University of Virginia Library

7. CHAPTER VII.
TESTIMONY OF NITROGEN.

IN order to complete my very imperfect sketch of the wonderful adaptations which the various qualities and functions of our atmosphere present, I wish in my lecture this evening to examine with you the properties of nitrogen gas. This aeriform substance is the chief constituent of the air, making up no less than four-fifths of its entire mass, and, although so seemingly inert, discharges functions no less important than those of oxygen gas to the well-being of man. Nitrogen is not, however, like oxygen, an element widely distributed in nature, and entering as a chief constituent into the composition of the globe. The atmosphere is the only great reservoir of nitrogen, and to this and to the bodies of organized beings its presence is almost exclusively confined. It seems to be the essential element of all the higher forms of corporeal vitality, and it is frequently called the zoögen, or life-generator. By some mysterious process it is constantly being withdrawn from the atmosphere, and entering into the composition of the numberless living forms which clothe the earth with verdure and crowd it with animal life; but these forms soon pass

away, and by the inevitable process of decay the nitrogen is restored to the great reservoir from which it was originally withdrawn. Science has not, as yet, been able to follow all the steps of this remarkable process; but, nevertheless, enough is known to show that the properties of nitrogen have been most admirably adapted to the numerous important ends which it has been appointed to subserve.

Nitrogen is, then, peculiarly the element of the atmosphere. It not only constitutes the greater part of the aerial ocean, but it exists there in a perfectly free and uncombined condition, and—with the self-limiting exception just noticed—is found nowhere else. We should naturally expect to find in nitrogen gas, occupying so important a place as it does in the scheme of creation, a substance full of the highest interest. Yet nothing could be less inviting than its external properties. A permanent gas, even at the lowest temperatures, without color or odor, it is entirely devoid of every active property. It will extinguish a candle immersed in it, and will not sustain animal life: but these are merely negative qualities; for animals cannot live in an atmosphere of nitrogen, solely because it does not contain oxygen, and it will not support combustion because it is not endowed with active affinities. Moreover, in all other outward aspects nitrogen is equally inert. It exerts no action whatever upon the most delicate chemical compounds, and, with a few unimportant exceptions, will not enter into direct combination with any of the chemical elements. Consider also the nitrogen as it exists in the atmosphere. Although

in immediate contact with the most violent of the elements, and exposed to its action when in its fiercest state, under the varying influences of light, heat, and electricity, yet no combination between the two results, except to a very limited extent, and under peculiarly oblique conditions. Through an ordinary iron blast-furnace there pass, in the course of a single day, many tons of this mixture of nitrogen and oxygen called air. The oxygen, as we know, causes the most violent chemical action; but although the nitrogen is brought into contact with the same intensely heated coal and iron, no combination, at least of any importance, ensues.

Shall we then conclude that nitrogen is entirely unendowed with chemical affections,—that it is capable of forming no compounds, and of producing no powerful effects,—that it is, in fine, a mere dead weight in the atmosphere, placed there, for the want of something better, to fill up the void and to give the required density, as a ship is frequently loaded with ballast when there is a lack of freight? Such is the conclusion to which the appearances would naturally lead, and such is the conclusion at which the chemists arrived in the early stages of their inquiry. Yet no inference could be more at variance with actual facts; for so far is it from true that nitrogen is the uninteresting substance which these negative qualities would seem to indicate, that there are but few elements which form a larger number of compounds, or which are endowed with more varied powers when the necessary conditions of combination are fulfilled. Nitrogen

can be made to unite with the other elements only by indirect and circuitous processes. It is one of its most distinctive qualities to avoid direct combination; but when the necessary conditions are present, it surprises us by the readiness with which it combines, and by the great variety and remarkable character of the resulting compounds. When we should least expect it, we find, not single compounds, but whole classes, springing into existence which, while they often defy our investigations by their Protean and complex character, yet in other cases excite our admiration by the simplicity of their constitution and by the beauty of the plan according to which they have all been fashioned. The points, then, which especially characterize nitrogen, and in which the evidences of design in its constitution are to be traced, are, first, its unexampled inertness when in a free condition; secondly, the variety and remarkable nature of its compounds; thirdly, the peculiarly oblique processes by which all these compounds are formed; and, lastly, their very great instability.

