University of Virginia Library

6. CHAPTER VI.
TESTIMONY OF CARBONIC DIOXIDE.

WHEN standing by some quiet mill-stream, have you ever speculated on the origin of the power which is animating the machinery of the great factory on its banks, spinning and weaving the crude cotton into miles of cloth every week? Or at Niagara, did the thought ever strike you, when gazing up at those floods of water which come tumbling over the rocky cliffs, and plunging into the seething sea at your feet, that similar floods had been pouring over that ancient river-bed for countless ages without diminishing the inexhaustible supply? Or, if it has been at once your privilege and your terror to witness that grandest sight of nature, a violent storm at sea, have you been impressed by the untiring might of that mysterious agent which impels the raging winds and upheaves the roaring billows? Whence can come all the power? and what keeps in motion that wonderful aqueous circulation, which we studied in the last chapter?

Although the origin of this never-ceasing motion may be still concealed, we have at least traced back

the power to a proximate source in the great central luminary of our system. It is the gentle influences of the sunbeam that raise the water in vapor, and it is the same solar rays that keep in motion the great aerial currents, spreading the clouds over the earth, and distilling their liquid treasures "to satisfy the desolate and waste ground, and to cause the bud of the tender herb to spring forth.'' Incredible as it may appear, it is actually the sun that weaves the cloth, that feeds the fountains of Niagara, and it is his delicate rays that rule in the tempest and direct the storm. But there are other influences of the sunbeam still more subtle than these, and there are other cycles of changes, as grand as the aqueous circulation, of which the sun is also the ever-active cause.

Referring to the table before given, representing the composition of the atmosphere, you will notice that the great aerial ocean contains more than five million billions of tons of an aeriform substance called carbonic dioxide. This amount, although absolutely very great, is nevertheless only a small fraction of the whole atmosphere, making up less than a thousandth part of its total mass. A cubic foot of air does not contain more than a quarter of a grain of carbonic dioxide; yet there is not one of the atmospheric constituents more intimately associated with organic life, or which discharges more important functions. Although itself a colorless gas, carbonic dioxide consists of ordinary black charcoal combined with oxygen, and these elements are united by one of the strongest affinities

known in nature; yet, intense as this force is, the power of the sun is greater, and his rays, acting on the green leaves of the plants, are constantly decomposing the gas and liberating the carbon, to be incorporated into the various forms of vegetable life. Here, however, it remains only for a brief period; for when the plants have finished their allotted term of life, the carbon again unites with oxygen, and, in the form of carbonic dioxide, is restored to the atmosphere by the process of combustion or decay. But frequently, before these destructive changes complete the cycle, the carbon undergoes a further transformation, and through the process of digestion becomes a part of the body of the animal. Yet this transmutation, as a general rule, only hastens the final result; since the processes of animal life are very rapid, and sooner or later the carbon is burnt up in the body, and breathed out into the atmosphere, ready to renew the same series of changes. In this lecture I wish to ask your attention to the evidences of design which may be discovered by studying this wonderful circulation of carbon; and we shall find that the properties both of carbon and carbonic dioxide have been most carefully adjusted to the part which they play in the great scheme of nature. We might begin our study at any link of this endless chain of phenomena; but to bring the subject into orderly connection with our previous trains of thought, let us return to the phenomena of combustion, which we considered in the third chapter, and study the details of this familiar process a little more closely.

All fuel, without exception, contains charcoal, or, as the chemists call it, carbon. Wood, soft coal, oil, wax, similar combustibles, which burn with flame, contain, besides carbon, a variable quantity of hydrogen and oxygen; but hard coal, coke, and common wood charcoal are almost pure carbon. The adaptations of each of these classes of combustibles demand special notice, and let us begin by studying the evidences of design which are to be found in an ordinary hard-coal fire; and while, in imagination, we are preparing the fire to be lighted on the grate, we may study with profit some of the external properties of the coal, for even they betray the master-hand of the Great Architect.

Examining closely this lump of charcoal, you will notice that it retains all the delicate structure of the wood from which it was prepared. Here is the fibrous bark next the sap-wood, and then the annual rings, all as on a stick of beech; and if you will take the pains to make a thin section of the charcoal, you will find, on examining it with a microscope, that the minutest cells have been preserved. You know how charcoal is made. The wood is exposed to a high temperature in the charcoal mounds or furnaces, by which the gases which it contains are driven off, while the charcoal, not being volatile, remains behind. Here, then, is a remarkable fact,—that, although the wood has been exposed to a red heat in the process of carbonization, yet the minutest cells have not been destroyed; and it illustrates an equally remarkable quality of charcoal, on which, as we shall see, its usefulness as fuel

very greatly depends. Carbon, in all its forms, is absolutely infusible. It does not even soften at the highest temperatures which can be attained by art, and it is for this reason that the charcoal retains so perfectly the structure of the wood. Were carbon fusible at a red heat, the charcoal would run together into a shapeless mass in the mounds or furnaces in which it is prepared, and did it even soften at this temperature, the forms of these delicate cells could never have been preserved. Viewed in connection with the volatile qualities of the other elements of organized beings, the extreme fixity of carbon in its uncombined condition is worthy of your special attention. The only other essential elements of organic matter are oxygen, hydrogen, and nitrogen; and these three substances are not only gases, but gases which, even at the lowest natural temperature cannot be condensed to the liquid condition by pressure alone; yet so strong is the tendency of carbon to remain solid, that it condenses these gases around itself in every organized substance that exists. Carbon is thus the solid substratum of organized matter, the skeleton, as it were, of every organic form. How evidently, then, has the attribute of infusibility been adapted to this important function which carbon has been appointed to subserve!

