.--Transactions of the Royal Irish Academy, vol. xxi. part 2.

[72] _An Inquiry concerning the Chemical Properties that have been attributed to Light_: by Benjamin, Count of Rumford.--Philosophical Transactions, vol. lxxxviii. p. 449.--The results obtained by Count Rumford were probably due to the non-luminous heat-rays--parathermic rays--which are known to be given off by boiling water.

[73] For Dr. Drapers paper, see Philosophical Magazine for May, 1847, vol. xxx. 3rd series.

[74] _On the Action of the Rays of the Solar Spectrum on Vegetable Colours_: by Sir J. F. W. Herschel, Bart.

The proof of the continuation of the visible prismatic spectrum beyond the extreme violet may be witnessed in the following manner:--“Paper stained with tincture of turmeric is of a yellow colour; and, in consequence, the spectrum thrown in it, if exposed in open daylight, is considerably affected in its apparent colours, the blue portion appearing violet, and the violet very pale and faint; but beyond the region occupied by the violet rays, is distinctly to be seen a faint prolongation of the spectrum, terminated laterally, like the rest of it, by straight and sharp outlines, and which, in this case, affects the eye with the sensation of a pale yellow colour.”--Philosophical Transactions, p. 133.