Nitrogen may be very appropriately termed the ballast of the atmosphere, and this is undoubtedly the most obvious of its functions. Air, you will remember, is not, in any proper sense of the term, a distinct substance. It is a mixture of several substances, or rather there coexist around the globe at least three different atmospheres—one of nitrogen, one of oxygen, one of aqueous vapor, and perhaps we should add, as a fourth, one of carbonic dioxide —each with its own peculiar characteristics, and so

entirely distinct that it would retain all its essential properties were the rest removed. Again, when studying in our fifth lecture the general features of the great aqueous circulation on the earth, we discovered that the whole plan turns on the fact that the atmosphere of aqueous vapor is mixed with a large mass of other aeriform matter, which moderates all atmospheric changes and mitigates the violence of their effects. It also appeared in the third lecture that the atmosphere of oxygen had been subjected to a similar restraint, and that the aroused energies of this terrible destroyer had been most carefully tempered by great dilution. As the atmosphere is constituted, the oxygen cannot reach the burning combustible without carrying with it the whole mass of the surrounding air; but if this mass of aeriform matter were not present, the devouring element would rush upon its prey with a fury which nothing could withstand, and iron [*] would burn as readily as straw. Moreover, in several other connections we have shown that it is an essential condition in the scheme of terrestrial nature that the air should have its actual density. See now how beautifully all the conditions are fulfilled in the atmosphere. The proportion of oxygen has been most carefully adjusted to the necessities of animal life, and made so small that the violence of the fire-element may be restrained within due limits. The amounts of aqueous vapor and of carbonic dioxide have in like manner each been accurately adjusted to the purposes which they were appointed to subserve, and then, in order to make up the required density, a large mass of a perfectly inert gas has been added. Thus in the very inertness of nitrogen we find the most obvious evidence of adaptation. Its negative qualities are precisely those required in a substance which is designed to act as so much dead material, adding to the density of the atmosphere without interfering with the functions of its active agents.

Consider, also, how very greatly this evidence of design is enhanced by the fact that nitrogen is found only in the atmosphere and in the bodies of organized beings, into which it has been temporarily withdrawn. It is not, like oxygen, carbonic acid, or water, a main constituent of the globe, and cannot therefore be regarded, as the fatalists would have us believe, as so much material left over after the solid globe had been condensed by the molecular forces from a chaotic nebula. Nitrogen is not only exactly adapted to the functions it subserves in the atmosphere, but, moreover, these are its only uses, and I cannot see how it is possible to resist the conclusion that it was especially designed for the place it fills. That you may appreciate the strength of this evidence, let me illustrate the subject by an example from common life, which will be more to our purpose than a philosophical analysis of the argument itself.

It does not follow that the square granite blocks which form the greater part of the front of yonder magnificent warehouse, however well adjusted they may be, were actually cut with reference to this

building, although the strong presumption is that they were. Nor does it follow that those highly ornamented window-caps and that elaborate cornice were originally designed for this particular edifice, although the presumption that such was the case is still stronger than before. Nay, more, it is not even absolutely certain that those skilfully carved ornaments which adorn the front, and are built into the walls, were originally intended to be placed where they are, although to doubt this conclusion would be the extreme of incredulity. I admit, it is barely possible that they were originally made for another building, rejected, perhaps, for some defect, and afterwards put up here. But I will show you where there is an evidence of design in the building-material of this warehouse which you will be forced to accept. It is not conspicuous, and might be overlooked. Just here at the corner of the building there is a very peculiarly shaped block of stone. You never saw one like it before. This extraordinary shape was required by the peculiar form of the building lot and the position of the walls on the adjoining estate. The sides of the lot are not perpendicular to the front, and the block has been cut to the precise angle of the bevel, and at the same time exactly fits the adjacent walls. The conclusion that this block was designed for that place is irresistible. No sane mind would doubt it for a moment. I do not say there is not one chance in many millions, estimated on the doctrine of probabilities, that a block of this exact size and shape might have been found among the refuse stock of the stone-cutter's yards; but I do say, that, in the absence of absolute proof to the contrary, the certainty that this granite block was wrought with reference to the place it fills, and that the exact correspondence of its dimension and angles was the result of measurement, is as great as it is possible to attain by any process of reasoning short of a mathematical demonstration; moreover, it is as great as can be obtained in physical science, or in any department of human knowledge one step removed from the facts of consciousness or of observation.