Examining again this lump of charcoal which we are using to kindle the fire, mark that it has a black color and is perfectly opaque. These qualities are so evident to the most superficial observation that they are apt to pass unnoticed, and yet it is these qualities of charcoal which make it so valuable

able as a basis of printing ink. All may not know that printing ink is a mixture of lamp-black and oil, and that the letters on a printed page are formed by thin layers of black charcoal spread over the white paper; and charcoal is peculiarly well adapted for this use, since, however finely sub-divided, it never loses its dead black color and perfect opacity. But this property of charcoal would be useless to the scholar for diffusing knowledge, were it not combined with qualities still more remarkable, and almost unique. Carbon is not acted upon by atmospheric agents, and, moreover, is absolutely insoluble in any liquid, with the exception of melted iron. The letters of the first Bible ever printed are as black as they were the day they left the types. They have been exposed to the action of atmospheric air for four hundred years, and, were carbon in the slightest degree acted upon by the atmosphere, they would long since have disappeared. As it is, they will endure as long as the paper on which they are printed lasts.

The almost unparalleled insolubility of charcoal is a quality equally important in this relation, for, were charcoal, even to a slight extent, soluble in water, the books of our fathers would have been rendered illegible by the dampness to which all libraries are more or less exposed; and were carbon soluble even in such liquids as alcohol, ether, or the volatile oils, the printed page would not have been, as now, safe from alteration, and all kinds of fraud would have been easy. We justly honor the names of Gutenberg and Faust, whose art has done so much to enlighten

and civilize the globe, and we bestow due admiration on those improvements in the art of printing, nowhere more developed than in our own land, which have made the press the great agent of power, and elevated the moral and intellectual above the physical man; but while we pay just tribute to the genius of these benefactors of the race, let us not forget that greater Benefactor, who was before them all; for in the most familiar qualities of this piece of charcoal, on which the art of printing so greatly depends, there has been displayed, since the foundation of the globe, an evidence of wisdom and skill before which all human ingenuity sinks into insignificance.

But the most remarkable attribute of carbon does not appear in this piece of charcoal; for of all the chemical elements carbon is by far the most Protean in its aspects, and charcoal is but one of its many manifestations. In the first place, there are the numerous varieties of coal, including charcoal, coke, lamp-black, and bone-black, all having the same general properties, and most of them partaking more or less of the structure of the organic tissues from which they were made. But, besides these varieties, which, although differing so much in their outward aspect, have all essentially the same properties, there are two entirely different states of carbon, differing as much from each other and from common charcoal as any two solids possibly could.

Are you aware that the brilliant gem you prize so highly is the same chemical element as these black coals? The diamond is simply crystallized carbon,

and although we do not know certainly how diamonds are made in the great laboratory of nature, yet there is no fact of chemistry better established than this. [*] To those who are not familiar with the results of modern chemistry, it seems almost incredible, and even the chemist can hardly believe the truth while he affirms it. It is at utter variance with the former doctrine of his science; it cannot be reconciled with any of his previous conceptions, and constantly reminds him of the limitations of his knowledge and the uncertainty of his philosophy. And, turning to the more familiar aspects of the subject, how singular the fact, and how typical of the universality of Christian brotherhood, that He, who "hath made of one blood all nations of men,'' should have also made of the same material the priceless brilliant which adorns the diadem of the prince, and the soot which begrimes the cabin of the humblest peasant! How different the estimation in which these two forms of carbon are held! and yet, if the marks of Divine wisdom can give nobility to a substance, the one is as excellent as the other.

But carbon exists in still a third modification, differing as much from the diamond as that differs from charcoal. Every one who has used a common lead pencil is familiar with graphite, and it is a fact as remarkable as the one just noticed, that the same carbon which forms the letters of a printed page fills

also the lines of the pencil sketch. Graphite is simply a modification of charcoal, and if this fact is not so well known as the humble relationship of the diamond, it probably arises from the circumstance that graphite has been generally called plumbago, or black-lead, a misnomer which has given a very erroneous conception of its nature. Graphite is frequently mixed with impurities, but it never contains lead, and in its finest condition it is nearly pure carbon. Compare now graphite with the diamond. Could there be two substances more unlike? the one the softest of minerals, the other the very hardest; the one dull and opaque, the other brilliant and transparent. But besides these external differences they have also a different crystalline form, a different specific gravity, a different capacity for heat, and, in fine, excepting that they are both infusible and combustible, there is not one point of resemblance between them. How then, you will ask, do we know that they are both the same elementary substance? Simply because, when combined with oxygen, they both yield the self-same compound.

All three of the modifications of carbon are combustible, although they take fire at very different temperatures. Charcoal will burn at a red heat, the diamond at a white heat, while graphite requires the highest temperature which can be attained by art. But however different may be the temperatures required, the process is the same in all cases, and the result is the same. The burning is simply combination with the oxygen of the air, and the result of that combination is carbonic dioxide gas. Moreover,

it has been proved by the most careful experiments that a given weight of either substance yields precisely the same weight of carbonic dioxide. Chemically considered, then, the diamond, graphite, and charcoal are the same substance, although, physically regarded, no substances could be more unlike. Chemical identity, therefore, does not consist in identity of properties, and we must admit that the same chemical element may manifest itself under utterly different physical aspects.

This remarkable phenomenon, which has been fully recognized only of late years, has been called by chemists allotropism, [*] and the diamond, plumbago, and charcoal are different allotropic modifications of the element carbon. Such differences of manifestation, moreover, are not confined to carbon, nor are they exceptional occurrences among the elements. We have already seen that oxygen may exist in an active and in a passive modification, which stand in as striking antithesis to each other as the diamond and charcoal, and the same is true of the different conditions of sulphur, phosphorus, and silicon. Again, these phenomena are not limited to the elementary substances, for they have been observed in many compounds as well, and every year enriches our knowledge with fresh examples. In what, then, are such developments to end? If substances so utterly unlike as the diamond, graphite, and charcoal are merely modifications of the same element, why may not all substances

be merely different allotropic states of a few universal principles, or possibly of only one single essence? Such, and many similar questions, arise in the mind of the chemist while contemplating these obscure phenomena. They cannot be satisfactorily answered in the present state of chemistry, [*] and they throw a degree of uncertainty and doubt on its whole philosophy. I shall have occasion to dwell upon this subject more at length in another lecture, and have adduced the facts at this time chiefly as further illustrations of that fertility of resources which so strikingly marks all the results of creative skill. To me this characteristic of the works of nature is one of the most convincing evidences of divinity. While studying the simple adaptation of means to ends which we find everywhere around us, we recognize in the plan something analogous to the creations of human skill, and we almost feel a conscious relation ship with its Author. But when we consider this incomprehensible power, by which the same element has been endowed with entirely different and incompatible properties, and not only this, but has been adapted in each condition with equal skill to produce the most opposite and seemingly irreconcilable results, we are also made to feel most keenly that, although man was created in the image of his Maker, he resembles the Divine Original only as the finite can resemble the Infinite. "For my thoughts are not your thoughts, neither are your ways my ways, saith the Lord. For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts.''

Of all the properties of coal, the one with which we are most familiar is its combustibility; and while we have been discussing its external properties, the hard-coal fire has been built in the grate, and it is ready to be lighted. The combustion of coal in one or the other of its varieties is the great source of all the artificial heat used by man. Although so entirely passive towards atmospheric agents at the ordinary temperature, yet when heated to a red heat it takes fire and combines with the oxygen of the air with great rapidity. The burning of coal is so familiar to every one that it would seem hardly necessary to dwell upon the subject here. But although the experiment is repeated every day in every grate of the city, and although it has been familiar to you all from infancy, there are, nevertheless, phenomena connected with it which few have observed and still fewer fully appreciated. It is a great mistake, but a mistake too frequently made even by scientific men, to suppose that new knowledge can be gathered only from the unexplored fields of science, when by the most familiar walks of life there are countless riches of truth which the reapers in the hurry of the harvest have passed unnoticed, and which will abundantly reward the careful gleaners. In the coal fire on which you daily gaze, there is enough to be discovered to engross the attention of the most diligent student of nature. Let us see, therefore, if we, too, cannot learn something new, at least to us, from the burning coals.

The first fact to which I would call your attention is the difficulty experienced in lighting coal. In order to kindle the fire we have placed on the bottom of the grate, first, some shavings, then some charcoal, and, last of all, the hard anthracite coal. We can readily set fire to the shavings with a match, and they in their turn will ignite the charcoal; but it requires the intense heat of the burning charcoal to ignite the anthracite. Charcoal will not burn unless at a full red-heat, and hard coal requires a still higher temperature. But notice now another fact: when once inflamed, the heat evolved by the combination of the carbon with oxygen is sufficient to sustain the temperature at the point of ignition. Here, again, we see most admirably illustrated the adaptation of the properties of the chemical elements to entirely different ends. In order that carbon might serve as the solid substratum of all organized beings, it was necessary that it should be made unalterable by the air within the limits of terrestialtemperature, but at the same time the economy of nature required that it should be made combustible, that is, endowed with strong affinities for oxygen; yet these affinities have been so carefully regulated, that they are called into play only at a high temperature, and are thus placed entirely under the control of man.

Now that the coal is in violent combustion, combining rapidly with oxygen, notice that it burns entirely without flame. We have here rapid chemical combination, with all the phenomena of active burning, and yet no flame, simply because flame is always

burning gas, and in a hard-coal fire it is not gas, but a highly fixed solid, that is burning. Charcoal and anthracite are almost the only combustibles which burn in this way. Most others, even when naturally solids, are converted into gases at a high temperature, and therefore burn with flame; but carbon in all its forms, when uncombined, persistently retains its solid condition, even in the hottest fire.

Remark, also, that this combustion is attended with a very bright white light, and compare it with the more violent combustion of hydrogen, with which most of the audience must be familiar. Hydrogen burns with a flame because it is a gas; but this flame is almost invisible because gases, however intensely heated, do not emit a bright light. The charcoal burns without flame because it is a permanent solid; but for this very reason it emits a great amount of pure white light. So far, at least, as ordinary experience extends, white light is emitted only from ignited solid matter. [*] Therefore neither white light nor flame is a necessary concomitant even of the most rapid combustion, the first depending solely on the solid, and the last on the

aeriform, condition of the burning substance. If, as in the burning of a candle, both flame and white light attend the process, it is because both solid and aeriform matter are there burning; and when we come to examine this phenomenon more closely, we shall find that the result is produced by a most delicate adaptation of properties.

Let me next call your attention to the importance of the infusibility of charcoal in connection with its use as fuel. However high the temperature at which it burns, however intense the furnace heat, charcoal never loses its solid condition, and on this wholly depends its application for generating heat. Were coal fusible, even at a very high temperature, it would melt and run out from our grates and furnaces, and the genial fire could not, as now, have been localized on the hearth. The enjoyment of the social fireside is thus closely connected with a familiar property of this wonderful element.