The evidence that nitrogen was designed for the place which it fills in the atmosphere is vastly stronger than this. The force of the argument in the illustration just cited evidently increases very rapidly the more singular the shape of the granite block, and the more accurately its form has been adjusted to the place it fills. Now nitrogen is as unique among the chemical elements as water is among the compounds. Its external properties are so entirely different from those even of the class of elements to which it belongs, that chemists can hardly believe that it is a simple substance, and for the last fifty years have been vainly attempting to decompose it; but it has resisted all their efforts, and the more intimately they have become acquainted with its properties, the more singular and exceptional it has appeared. At the same time, while presenting these remarkable anomalies, nitrogen has been fitted to the unique place which it fills in the scheme of creation, with a nicety and precision which it is as much beyond our powers of

thought to conceive, as it is beyond my feeble language to describe. It is not only that one or two of the corners of this block of nature's edifice have been bevelled to an exact angle, but it has been adjusted at every point to the ten thousand conditions of that complex structure I have been imperfectly describing during this course of lectures, with a skill immeasurably beyond all human art, and with an intelligence which "looketh to the ends of the earth and seeth under the whole heaven.'' If this be so,—and you will find that my guarded expressions fall far short of the truth,—why not use in these matters of faith the same common sense which we apply with so much success in common life, and which in our daily intercourse it would be nothing short of madness to disregard? We do not hesitate to trust the skill and honesty of a fellow-man, whom we not only have never seen, but even as to whose character our sole evidence is the most indefinite testimony. Why, then, not accept the precious and comforting truths of religion, and repose equal faith in the providence of our Heavenly Father, on evidence which, we must admit, is ten thousand-fold stronger, and when we have everything to gain, and nothing to lose? Is it said, There is still room for doubt? Of course there is. God be thanked! there is no relation in life in which there is not doubt. Were there no doubt, there would be no faith, no trust, no confidence, no love; the heart would be absorbed in the intellect, religion would become an axiom, and morality a formula of mathematics. Use but one-half of the observation, one-half of the intelligence, which are never at fault in the business of life, and these marks of the Creator's wisdom and providence which lie all around us will become as evident as the sun. Act on this evidence, and the door of grace will be opened, new light will stream into the soul, and all nature will be seen radiant with a Father's love.

All this striking evidence of design and adaptation we have discovered in the most obvious of the attributes of nitrogen,—in those merely negative qualities in virtue of which it increases the density of the atmosphere without interfering with the functions of its active constituents. It would not, however, be in accordance with that economy of resources which we find everywhere in nature, that the uses of nitrogen should be limited to this single object; and after what we have already seen to be true in the case of oxygen, we shall not be surprised to find this singular element suddenly changing its character and appearing in a new condition. The second point, as you will remember, which I am to illustrate in regard to nitrogen, is the variety and remarkable nature of its compounds, as well as the singularly oblique processes by which they are formed; and, having examined the marks of design it bears in its first manifestations, let us now study the no less impressive evidence presented by the second. It would be entirely out of place in a popular work like the present, to describe in detail any of the countless nitrogenized compounds which are known to chemistry, and it would require a separate volume merely to illustrate the

characteristic features of the great classes into which they may be subdivided. I shall be able only to glance at a few general facts which illustrate the point now under discussion, and also the part which nitrogen plays in organic nature.

Although nitrogen presents such an indifferent exterior towards the oxygen of the atmosphere, it can, nevertheless, be made to combine with it by resorting to certain oblique processes, and there may thus result no less than five different compounds. Every one is familiar with that highly corrosive liquid called nitric acid, and this is formed by the union with water of one of the compounds in question. Under certain conditions this acid results from the union of the oxygen, nitrogen, and aqueous vapor which are mixed together in the air. Indeed, the only essential difference between the bland atmospheric air and this highly active chemical agent consists in the fact that while in air the elements are only mixed together, in the acid they are chemically combined. Were nitrogen to be suddenly endowed with the active affinities which from its position among the chemical elements we might naturally expect it to possess, then nitric acid would be formed in the atmosphere in large quantities, and it is only the unexampled inertness of nitrogen which prevents a result which would be fatal to all organic life. But although so corrosive when pure, nitric acid when immensely diluted is one of the few materials which nourish and sustain the plant, and therefore provision has been made that it should be formed in the atmosphere, but