But our fire is slowly burning away, and already more than one-half of the coal has been consumed. What has become of it? Do you point to the ashes? These are only the earthy impurities, which are more or less mixed with the pure carbon, and constitute but a small fraction of the whole mass of the coal. The carbon itself has combined with the oxygen of the air and formed a colorless and invisible gas, which has escaped by the chimney, which, as I have already stated in the lecture on Oxygen, is called carbonic dioxide. Reflect now on the importance of the circumstance, that this compound of oxygen and carbon is aeriform, and consider what

a marked evidence of design and adaptation is to be found in the very fact that the products of ordinary combustion are invisible gases, which ascend our chimneys and are wafted away by the currents of the atmosphere. As common experience is confined to the burning of coal, wood, oil, and similar combustibles, consisting mainly of carbon and hydrogen, men naturally associate with smoke the idea of a gas, and are apt to think that the aeriform condition is a necessary result of the nature of things. But it is not so. This peculiar provision in the case of carbon and hydrogen is an exception to the general rule. The two combustible elements which are most closely allied to carbon in all their properties —boron and silicon—not only form solids by burning, but two of the most fixed solids known in nature, one of which—silica—constitutes, as we have seen, at least one-half of the rocky crust of our globe; and the same is true of almost all the other combustible elements. A very interesting experiment in illustration of this fact may be made by burning a piece of phosphorus under a dry glass receiver. The smoke of phosphorus is solid, and it will fall in thick white flakes, producing within the glass the appearance of a miniature snow-storm. Picture to yourself the desolation which would be produced were the order of nature so far changed as to make the products of burning coal like those of burning phosphorus. Every furnace would become a volcano, and we should soon be buried beneath the smoke of our own fires. When, now, we consider that a special provision has been made in the case of that substance whose combustion administers to our wants by evolving light and heat, what evidence does it open to us of the all-wise forethought of the Great Original!

But this is not all. Let me now call your attention to an additional fact in regard to the carbonic dioxide which is escaping from our coal fire. The gas is entirely devoid both of odor and of taste, and, moreover, when in a sufficiently diluted condition it can be breathed with impunity. Consider what an amount of this product is daily formed, and you will then be able to appreciate the importance of this circumstance. The amount of carbonic dioxide which escapes from an average-sized iron blast-furnace in the course of a single hour is equal to at least two tons, and the amount which is generated even by our coal fire is surprisingly large. Moreover, no less than two hundred tons [*] of this gas are breathed into the air by the population of this city in a single day. If carbonic dioxide had been a pungent or corrosive gas, coal could not have been used as fuel; for its combustion, like that of sulphur, would soon have rendered the air irrespirable. But so entirely destitute is it of any perceptible odor or taste, that, although it has been evolved in these immense quantities from every fire lighted by man since he appeared on the globe, it so entirely escaped notice that its existence was not even suspected until it was discovered by Dr. Black about a century ago.

There is still another remarkable phenomenon attending a coal fire, which, although it cannot be made evident to the senses, has been substantiated again and again by the most accurate experiments. The volume of carbonic dioxide gas formed by the combustion is exactly equal to the volume of oxygen consumed. It is a consequence of this fact, that the volume of the air is not in the slightest degree increased by the vast quantity of carbonic dioxide gas which is daily poured into it. The gas occupies precisely the same space as the oxygen removed during the combustion, and thus the equilibrium of the atmosphere is not disturbed. It is true that we probably cannot see all the bearings of this simple provision; but we know enough to recognize in it a most marked evidence of design.

The last fact in connection with the coal fire to which I would direct your attention is the large amount of heat which the combustion of coal liberates, and on which its use as fuel very largely depends. One pound of charcoal, in burning completely, generates sufficient heat to raise the temperature of 80.8 pounds of water from the freezing to the boiling point. Every pound of charcoal may, therefore, be regarded as containing sufficient heat to boil eighty pounds of ice-cold water. What a vast amount of heat then lies buried in those inexhaustible beds of mineral coal, in which our country is so rich! And have we not another remarkable evidence of Divine wisdom in the fact that carbon, a substance which, on account of its infusibility and other qualities, is so well adapted for fuel, has been

made a great reservoir of heat, from which man can draw an unlimited supply? When we remember that this heat, through the expansion of steam, may be converted into mechanical force, and that hence these beds of coal are not only magazines of heat, but stores of force, which have been accumulating from the foundation of the globe for the use of civilized man, and when we reflect that it is this force which is animating our commerce, weaving our cloth, forging our iron, and impelling the printing-press, how can we express our praise at the foresight of that Providence which endowed coal with such wonderful qualities, made it a vast repository of heat and of force, and then spread it bountifully over the globe?

We have discovered all these wonderful indications of design and adaptation in this simple experiment, so familiar as to be almost trite, so frequently repeated as to pass unnoticed, and they are constantly speaking to us of the great Author of nature from the fireside of every home, and from the furnace of every workshop in the land. The followers of Zoroaster still worship, in India, fire as divinity, and regard these burning coals as sacred. Behind this superstition and idolatry there is concealed true wisdom, by which we may well profit. Fire is neither divinity, nor yet its emblem. It has no other reality than as a phenomenon attending a chemical change; but in the qualities with which carbon has been endowed in order to produce this phenomenon, in the delicate adjustment of forces by which the destructive change

is confined within due limits, there are indications of divinity which may well make us thoughtful, and consecrate with additional sanctity the family hearth; and if I have succeeded, however imperfectly, in making audible to your intellectual ear this mute eloquence of burning coal, our time has not been spent in vain.

I have thus far drawn all my illustrations from the burning of charcoal and hard coal, simply because these familiar forms of fuel are nearly pure carbon, and the phenomena attending their combustion are comparatively simple. They burn, as we have seen, without flame, for the reason that carbon does not volatilize, even at the highest temperatures. It is different, however, with soft coal, wood, oil, wax, and all other combustible materials which are used for generating light. These do not consist wholly of carbon; but this latter element is always combined with hydrogen, and most of the combustibles named contain also, in addition, a limited amount of oxygen. When heated, they all evolve common illuminating-gas, and for this reason burn with a flame. In fact, the gas we are burning here to-night was made from just such materials. If you visit the gas-works of this city, you will see long rows of iron retorts, firmly built into large brick furnaces. In these retorts the gas is made, and they are connected by means of a complicated system of tubes with all the numberless gas-burners of this large city. Every few hours the retorts are charged with soft coal, which soon becomes heated to a low red heat. At this temperature it slowly gives off

gas, and it is the gas thus formed which is now illuminating this hall. After three or four hours, the gas has been all driven off, but there is still left in the retorts the greater part of the carbon of the coal, in a condition which is called coke. This is then removed and used for feeding the furnaces, and a new charge of soft coal is introduced in its place. Coke is an excellent fuel, but, like charcoal, it burns without flame.

The processes which, in the manufacture and use of illuminating-gas, are spread over a whole city, are united in every soft-coal fire. The gas which is burning at this jet was generated in the retorts of the gas-works, and brought here in iron tubes to be burnt. In the grate the gas is made and burnt in successive moments, but the process is identical in both cases. When you throw a fresh supply of soft coal on the grate, the first effect of the heat is to generate illuminating-gas, which at once takes fire and burns with a brilliant blaze. But after some time the flame ceases, because all the volatile elements of the coal have been expelled, and the coke which is left merely smoulders, like charcoal or anthracite. What is true of soft coal is also true of wood and of all this class of combustibles.

Flame, as I have before stated, is in all cases burning gas. As we are generally familiar with it, flame is a cloud of illuminating-gas combining on its exterior surface with the oxygen of the air. In a gas lamp the gas is supplied ready made at the jet. In an oil lamp or a candle, the gas is manufactured as fast as it burns. The use which we make of the flame, in

all these cases, is to generate light, and the qualities of carbon have been most admirably adjusted to produce that result. This is the point which I wish next to illustrate, and we shall understand this beautiful example of adaptation more readily by analyzing the burning of some one of the light-generating materials. I will, therefore, select a common wax candle as my example, because it is familiar to every one, and illustrates all the points I have in view.

Nothing could be simpler than the candle itself. It is a long cylinder of wax formed around a string made of loose cotton threads, which we call the wick. The wax, that familiar secretion of the honeybee, is composed, chemically, of carbon, hydrogen, and a little oxygen; the wick, as the microscope would show us, is merely a collection of fine vegetable tubes. Let us now light the candle. For that purpose we apply the flame of a friction match to the end of the wick, and mark the result. The heat of the match melts the wax around the base of the wick, and now the peculiar virtue of these vegetable tubes come into play. All fine tubes have the power of sucking up liquid, and the finer the tube, the greater the height to which the liquid is thus elevated. The tubes of the wick act in this way, and the melted wax is at once drawn up to the flame of the match. There it is volatilized by the high temperature, and a cloud of red-hot combustible gas forms around the summit of the wick. Like the rain-drop, or any other fluid body in a free state, it assumes a spherical form, but being much lighter

than the air, this sphere of gas no sooner forms than it begins to ascend, and, being very combustible, is burnt up by the oxygen of the air with great rapidity, so that before it has risen an inch from the wick it is reduced to a point. Meanwhile, however, the first sphere is followed by others, which in rapid succession meet with the same fate, and at any moment we have a large number of these little spheres, one above the other, rapidly diminishing in size from the lowest to the highest, which has then become a mere point. Hence results the familiar conical form of the flame. But our match is long since burnt out, and what, you will ask, now volatilizes the wax? Solely the heat evolved by the burning gas. This heat converts the wax into vapor as fast as it creeps up the wick, and thus the flame being constantly supplied with combustible gas, the candle continues to burn until it is all consumed. The candle-flame is, then, merely a cone of volatilized wax, rapidly combining on its exterior surface with the oxygen of the air, and as rapidly replenished from below by the constant conversion of fresh wax into vapor. In this process light and heat are evolved; but these are generated solely on the exterior surface of the flame, where the burning takes place. Within it is perfectly dark, as can be easily shown by pressing down upon it a piece of window glass, through which the interior may be seen. Let us now study this chemical process more carefully, as the whole illuminating power of the flame depends on a very delicate play of affinities.

The combustible gas formed from wax is composed

essentially of charcoal and hydrogen. The light and combustible hydrogen has so great a tendency to retain its aeriform condition, that, when combined with carbon, it renders even this, the most fixed of all the elements, aeriform; but the moment the bonds of chemical affinity are loosened, the carbon resumes its solid condition. Such a change takes place in the flame, and it is the particles of solid charcoal thus liberated that render it luminous. Of the two elements of the gas, hydrogen has the greatest affinity for oxygen, and therefore burns first, momentarily setting free the carbon, which is sprinkled in a fine powder through the burning gas. This is at once intensely heated, and each glowing particle becomes a centre of radiation, throwing out its luminous pulsations in every direction. The sparks last, however, but an instant; for the next moment the charcoal is itself consumed by the fierce oxygen, now aroused to full activity, and nothing but a transparent gas rises from the flame. But the same process continues; other particles succeed, which become ignited in their turn, and hence, although the sparks are evanescent, the light is continuous.

Thus it appears that all our artificial light, the light which we are enjoying this evening, depends upon this provision, by which the particles of charcoal linger for a moment in the flame before they are burnt. Let me again repeat, white light is emitted by ignited solid matter. The flame of pure hydrogen gives very little light, because there are in it no solid particles, and were the affinity of oxygen

for carbon slightly greater than at present, the flame of the candle would be as little luminous: then the carbon would burn simultaneously with the hydrogen, and there would be no pulverized charcoal in the flame to radiate light. On the other hand, were the affinity of oxygen for carbon a little less than at present, the carbon particles would not burn in the flame, but would escape from it in clouds of dense soot. Our Heavenly Father has so carefully adjusted the relative affinity of oxygen for the two elements of these light-giving gases, that the hydrogen should burn a small fractional part of a second before the carbon. During this brief interval of time, imperceptible to our unaided senses, the solid particles of charcoal are set free, become ignited, and give motion, perhaps, to a single wave of light; but the instant after, they too rush into combination with the great fire-element, and not a particle is left to dim the transparency of the air. The smallest variation in either force would destroy the adjustment by which this result is produced, and our lamps and candles would cease to give their light. How delicate the adjustment! How beneficent the result! How evident the design!

To me the marks of God's designing hand are more conspicuous in that familiar candle-flame than in the grand cycles of astronomy, or in the wonderful mechanism of the human body. I return to it again and again with renewed confidence, and always find fresh satisfaction and increasing faith. There are many who believe, with Laplace, that this glorious system of suns and planets, with all its

complex movements and adjustments, might be evolved out of a nebulous chaos by the sole action of the primary laws of motion; and now, after the great French mathematician has furnished a world to begin with, a modern naturalist asks us to believe that this hand of mine, with all its wonderful combination of nerves, bones, and muscles, was developed out of the claw of an animalcule, or some such thing, by what he calls "the law of natural selection;'' and although these and similar theories may be held consistently with a belief in a Divine Disposer, yet it is too true that to many of their advocates the order of nature signifies nothing higher than self-existing matter, directed by inexorable necessity. But no cosmogonist has been able to go behind the chemical elements, and until human philosophy can show how these forms of matter, with all the marvellous adjustments among their properties, have been evolved out of the "star dust'' of the original chaos, or out of nothing, and can adjust by natural causes the delicate play of forces in that most familiar of all phenomena, a candle-flame, it will not be able to overthrow the evidence of design afforded by this genial winter-evening light. The fact that these would-be world-makers explain most satisfactorily what men know least about, is, it must be confessed, not in favor of their theories. Yes, my friends, it is these most familiar evidences of design which are the most impregnable against the attacks of materialism. It is these household altars that we find always burning to enlighten our dull understanding, to disperse our gloomy doubts, and to reveal to us the presence of our God.

The delicacy with which the affinity of oxygen for carbon has been adjusted appears still more wonderful when we consider another of the uses of this force in nature. The useful metals, which may be said to be the tools of civilized life, are seldom found in nature in a pure state. They generally occur combined with oxygen, and this compound, which is called the ore of the metal, is found in beds or veins of the rocks, where it has been deposited through the agency of water. After the miner has dug out the ore from the earth, and washed it free from impurities, it is the business of the smelter to melt out the pure metal. Now in this ore the metal is combined with oxygen, and unless the smelter could break this bond, the highest temperature of his furnace would be unavailing. But the merciful Parent of mankind, when he thus locked up these his choicest gifts, gave to man a key which would unlock the treasure-house, but left him to find out its use; and as in the progress of humanity the metals were required to advance civilization and multiply the comforts of life, the secret was discovered, and the treasures one by one were brought to light. This needed key was charcoal. The Creator has endowed carbon with a power so strong, that it readily overcomes the force by which the metals are united to oxygen, and by simply heating the ore with charcoal the metal is set free. Would that I could give you an idea of the strength of the force which is required to produce

this result. The affinity of carbon for oxygen is one of the most powerful forces known in nature, so great as to be immeasurable by our ordinary human standards, and yet it is this same force which produces that delicate result, the light of a candle-flame. With such wonderful skill does God wield these mighty agents of his power.

Consider, finally, how this power of reducing the metallic ores has been united in charcoal to those other qualities which render it so valuable as fuel. The smelter heats his furnace with the self-same coals which reduce the ore. These coals remain unchanged in contact with the ore until they have done their work, and then are converted into a colorless and harmless gas, which escapes by the chimney and is wafted away by the air; while, on the other hand, the melted metal, freed from its long imprisonment, flows out below in glowing streams, ready to be cast into thousands of useful forms.

Review now, for a moment, the qualities of carbon, and notice how manifold and important are the functions which this element has been appointed to subserve. It has been made hard and brilliant, for the glazier's diamond and the monarch's crown. It has also been made soft and black, for the artist's pencil and the printer's ink. It has been made indestructible by atmospheric agents, and thus has preserved for us the wisdom of past ages, and will transmit our bequests of knowledge to those that are to come. It has been made combustible, and at the same time infusible, in order to localize our fires and

confine them within their appointed bounds. It has been made a great reservoir of heat, in order that it might protect us from the winter's cold, and shed its enlivening warmth around the family hearth. It has been endowed with a strong affinity for oxygen, in order that it might reduce the metallic ores; but at the same time this affinity has been so carefully adjusted that the carbon particles linger in the flame for a moment before passing into invisible gas, and thus become a source of light as well as of heat. Lastly, the product of its combustion is a gas so transparent that it does not even cloud the atmosphere, and so bland that it bathes the most delicate organisms without harm. What an array of evidence have we here! But this, my friends, is only the first stage of that grand circulation of carbon in nature, which we proposed to ourselves as our subject this evening. The product of all these various processes of combustion is carbonic dioxide, and let us now follow this gas into the atmosphere, and examine some of its more familiar qualities.

Carbonic dioxide is so perfectly transparent and so devoid of every active quality that its presence cannot be recognized by any of our senses, and we must therefore call in the aid of experiment to make evident its existence. This is the reason why it remained so long unknown, the method we now use for detecting its presence having been first discovered by Dr. Black only a little more than a century ago. The method is very simple. Carbonic dioxide has a great tendency to combine with lime, and the

result of this combination is the familiar white solid called chalk. Now lime is, to a certain extent, soluble in water, while chalk is insoluble; and hence, if lime-water is exposed to an atmosphere containing carbonic dioxide, the formation of particles of chalk, rendering the transparent solution turbid, will indicate the presence of the gas. Such a result is actually obtained by exposing lime-water in a saucer for a few days to the atmosphere, and any one can convince himself by this simple experiment of the existence of carbonic dioxide in the medium around us, as well as in the air which is exhaled from the lungs. Indeed, the breath is so loaded with this product of combustion that lime-water is rendered milky by blowing into it for only a few minutes. The quantity of carbonic dioxide in the atmosphere, however, is relatively very small, not amounting to more than a few ten-thousandths of its whole weight. It enters to a far greater extent into the composition of many rocks. All limestones have the same composition as chalk, and contain nearly one-half of their weight of carbonic dioxide, rendered solid by the force of chemical affinity. These rocks, indeed, are the great reservoirs of this aeriform compound, and when you consider how widely the limestones are distributed, underlying whole districts of country, reaching down to unknown depths, and piled up into vast mountain chains, you can form some appreciation of the extent to which carbonic dioxide gas was used in laying the foundations of the globe.

When pure, carbonic dioxide gas will instantly extinguish flame, and is perfectly irrespirable, causing

the epiglottis to close spasmodically and producing immediate death by asphyxia. When so far diluted as to admit of being received into the lungs, it acts like a narcotic poison, causing drowsiness and insensibility, and this even when a candle will burn in the gas. Carbonic dioxide is not, however, poisonous in the strict sense of that term. On the contrary, it is always present in the blood in large quantities, and with it bathes all the tissues of the body. The carbonic dioxide results, as we have seen, from that slow combustion constantly going on in the blood, by which the animal heat is maintained, and it is an essential condition of life that this product should be secreted from the body as fast as it is formed. If the atmosphere contains more than a small percentage of the gas, the process of secretion is arrested, and fatal results necessarily ensue.

The density of carbonic dioxide is much greater than that of either of the other constituents of the atmosphere, the same volume weighing one-half as much again as common air. Indeed, it is so heavy that it can be poured from one vessel to another like water, and the immense volumes of carbonic dioxide which are constantly flowing from our lungs and furnaces would cover the whole surface of the earth with their deadly vapor, were it not that the Creator has provided, by those simple laws of diffusion, which we studied in a former chapter, that this noxious gas should be dispersed as fast as generated, and so mixed with the great mass of the atmosphere as to be rendered harmless by extreme dilution. The unfortunate accidents which sometimes occur

to persons who descend incautiously into cellars or wells, where the carbonic dioxide is generated more rapidly than it can be dissipated, constantly remind us that the existence of animal life on the globe depends upon this beneficent provision. The large kilns in which lime is burnt into quicklime are constantly pouring out streams of carbonic dioxide gas, and more than one poor, houseless wanderer, attracted by the heat of the kiln, has laid down to rest in the stream, and slept to wake no more. Were the force of diffusion much less than it is, we should all be constantly exposed to a similar fate; and when we lie down at night, it is only this guardian angel which prevents the deadly fumes of our own fires from descending on our beds.

Carbonic dioxide is soluble in water, a given volume of this liquid being capable of absorbing its own volume of the gas, irrespective of the temperature or pressure. We should therefore expect to find carbonic dioxide in solution in all water exposed to the air, and in fact a cubic foot of river, lake, or ocean water generally contains a very much greater amount of this gas than an equal volume of the atmosphere. Water, when holding carbonic dioxide in solution, has its solvent power very greatly increased. It then dissolves, in large quantities, all the varieties of limestone, and even granite rocks cannot wholly resist its action; but these solutions, when exposed to the air, gradually lose the carbonic dioxide, and with it their solvent power, incrusting with calcareous matter the moss, the twigs, or the walls of caverns on which the liquid may chance to

rest. It is the solvent power of such water, acting slowly through ages of time, that has hollowed out that immense cavern in the limestone strata of Kentucky, and it is from the solution thus made that those stalactitic ornaments have been formed which add so much to its beauty and interest. It is also this same agency which in other places has deposited beds of calcareous tufa over great areas, and cemented together loose sands into firm rocks; and, finally, it is from the lime dissolved in the water of the ocean, that the crustacea form their shells and the coral polyps build their reefs. [*]

The origin of carbonic dioxide is the same in water as in air. In the water we have not, of course, active combustion; but this, as has been shown, is an insignificant source of carbonic dioxide when compared with the never-ceasing functions of respiration and decay, and these are as active in the rivers, the lakes, and the oceans as in the atmosphere. Moreover, the purpose which the carbonic dioxide subserves is the same in both cases, and this demands our attentive study.

I have already intimated that carbonic dioxide is one of the few articles of which the food of plants consists. Let us trace, for a moment, the history of the plant. The seed containing the germ is placed in the soil. The genial warmth of the sun calls it into activity, and it shoots forth its small leaflets

into the air. For a short time the small stock of starch and similar nourishment stored in the seed by a wise Providence serves for its support; but this is soon exhausted, and for the future the plant must depend for its food upon the soil and upon the air. The articles which compose its diet are exceedingly simple. They are water, carbonic dioxide, and ammonia, substances always present in the atmosphere and in every fertile soil. As soon as the young plant has expanded its green leaves it absorbs these substances, partly through its rootlets from the soil, and partly through its leaves from the air. The leaf, a tissue of minute organic cells, is the laboratory in which, from these few compounds, are elaborated the different organs of the plant. The sun's rays, acting upon the green parts of the leaf, give them the power of absorbing water, carbonic dioxide, and ammonia, and of constructing from the materials thus obtained the woody fibre, starch, sugar, and other compounds of which the plant consists. We have analyzed the woody fibre, and we know that it is composed of carbon and water. Twenty-seven ounces of wood contain twelve ounces of carbon and fifteen ounces of water. Moreover, the amount of carbon required to make twenty-seven ounces of wood is contained in forty-four ounces of carbonic dioxide. If, then, we add together forty-four ounces of carbonic dioxide and fifteen ounces of water, and subtract from this sum thirty-two ounces of oxygen, we shall have just the composition of wood. This is what the sun's light accomplishes in the leaves of the plant. It decomposes the carbonic dioxide, and unites its carbon to the elements of water to form the wood.

What I have stated to be true of wood is equally true of starch, gum, sugar, and most of the products of vegetable life. All these, with a few exceptions, which I shall notice in the next lecture, are prepared by the plant from carbonic dioxide and water, under the influence of the sun's light. Why it is that starch is deposited in the cells of the potato, sugar in those of the sugar-cane, and gum and woody fibre, more or less, in all plants, we do not know. These are the mysteries of organic life which no science has been able to solve. This much, however, is certain. The acorn, buried in the ground, grows into the noble oak. Of that wide-spreading tree, at least nine-tenths consist of carbon and water. The water is absorbed, as such, directly from the atmosphere; the carbon was recovered from the carbonic dioxide decomposed by the sun's rays. Here is the wonderful fact. The gentle influences of the sunbeam have the power of reversing the process of combustion, of overcoming the intense affinity of the fire-element, tearing it apart from the carbon, and restoring it to the air. How great this power is, I have already endeavored to illustrate. I have stated that the affinity of oxygen for carbon is one of the strongest affinities known to nature, immeasurable by any human standard. In order to decompose carbonic dioxide in our laboratories, we are obliged to resort to the most powerful chemical agents, and to conduct the process in vessels composed of the

most resisting materials, under all the violent manifestations of light and heat, and we then succeed in liberating the carbon only by shutting up the oxygen in a still stronger prison; but under the quiet influences of the sunbeam, and in that most delicate of all structures, a vegetable cell, the chains which unite together the two elements fall off, and while the solid carbon is retained to build up the organic structure, the oxygen is allowed to return to its home in the atmosphere. There is not in the whole range of chemistry a process more wonderful than this. We return to it again and again, with ever-increasing wonder and admiration, amazed at the apparent inefficiency of the means, and the stupendous magnitude of the result. When standing before a grand conflagration, witnessing the display of mighty energies there in action, and seeing the elements rushing into combination with a force which no human agency can withstand, does it seem as if any power could undo that work of destruction, and rebuild those beams and rafters which are disappearing in the flames? Yet in a few years they will be rebuilt. This mighty force will be overcome; not, however, as we might expect, amidst the convulsion of nature, or the clash of the elements, but silently, in a delicate leaf waving in the sunshine. And this is not all. Those luminous waves which beat upon the green surface of the leaf are there arrested, and their moving power so completely absorbed, that the reflected rays will not even affect the exquisitely sensitive plate of the photographer. But the power of the light has not been lost, and when the wood is burnt and the carbon converted back into carbonic dioxide, this power reappears undiminished in the heat which radiates from the burning embers. The heat, therefore, which the wood contains, and which it gives forth on burning, comes from the sun. What a beautiful provision of Providence have we here! During the summer, when the sun is warming us with his genial rays, he is also laying up in the growing wood vast stores of heat, with which to warm us at the winter evening fireside, when his rays have been withdrawn.

But you will tell me, it is not wood, it is coal which is burning in the grate, and you will lead me, perhaps, to the mouth of some black coal-pit, and ask if those dismal regions below ever saw the sun. Certainly! and it is one of the most remarkable revelations of modern science, that the stone-like coal was once alive. Coal is the remains of an ancient vegetation, which flourished on the earth ages before man first walked in Eden. The process by which it has been formed and buried in the earth is well known. You can see it now forming in many tropical swamps. There you will find a vast mass of vegetable matter, the result of a rank vegetation, gradually decaying under water. The land is slowly sinking, and as this bed of peat sinks with it, it becomes covered with mud and sand, which numerous streams are constantly washing into the swamp. This goes on year after year, century after century, age after age, until the bed is buried hundreds of feet beneath the surface. In the meantime the vegetable tissues undergo a sort of internal combustion,

similar to that which takes place in a charcoal mound. Wood consists, you will remember, of carbon and the elements of water. The oxygen which it contains reacts on the carbon and hydrogen. Carbonic dioxide and water are formed, which escape, while the rest of the hydrogen and carbon unite together to form the coal. The reaction is a true process of combustion, and the heat thus evolved aids the chemical change, and gives to the coal its baked appearance. This change it requires long ages to complete. Millions and millions of times has the earth repeated its annual revolution around the sun, and the whole external appearance of the globe has changed since those mighty forests grew, which have been petrified in the coal. But though such long intervals have elapsed, their history has not been lost. It has been written on the rocks, the mighty monuments of past ages. The geologists have read it, and we know with as much certainty the form of the leaves and the structure of the stems of those ancient trees, as we do those of the oak or the chestnut. We know, also, that every atom of coal which now lies buried hundreds of feet beneath the surface was once a part of the atmosphere, and that the heat which it evolves by burning was received from the sun, when the carbonic dioxide was decomposed by the light in the leaves of the ancient trees. Consider for a moment of what immense value to man are those beds of coal. Without them modern civilization would have been impossible. Remember that since the dawn of creation the sun has been employed in accumulating these vast stores of force, and thus preparing the globe for civilized man. We may admire the genius of a Papin and a Watt, who have told us how to use this force, and who have thus covered the ocean with steamships and the land with railways; but let us not forget that infinitely greater wisdom which saw the end from the beginning, and before the mountains were brought forth, or ever the continents were formed, laid up the beds of coal in the early strata, and preserved them through the long ages of geological time until the earth had become fitted to be the abode of man.

I have now glanced at some of the distinctive features of the great circulation of carbon in nature, and have endeavored to show that the sun's rays are the prime moving power of the whole. I trust that you have been impressed with the grandeur of its cycles, the delicacy of its adjustments, and the mighty power of that mysterious influence by which it is sustained; but above all, that I have succeeded in making clear to your intellectual vision those marks of wisdom and of power which have been so visibly stamped upon this Divine economy.