only under very restricted conditions, and to a very limited extent. When electrical sparks are passed through a confined quantity of air, in the presence of some alkaline substance, such as potash, soda, or lime, a very partial combination takes place between the two elements, and an infinitesimal quantity of nitric acid is formed. So, also, when organic matter decays in the presence of these same alkalies, a similar combination, although to a very slight extent, results. Nitric acid is endowed with such violent affinities that it does not remain in a free state. It at once enters into combination with the alkalies, forming a class of salts, of which saltpetre is the best known example, and from which the common nitric acid is extracted for the uses of the arts. Nitrogen, you will notice, acts here very much like a self-willed child. All the powers of nature cannot compel it to combine directly with oxygen; but if you offer to it these alkalies as an inducement, and make your approaches sufficiently indirect, you can coax it to combine, and nitric acid is then formed. We do not understand how the peculiar conditions just mentioned conspire to produce the result; but the whole phenomenon seems to be mysteriously connected with ozonized oxygen, and is undoubtedly another phase of that obscure subject, allotropism, to which we alluded in a previous lecture. See now how beautifully this attribute of nitrogen has been adapted to the conditions of vegetable life, and made the means by which the plant is furnished with one of the articles of its food. Every discharge of lightning is accompanied by a partial combination of the elements of the atmosphere, and the nitric acid which is thus formed and washed down by the rain-water serves to fertilize the soil and bring the growing corn to maturity. So in like manner, when life is extinct, and the organized forms are resolved into their original elements, the very process of decay causes a similar combination, and thus sweetens the flowers which spring from the grave.

But not only does nitrogen combine with oxygen. It unites also with hydrogen, that element which is the very antithesis of oxygen, and forms a most remarkable compound called ammonia. This substance is the very reverse of nitric acid in all its chemical relations, but, like nitric acid, it is a highly active and caustic agent. I need not dwell upon this fact; for the common smelling-bottle has made every one familiar with this pungent substance. Nitrogen manifests the same indifference towards hydrogen that it does towards oxygen, and the two elements can be made to unite only by indirect processes, which are not well understood. The most important of these is the process of decay. This destructive change in all the higher forms of organized beings is attended with the formation of ammonia, and the same nitrogenized compound is a uniform result of the normal functions of animal life. You will not, therefore, be surprised to learn that traces of ammonia, as of nitric acid, are found in the atmosphere and in all rain-water. Indeed, it is generally supposed that the two are in combination, forming a salt called nitrate of ammonia, but the amount present is, at best, very small.

Ammonia is thought by many to be a more important article of vegetable diet than nitric acid; but our knowledge of agricultural chemistry is very imperfect, and chemists are not agreed on many of the most fundamental points. [*] Still, as I have before stated, nitrogen is an essential element of all the higher forms of corporeal vitality, and compounds like those we have been considering are the appointed channels by which it is introduced into the organization of the plant. Had these compounds been allowed to form to any extent in the atmosphere, they would soon have rendered the globe uninhabitable. It was therefore essential that nitrogen

should be endowed with that unexampled inertness which it manifests in its gaseous state. But had not at the same time a power of combination, under certain restricted conditions, been granted, this chemical element would not only have been an isolated phenomenon in nature, an exception to its general laws, but its usefulness would have been restricted to the least remarkable of its functions. Unlike the results of human skill, this creation of Divine wisdom has been adapted to the most varied and apparently incompatible ends; and while in the atmosphere it is a mere dead weight, it is also the most plastic of the elements, is capable of entering into the most complex relations, and thus serves as the peculiar substratum of all the higher forms of organized being.

The last point I am to illustrate in regard to nitrogen is, perhaps, the most characteristic of its features, and it is one on which its relations in the scheme of organized nature very greatly depend. All the compounds of nitrogen are very unstable, and the slightest force is generally sufficient to overpower the delicate affinities by which the elements are held together, when the nitrogen at once returns to its home in the atmosphere. Although this inert element may be coaxed into combination, it never forms strong compounds. Its affinities, although so varied, are at best very feeble and delicate. It is always a weak timber in a chemical structure, and when this timber breaks, as it certainly will, sooner or later, the whole falls. You will need no further illustration of this fact than to be told that gunpowder,

percussion-powder, and gun-cotton are all nitrogenized compounds, and owe their well-known properties to the weak affinities of this element. Nitric acid is only a little more stable than these explosive agents, and ammonia, although one of the most permanent of nitrogenized compounds, is still very easily decomposed. Passing next to organized substances, we find this distinguishing character still more conspicuous. As we have already seen, it is always the nitrogenized compounds which start the decay in vegetable or animal structures; and thus the great characteristic feature of all organized matter, its proneness to change and decay, nay, even death itself, is clearly foreshadowed in the properties of nitrogen. When the Creator first endowed this element with its feeble affinities, He also passed the doom of all living creatures: "Dust thou art, and unto dust shalt thou return.''

Here I must leave this division of my subject. It would be highly interesting to study the innumerable phases in which nitrogen manifests itself in the world of living matter; to trace how, under the guidance of that mysterious principle of life, the most complex organic compounds are educed from such simple materials as water, carbonic dioxide, ammonia, and nitric acid; to follow these nitrogenized compounds through their varied history, from the time they are first generated in the plant until they are incorporated into the brain, the muscles, and the bones of man; to notice at every stage the same instability which so strikingly characterizes all the compounds of this singular element, capable of

existing only under the continued influence of the vital principle, and, when that ceases to act, gradually degenerating and falling back into the simple products from which they sprang; but all such details would be incompatible with the plan of these lectures, and must therefore be reluctantly passed by. If, however, I have been able to place before you in a clear light the main features of this remarkable element,—its isolated existence in the atmosphere, its unparalleled inertness in the aeriform condition, its power of combination under restricted conditions, the great variety and complexity of its compounds, and, finally, their singular proneness to decomposition and decay,—it is all that I could expect. We have seen that in each of these respects nitrogen has been adapted with exquisite skill to the unique part which it plays in the scheme of the world; and this element, although outwardly so unattractive and dull, has borne the richest testimony to the wisdom of the Maker.

Having now become acquainted with the characteristic features of nitrogen, let us next consider the part which this element plays in that grand circulation of matter in organic nature, which has been already in part described. I have before stated that the plant is a true apparatus of reduction, in whose leaves carbonic dioxide is decomposed by the solar light. The plant absorbs carbonic dioxide partly through its leaves from the air. and partly through its roots from the soil. The sun's rays, acting upon the green surface of the leaf, decompose in some

mysterious way the carbonic dioxide, overcoming the intense affinities of its elements, fixing the carbon, and setting free the oxygen, to be restored to the air. From the carbon thus obtained, and from the water, ammonia, and nitric acid which are the other articles of its food, together with a few inorganic salts, the plant constructs its tissues. If in their production carbonic dioxide and water alone take part, there result such substances as woody fibre, starch, gum, and sugar, and of these nine-tenths of all vegetable structures consist. If the nitrogen compounds are likewise employed in the process, there are formed, besides, such nitrogenized products as albumen, caseine, and fibrine. These last names may not be so familiar to you as the first, but you are equally familiar with the substances, and will recognize them at once when told that the white of an egg is nearly pure albumen, that cheese consists almost entirely of caseine, and meat of fibrine. Although these substances are best known to us as animal products, they are likewise found in all those vegetables which are articles of food. Albumen and caseine can readily be extracted from either peas or potatoes, and gluten, the substance which gives tenacity to flour-paste, has essentially the same composition as animal fibrine.

The animal, unlike the plant, has not the power of forming the substance of its tissues from inorganic compounds, but it receives them ready formed from the vegetable kingdom. It transmutes the vegetable products into a thousand shapes in order to adapt them to its uses, but its peculiar province

is to assimilate and consume, not to produce. The nitrogenized compounds just referred to are the portion of its food which supplies the constant waste attending all the vital processes. The non-nitrogenized starch and sugar, although they form the greater part of our food, are never actually incorporated into the tissues of the body, and, as we have already seen, are merely the fuel by which its temperature is maintained. The animal may either receive its nitrogenized food directly from the plant, as is the case with all herbivora, or only indirectly, like the carnivora; but in either case the origin is the same, and by the process of digestion these, originally at least, vegetable products are assimilated and converted into bones, muscles, or nerves, as the necessities of the animal may require. We find that during this process these substances do not undergo any fundamental change, but merely become parts of more finely organized tissues. We discover in the blood albumen and caseine, having precisely the same composition as that which may be prepared from potatoes, and the substance of the muscle does not differ essentially from the gluten of flour-meal.

Do not, however, suppose that the part played by the animal is less noble than that of the plant. It is really much higher. We must be careful to make a distinction, too frequently overlooked, between the organized structure and the material of which it consists. There is the same difference here as between a house and the bricks of which it is built. It was formerly supposed that organic matter was

formed under peculiar influences, and subject to special laws. But it is now known that animal and vegetable substances obey the same laws of affinity as mineral matter, and the recent progress of chemistry has given us great reason to believe that we may be able one day to prepare all the materials of which plants and animals build their cells. Here, however, chemistry stops and creation begins. The great Architect of nature alone can fashion this dead material into living forms. [*] The vegetable kingdom is a great laboratory, in which the sun's rays manufacture from the gases of the atmosphere, and from a few earthy salts of the soil, the different materials which the organic builders employ. There the bricks are made, and from these the animal builds his bones and muscles. He does not make the bricks, but he does what is far more glorious, he builds with them his delicate frame, and as the work of the builder is higher than that of the brick-maker, so in the scale of being is the animal higher than the plant, and the more noble in proportion as its structure is more intricate and elaborate.

While the plant is a true apparatus of reduction, the animal is a true apparatus of combustion, in which the substances it has derived from the vegetable are burnt and restored to the atmosphere in the

form of carbonic dioxide, water, and ammonia, ready to be again absorbed by the plant and to repass through the phases of organic life. Our bodies are furnaces,—furnaces continually burning,—whose fuel is our flesh, and whose smoke is the breath of our nostrils. Every time I strike a blow a portion of the muscle is consumed, actually burnt up in producing the force. In every muscular effort I make, in every word I utter, in every step I take, a portion of the muscles concerned is burnt, and motion can no more be produced in the animal body without a combustion of its tissues, than it can be generated in a steam-engine without burning fuel under its boiler. As in the steam-engine the burning fuel is the source of its power, so in the animal body the burning muscle is the immediate cause of all its motions. I will to strike a blow, but my will is not the moving power. The power is in the muscle, and in the exertion the muscle is consumed. The muscle, however, does not originate the motion, any more than the fuel originates the motion of the steam-engine. The fuel, we have seen, does not originate heat. It is merely a reservoir of heat, and in burning it merely gives up the heat it once received from the sun. So the muscle is merely a reservoir of force, and in burning it gives out the force it contains. The force it contains it also received from the sun, when its substance was formed by the sun's rays acting upon the leaves of the plants.

What a wonderful revelation is this! Muscular power originates in the sun. We do not create the force; we do not originate it; we merely excite it.

The force which originally came from the sun lies dormant in the muscles until our will calls it into activity. Our bodies are machines, perfect machines it is true, but yet machines. Like all other machines, they merely transmit power, they cannot create it. They very closely resemble a steam-engine. As we must constantly feed the engine with fuel, so we must supply our bodies with food in order to repair the muscle burnt, and we can no more be said to originate that force which manifests itself in our bodies, than the stoker, who shovels the fuel into the grate, can be said to originate the force of the steam-engine. We are not our bodies, although we live in them, and direct their motions. They move by forces which emanate from a source far higher than we, and we stand in the same relation to them in which an engineer does to his machine. Certainly Lavoisier, the great father of modern chemistry, had caught a glimpse of the results which it was left for more modern science to establish, when he wrote: "Organization, sensation, voluntary motion, life, only exist on the surface of the earth, and in places exposed to the light. It might be said, indeed, that the fable of Prometheus was an expression of a philosophical truth, which had not escaped the penetration of the ancients. Without light, nature were without life and without soul; a beneficent God, in shedding light over creation, strewed the surface of the earth with organization, with sensation, and with thought.''

Although it thus appears that our bodies are mere channels of force, machines whose motive

power emanates from the great centre of the solar system, let us, while we recognize this startling result of science, remember the no less certain fact of consciousness,—that we are not our bodies, though we live in them,—that this conscious personality is something entirely apart from, and infinitely superior to, these corporeal atoms in which it is temporarily enshrined, surviving as it does all their changes. Let us also keep clearly in view the still more glorious truth, that this machine, with all its infinite capabilities of good and evil, is put entirely at our command; that not one conscious motion can take place unless we will it; and that this will of ours can set in action a chain of causes which no space can bound and no time can limit. Let us then well consider how great is the power which has thus been delegated to us, let us duly weigh the awful responsibility it involves, and so act that, when the Master claims his own, we may not be ashamed to render up the account of our stewardship.

Moreover, although it is true that these bodies themselves are constantly dissolving into air, that the material atoms which compose them will in a few short weeks all be gone, and that there is nothing but the shadow of our forms which we can call our own, we must also remember that there is a mysterious principle within, constantly renewing and repairing our wasting frames,—a cunning architect superintending a thousand builders who are constantly reconstructing, with materials prepared by vegetation, the bones, the muscles, and the nerves, as fast as they are wasted and consumed; making,

in a most mysterious way, beyond all human comprehension, here the fibre of a muscle, there the filament of a nerve, here building up a bone, there uniting a tendon, fashioning each with scrupulous nicety, and fitting each to its place with never-failing skill. But no sooner is the work of the architect done, than another great power comes in to destroy it. The oxygen gas which the blood absorbs in the lungs and carries to the different parts of the body burns up these carefully elaborated tissues, converting them into carbonic dioxide, water, and ammonia, which pass into the atmosphere, from which they originally came. Life is, in fact, a constant struggle between the builders and the destroying element of the air; and when its short term is ended, and the builders cease because they are wearied and few, then "the dust returns to the earth as it was, and the spirit returns unto God who gave it.''

But let us not sorrow as those who have no hope; "for we know that if our earthly house of this tabernacle were dissolved, we have a building of God, an house not made with hands, eternal in the heavens.'' And cannot He who hath clothed us with our earthly house provide for us a better and more enduring mansion? and are not all these wonderful changes in our present bodies a foreshadowing of the final consummation, when our earnest desire "to be clothed upon'' shall be satisfied, and "mortality shall be swallowed up of life''?

Such is a very imperfect sketch of that great cycle of changes, of which all organic nature is merely a passing phase. Let us review for a moment its main

features. When the foundations of the globe were laid, there were collected in the atmosphere all the essential elements of organized beings. From this inexhaustible storehouse the plant absorbs water, carbonic dioxide, and ammonia, which were placed there for its use, and which have been made to serve as its nourishment and food. It is the special office of the plants to elaborate from these few mineral substances, and a small amount of earthy salts, all the materials of organized beings. The animal receives these crude materials already prepared, and builds with them its various tissues; but no sooner are the cell-walls finished, and the structure ready to discharge its vital functions, than it is consumed by almost the very act which gave it life. The carbonic dioxide, water, and ammonia are restored to the atmosphere, and the cycle is complete.

Of this Divine economy the sun's rays are the great moving cause, and it is their mysterious power which is constantly reappearing in all the varied phases of organic life. And not in these alone; for, as we have seen, this same gentle influence keeps in motion the aerial currents which blow our ships across the ocean. It raises the water which turns the wheels of our factories. It drives the locomotive over the iron road, and impels the steamer through the waves. It roars at the cannon's mouth, and charges the grander artillery of the skies. There is no motion on the globe which cannot be traced directly or indirectly to the sun, and were his rays to lose their mysterious power, all nature would be come silent, motionless, and dead.

But in thus tracing to the sun all these varied phenomena, let us not forget that we have not yet found the great First Cause. The problem is not yet solved; the profoundest truth has yet to be told. This mysterious force, which the sun pours in ceaseless floods upon the earth,—whence comes it? You have already answered the question. The answer is on your lips. I have but to re-echo it, and how can I better do this than in the words of that blind poet to whom misfortune had revealed the true meaning of light: