University of Virginia Library

3. CHAPTER III.
TESTIMONY OF OXYGEN.

WERE we to limit our regards to those physical qualities of the atmosphere which we studied in the first two chapters, we should overlook the most wonderful adaptations in its divine economy. These properties belong to the atmosphere, in great measure at least, in virtue of its aeriform condition, and, so far as we know, an atmosphere composed of other gases, and still having the same density, would soften the intensity of the light, and diffuse the genial influences of the sun's heat, as well as air. Not so, however, with the chemical qualities of the atmosphere, which we are next to consider. These belong to the atmosphere solely as air, and could not have been obtained with any other known materials.

When a chemist wishes to investigate the nature of a new substance, his first step is to analyze it. Let us, therefore, as a preliminary to our present inquiry, ascertain what is the composition of this aeriform matter we call air. The air has been analyzed hundreds of times in every latitude and in every climate; and the result has been uniformly that which is given in the following table:—

Composition of the Atmosphere. [*]

               

Oxygen	20.61
Nitrogen	77.95
Carbonic Dioxide	.04
Aqueous Vapor (average)	1.40
Nitric Acid,}
Ammonia,}	traces.
Carburetted Hydrogen,}	________
	100.00

Composition in Tons.

       

Oxygen	1,233,010 billions of tons.
Nitrogen	3,994,593 '' ''
Carbonic Dioxide	5,287 '' ''
Aqueous Vapor	54,460 '' ''

Besides oxygen and nitrogen gases, which, as you will notice, are the chief constituents, there are always present in the atmosphere the vapor of water, carbonic dioxide, and ammonia gas; and if we add to these uniform constituents the various exhalations constantly arising from the earth, we shall have as accurate an idea of the composition of the air as chemistry can give. While, however, the proportions of oxygen and nitrogen are almost absolutely constant, those of the other ingredients are very fluctuating, and the total quantity exceedingly small, never amounting in all, exclusive of aqueous vapor, to more than one part in a thousand, unless in some confined locality, and under very unusual circumstances.

Do not, however, measure the importance of these variable, and in a degree accidental, constituents by their amount, for, although present in such small quantities, they are not less essential in the atmosphere than the two gases which make up almost its entire mass.

Moreover, we must carefully avoid the error of considering air as a distinct substance, like water or coal. On the contrary, it is merely a mechanical mixture of its constituent gases, and is in no sense a definite chemical compound. Indeed, we may regard the globe as surrounded by at least three separate atmospheres,—one of oxygen, one of nitrogen, and one of aqueous vapor,—all existing simultaneously in the same space, yet each entirely distinct from the other two, and only very slightly influenced by their presence. To each of these atmospheres the Author of nature has assigned separate and different functions. They are like so many servants in a household, each with a distinct set of duties, which are discharged with a fidelity and diligence unknown to any earthly service. Let us consider what those duties are, and see how skilfully each is adapted to the offices which it is designed to fill.

Were all the other constituents of the air removed, the earth would still be surrounded by an atmosphere of oxygen, having about one-fifth of the density, and exerting at the surface of the globe about one-fifth of the pressure, of the present atmosphere. In studying the chemical relations of air, let us begin with some of the more

important functions of this remarkable substance, and these will fully occupy us during this and the succeeding chapter.

It is easy to prepare oxygen in a pure state. It is then a perfectly colorless and transparent gas, and so persistently does it retain its aeriform condition that it cannot be reduced to the liquid state by pressure alone. A German chemist, Natterer, submitted this gas to a pressure of over forty-five thousand pounds, or twenty tons on a square inch, but he did not succeed in changing its condition. More recently, by the combined action of great pressure and the most intense cold which can be artificially produced, all the gases formerly called permanent have been liquefied, and oxygen among the number. But this remarkable result, while it shows conclusively that the so-called permanent gases differed from other forms of aeriform matter in degree only, and not in kind, also brings into prominence the extreme qualities of these constituents of our atmosphere. Most aeriform substances may be reduced to liquids by pressure under a very moderate reduction of temperature; [*] but oxygen

and nitrogen retain their aeriform condition under the widest variations of temperature which exist on the earth.

The importance of this fact will be seen at once on comparing the condition of the oxygen and nitrogen in the atmosphere with that of the aqueous vapor. A fall of temperature of only a few degrees will generally condense a portion of the vapor, and, small as is its relative amount, the resulting rain is at times poured down upon the earth in deluging floods; and if you consider what must have been the destructive results had the whole mass of the atmosphere been liable to a similar fluctuation, even under extreme conditions, you will discover in the permanency of oxygen a most obvious adaptation of its properties to the thermal condition of our globe.

The permanently aeriform state of oxygen will appear still more remarkable when we consider how largely it enters into the composition of the solid crust of the earth. Oxygen belongs to that class

of substances which the chemists call elements, because they have never succeeded in resolving them into simpler parts, and of all the elements it is by far the most widely diffused. As we have already seen, one-fifth of the volume of the atmosphere consists of this gas; but this is a small amount compared with that which enters into the composition of most substances. You may be surprised at the statement, but it is nevertheless true, that between one-half and one-third of the crust of this globe and of the bodies of its inhabitants consists of oxygen. No less than eight-ninths of all water is formed of the same gas. It makes up three-fourths of our own bodies, not less than four-fifths of every plant, and at least one-half of the solid rocks. Remembering now that twenty tons of pressure on a square inch are not sufficient to reduce oxygen to a liquid condition, consider what must be the strength of that force which holds it thus imprisoned. In a tumbler of water there are no less than six cubic feet of oxygen gas, condensed to a liquid condition, and held there by the continuous action of a force which can be measured only by hundreds of tons of pressure. We call the force chemical affinity; but who shall measure its power? Who but He who could make with such a subtile material the rocks, with which he "laid the foundations of the earth,'' and the waters which roll over its surface?

Oxygen gas, like all other forms of aeriform matter, tends to expand, and can be prevented from obeying this natural tendency only by enclosing

it in an air-tight receiver. As it exists in our glass jars, under the ordinary conditions of temperature and pressure, one cubic foot of oxygen weighs 590.8 grains, although in its more expanded state, as it exists in the atmosphere at the surface of the globe, it has but one-fifth of this density. One cubic foot of nitrogen gas weighs, under the same circumstances, 517.5 grains; but although there is such a decided difference between the specific gravities of the two gases, yet so perfectly are they mixed together throughout the whole extent of the atmosphere, that analysis has been unable to detect more than a very slight difference in composition between the air brought from the summits of the Alps and that from the deepest mine in Cornwall. Why, you may ask, do not these gases obey the well-known laws of hydrostatics, the heavier oxygen sinking to the surface of the earth, and the lighter nitrogen floating above it? Simply because gases, unlike the other forms of matter, have the property of "diffusing'' through each other, and existing together in the same space. The presence of one gas does not prevent the entrance of another into the space which it occupies, and if two open jars, containing different gases, are placed together, mouth to mouth, each gas will expand until it fills the whole volume of both receivers. Moreover, the greater the difference between the densities of the gases, and the greater consequent disposition to separate, the stronger is their tendency to mix together. This process is known as "diffusion,'' and plays a very important part in the plan of creation. Were no such law in operation the two gases composing air would have separated partially, and the atmosphere have become unfitted for many of its important functions. Take, for example, the function of transmitting sound.

As the air is now constituted, there is a constancy of pitch, however far sound travels. Any tone once generated remains the same tone until it dies away. Its degree of loudness alters in proportion to the distance of the listener, but the pitch is constant. Were it not, however, for this law of diffusion,— were the atmosphere not perfectly homogeneous, and the gases of which it consists even partially separated,—there would have been a very different result. The constancy of pitch could no longer have been depended upon. The sound as it travelled would vary its pitch with the ever-varying medium through which it passed, and would arrive at the ear with a tone entirely different from that with which it started. Nor would it require any great difference in the medium to produce a sensible result and to confuse all those delicate differences of pitch on which the whole art of music depends. Whenever, therefore, you may be next enjoying the grand Pastoral Symphony of Beethoven or the Requiem of Mozart, recall the careful adjustment of forces by which alone these magnificent creations of genius were rendered possible, and you cannot fail to recognize in this simple law of nature the same hand that first strung the lyre and made the soul of man responsive to its seven notes.

Returning again to the qualities of oxygen, let us

notice, in the next place, that it is entirely destitute either of odor or of taste. This fact is a matter of common experience; for as oxygen exists in a free state in the atmosphere, it would there manifest these properties did they exist: and reflect how essential these negative qualities are to our comfort and well-being. Moreover, in its ordinary condition, oxygen seems entirely devoid of any active properties. It does not affect the most delicate and evanescent vegetable dyes, which the weakest chemical agents will either alter or destroy. And consider the oxygen as it exists in the air. How bland and seemingly inactive it is there? Reflect that it bathes the most delicate animal organisms, that it pervades the minutest air-passages of the lungs,— remember that it is in contact with all matter,— and every substance will seem to bear evidence to the fact that oxygen in the state of gas possesses no active properties, and is incapable of manifesting any strong chemical force. And yet, if you infer that oxygen always appears in the passive condition, and is under all circumstances incapable of violent action, you will be entirely deceived; for so far from being one of the weakest, it is the strongest of the chemical elements, and beneath this apparent mildness there is concealed an energy so violent, that, when once thoroughly aroused, nothing can withstand it. A single spark of fire will change the whole character of this element, and what was before inert and passive becomes in an instant violent and irrepressible. The gentle breeze which was waving the corn and fanning the browsing herds, becomes the next moment a consuming fire, before which the works of man melt away into air.

And here I must correct an erroneous, although very common impression, that there is something substantial in fire. This is one of those ideas, originating in an illusion of the senses, which we have inherited from a more ignorant age, and which our modern science cannot wholly dispel from the popular mind. Fire was formerly regarded as one of the elementary forms of matter, and all burning was supposed to consist in the escape of this principle of fire, previously pent up in the combustible substance. In support of this doctrine the old philosophers confidently pointed at flame as the visible manifestation of the escaping fire-element; and, childish as this doctrine may seem, it was the prevalent belief of the world for at least two thousand years.

The last phase which this doctrine assumed was the phlogiston theory of the last century. In the hands of Bergmann and Stahl, the vague ideas of the time received a more material form, and were embodied in a philosophical system. They termed the principle of fire phlogiston, and burning, or the escape of fire, dephlogistication, and their ingenious system did not a little to retard the progress of truth. The philosophers of that age either took no account of the increase of weight which results from burning, or attempted to explain the few instances in which the fact was forced upon their attention by the fanciful notion of Aristotle—that the essence of fire was specifically light. Hence, they reasoned,

phlogiston buoys up all bodies into which it enters, and after its escape in the process of burning, the burnt material must weigh more than before. It was not until 1783 that the true theory of combustion was discovered, and from this discovery modern chemistry dates. The fortunate discoverer was Lavoisier. He proved, by simply weighing the products of combustion, that burning, instead of being a loss of phlogiston, is a union of the burning substance with the oxygen of the air, and this theory is now one of the best established principles of science.

Burning is merely chemical change, and all combustion with which we are familiar in common life is a chemical combination of the burning substance, whether it be coal, wood, oil, or gas, with the oxygen of the air. Combustion is simply a process of chemical combination, and the light and heat which are evolved in the process are only the concomitants of the chemical change. Why those mysterious influences of light and heat are radiated from the coal which is combining with oxygen in our grates, we may understand better hereafter; but this much we already know,—the sensations of light and heat are caused by waves of an ethereal medium breaking upon the extremities of the delicate nerves of our human organism; and such waves are set in motion during the chemical change which we call combustion. What the chemist mostly studies, however, is the change itself, and to this we will for the present confine our attention.

The chief products of ordinary combustion, that is,

the compounds of oxygen with the elements of coal, wood, and illuminating gas, are only two in number. carbonic dioxide gas and aqueous vapor. These products, as is well known, are perfectly colorless and transparent aeriform substances, wholly without odor or taste, and entirely devoid of every active quality. For this reason they escape without observation from the burning wood, ascend our chimneys, and by the force of diffusion are spread throughout the atmosphere; but if, as may readily be done by chemical means, we collect the neglected smoke and weigh it, we shall find that it weighs much more than the burnt wood, and, as more careful experiments will show, its weight is exactly equal to that of the wood added to that of the oxygen of the air consumed during the burning.

Moreover, this smoke, though so long unnoticed by man, was not overlooked by the Author of nature. It is a part of his grand and beneficent design in the scheme of organic nature. No sooner do the products of that wood burning on the hearth escape into the free expanse of the outer air, than a new cycle of changes begins. The carbonic dioxide and the aqueous vapor, after roving at liberty for a time, are absorbed by the leaves of some wide-spreading tree, smiling in the sunshine, and in the tiny laboratory of their green cells are worked up by those wonderful agents, the sun-rays, into new wood, absorbing from the sun a fresh supply of power, which is destined, perhaps, to shed warmth and light around the fireside of a future generation.

But let us not anticipate our subject. In a future

chapter we shall discuss this wonderful cycle of changes at some length. At present I wish to direct your attention to the remarkable contrast of qualities presented by the element oxygen in its active and passive conditions. How is this complete inversion of properties to be explained? There is a cloud of mystery hanging over the subject, which the progress of knowledge has not as yet entirely dispelled; [*] but, so far as the cause is known, I will endeavor to make it intelligible. The difference in the action of oxygen in these two conditions depends on temperature. At the ordinary temperature of the air its chemical affinities are dormant, and, although endowed with forces which are irresistible when in action, it awaits the necessary conditions to call them forth. One of the grandest works of ancient art which have come down to us is the colossal statue of the Farnese Hercules. The hero of ancient mythology is represented in an erect form, leaning on his club, and ready for action; but for the moment every one of the well-developed muscles of his ponderous frame is fully relaxed, and the figure is a perfect ideal of repose, yet a wonderful embodiment of power. Here in this antique we have most perfectly typified the passive condition of oxygen, the hero of the chemical elements. Raise now the temperature to a red heat, and in a moment all is changed. The dormant energies of its mighty powers are aroused, and it rushes into combination with all combustible matter, surrounded by those glorious manifestations of light and heat which every conflagration presents.

In order to evoke the latent forces in the oxygen of the atmosphere, it is not necessary, however, to raise the temperature of any considerable portion either of the gas or of the combustible. There is a provision in nature by which chemical combination, once started at any portion of the combustible mass, is sustained until the whole is consumed. All chemical combination is attended by the evolution of heat, and in the combination of oxygen with most combustible substances the amount of heat thus generated is so great, that by the burning of one portion sufficient heat is evolved to raise the temperature of a second portion to the point of ignition, and thus the process is continued. Consider, for example, what takes place in the burning of a jet of gas. We start the combustion by bringing the flame of a lighted match over the orifice of the burner. By this the temperature of the gas and that of the air surrounding it are raised to a red heat, and chemical combination at once ensues. But the chemical union, as just stated, is attended with the evolution of great heat, which, before it is dissipated, raises to the point of ignition the temperature of the next portion of gas issuing from the burner. This, combining in its turn with oxygen, generates a fresh quantity of heat, and thus keeps up the combustion so long as the gas is supplied. What I have shown to be true of a gas

burner is equally true of all ordinary combustion, and so a single spark may be sufficient to light up a conflagration which will reduce to ashes a whole village or involve a city in ruin.

Thus it appears that burning is chemical combination with oxygen, that this union is attended with the evolution of heat, and that a high temperature is the condition under which oxygen manifests its latent power. But, you may say, these facts do not explain the difference between the two states of oxygen, they merely give the conditions under which these states are manifested; and this is true. Why it is that at one temperature oxygen is so completely passive, and at another temperature, a few hundred degrees higher, so highly active, we cannot fully explain; but the facts are undisputed.

The temperature at which oxygen assumes its active condition is called the point of ignition. Although fixed for each substance, it differs very greatly with the different kinds of combustible matter, being determined, apparently, by their relative affinities for the great fire-element. Thus phosphorus ignites at a temperature less than that of boiling water, sulphur at about 500°, wood only at a full red-heat, anthracite coal at a white-heat, while iron requires the highest heat of a blacksmith's forge. Beginning with a phosphorus match, which can be ignited by friction, and using the more combustible materials as kindlings, we can readily attain in our furnaces the highest temperature required, and thus the energies of this powerful agent are fully at the command of man. But notice at the same time

that the point of ignition of wood, coal, and common combustibles, has been placed sufficiently above the ordinary temperature of the air to insure the general safety of our combustible dwellings; and when we consider how liable they are, even now, to accidents from fire, we shall appreciate the care which has been taken by our Heavenly Father to guard us against this terrible danger.

But even this precaution would have been insufficient to secure safety, were it not that the active energies of oxygen, even when aroused, have been most carefully tempered by extreme dilution. It would be easy to show by experiment that the slowness of combustion depends on the fact that in the atmosphere oxygen is mixed with a great mass of an inert gas, and the proportions have been so adjusted in the scheme of creation as generally to restrain the awakened energies of the fire-element within the narrow limits which man appoints; but when, through his misfortune or carelessness, it overrides these limits, and, from administering to man's wants, becomes the agent of his destruction, we are reminded in the awful conflagration by what a delicate tenure we hold our earthly possessions, and how small a change would be sufficient to involve all organized matter in a general conflagration. Remember now that fire is one of the most valuable servants of mankind; that it is the source of all artificial heat and light; that in the steam-engine it is the apparent origin of that power which animates the commerce and the industry of the civilized world; that under its influence iron becomes plastic,

and the ores give up their metallic treasures; that it is, in fine, the agent of all the arts,—and you cannot wonder that in a ruder age the Romans should have enthroned its presiding deity on Olympus, or the Persians worshipped its supposed essence as divinity itself. Looking at it again, in the light of modern science, as merely the manifestation of the latent power of this bland and diffusive atmosphere, the truth seems almost incredible. To think that this, the strongest of the chemical elements,—which, although a permanent gas, forms more than one-half of the solid crust of the earth, and is endowed with such mighty affinities that it is retained securely in this solid state,—could have been so shorn of its energies as not to singe the down of the gossamer, and yet so tempered that its powers may be evoked at the will of man and made subservient to his wants! To me the double condition of oxygen is one of the most remarkable phenomena of nature. I ponder it again and again, with increasing wonder and admiration at the skill of the infinite Designer, who has been able to unite in the same element perfect mildness and immeasurable power. It seems as if the millennium of the Hebrew prophet were prefigured in the atmosphere. "The wolf also shall dwell with the lamb, and the leopard shall lie down with the kid, and the calf and the young lion and the fatling together, and a little child shall lead them.''

If I have succeeded in making clear the relations of this twofold character of oxygen to man and his works, I think that you cannot fail to have been

impressed with the evidence of design which the subject affords. This evidence is seen in the facts, first, that the same element is at different temperatures endowed with such opposite and apparently incompatible qualities; secondly, that in each of its conditions the properties are so skilfully adapted to the functions which it is appointed to perform; thirdly, that the temperature at which it assumes its active state has been so accurately adjusted to the thermal conditions of the globe; and lastly, that its active energies have been so carefully guarded, and placed to so great a degree under the control of man. But we have not as yet one-half exhausted the subject. Here, as everywhere else in nature, the argument is cumulative; the more we study, and the more our knowledge is enlarged, the more it grows upon us; and wherever we may leave the field, we always are conscious that there is a still richer harvest to be reaped beyond.

If the crust of the globe is a fair sample of the whole mass, oxygen was the chief material employed by the Great Architect in constructing our earth. Moreover, world-building was a process of burning, like those we have been studying, and the foundations of the earth were undoubtedly laid in flames.

When we attempt to break up the various materials around us into simpler parts, we soon reach a class of substances which cannot be further decomposed. Simple inspection will show that granite rock, for example, is a mixture of three minerals, called feldspar, mica, and quartz. We know,

also, that feldspar consists of alumina, potash, and silica, that mica contains the same materials in different proportions, and that quartz is silica alone. Lastly, the chemists have discovered that alumina is composed of aluminum and oxygen, potash of potassium and oxygen, and silica of silicon and oxygen. But here we must stop; for when you ask us of what these last-named materials are made, we find ourselves in the condition of the old philosopher, who got on very well with his flat earth, supporting it on an elephant, and the elephant on a tortoise, until he came to seek a resting-place for the tortoise; but then his theory failed. So is it with our science. These undecomposed materials are the blocks on which the whole is built; and we are totally ignorant of what lies below.

We call all substances which have never yet been decomposed, whatever may be their nature, chemical elements, and of such some seventy are now known. Setting apart oxygen as the supporter of combustion, the great mass of the remaining elements are combustible; that is, under certain conditions they combine rapidly with oxygen, evolving light and heat. Indeed, many of the combustible substances with which we are most familiar are elements. Charcoal is an element, phosphorus is an element, sulphur is an element, iron and all other metals are elements, and out of such combustible materials, together with oxygen, the world is made, but chiefly out of oxygen.

When we burn charcoal in air, or in pure oxygen gas, the burning is a process of world-making. The

charcoal combines with oxygen, and the result is a transparent, colorless gas, called carbonic dioxide. Many may not have heard of such a substance before, but it is always present in the atmosphere, at least in small quantities, and if we continue our process of world-making a little further, we shall find that it enters into the composition of some of the most familiar rocks and minerals.

I have at the bottom of this closed glass tube a small piece of a yellowish-white metal, looking very much like a flattened shot; and so it is, but the metal is not lead, although it resembles lead very closely. Like lead it is quite soft, and can be easily beaten into leaves thinner than writing paper; but it is very much lighter than lead, and tarnishes so rapidly in the air that we are obliged to keep it thus protected. We call the metal calcium, and although you may never have seen the substance before, it is one of the most abundant metals in nature, yet seldom seen, because of the extreme difficulty with which it is extracted from its ores. When heated to redness, calcium burns with a brilliant white light and a scintillating flame. In burning, it combines, of course, with oxygen, and the result is lime, common quick lime, such as is used for making mortar. This is a process which in the original world-making must have played a very important part, for lime rocks form a large portion of the earth's crust. None of these rocks, however, will slake like quick-lime, and we must go a step further in our world-building, and bring in the agency of water, before we can reach the actual condition of things.

We have now before us two products of burning, one a solid, called lime, made by uniting calcium with oxygen, the other a gas, called carbonic dioxide, made by uniting charcoal with oxygen. Both are soluble to a certain extent in water, and these clear solutions, called lime-water and soda-water respectively, are even more familiar to you than the substances themselves. Mix now the solutions together. The water becomes at once very turbid, and there soon settles from it a white powder. The lime and carbonic dioxide have united, and this is the result. If we collect and examine the white powder we shall find that it is chalk, and from the same material, spread in thick layers over the ocean-bed, and subsequently hardened by the mutual action of heat and water, have been formed limestone, marble, and the different varieties of lime rock, which are all ores of calcium.

But we may study with profit a second example of world-building. I have here a small quantity of another very abundant element, called silicon, but, like calcium, a comparative rarity, because it is with difficulty obtained pure. It resembles in many respects carbon, and has been observed in three different states, corresponding to charcoal, graphite, and diamond Like carbon, it also is combustible, combining with the oxygen of the air when heated to a high temperature, and forming a very hard white solid, called by chemists silica, which is the same thing as quartz, rock-crystal, agate, jasper, calcedony, opal, etc. All these familiar minerals are merely different conditions of this one material, and

contain over one-half their weight of oxygen gas. When ground to a coarse powder by the action of running streams, they become sand, and the grains of sand, compacted together, form sandstone and similar rocks; and you will begin to appreciate the enormous amount of silicon which must have been burnt up in the process of world-making, when you learn that at least one half of the solid crust of the earth consists of silica in its different varieties.

Setting aside the silica for a moment, let us turn to another very widely distributed element, called aluminum. This brilliant white metal, comparing favorably even with silver in lustre, was, until very recently, as great a rarity as calcium or silicon; but within a few years a process has been discovered by which it can be extracted from its ore at a cost sufficiently low to render the metal available in the arts, and it has now come into quite general use for making mathematical instruments, for jewelry, and for similar purposes. It forms also, with copper, a valuable alloy, which does not readily tarnish, and resembles gold so closely that the two cannot be distinguished by their external appearance.

Aluminum, like most of the metals, is combustible, although it does not burn readily in the air, unless the temperature is very high and the metal finely subdivided; but it then burns very brilliantly, emitting a vivid light, and forming a compound called by chemists alumina, which is melted by the intense heat to a yellowish transparent glass, and is the same substance from which nature makes the sapphire and the ruby. Emery also, which, on account of its great hardness, is used so largely for

polishing, is only a rougher form of the same material. Unite now the alumina to silica, add water, and we get clay. Burn the clay, and we have, according to the fineness of the materials, porcelain, pottery-ware, or bricks.

Taking next the element magnesium, which is also a brilliant white metal, allied to zinc, you notice that it takes fire even in the flame of a candle, and burns with dazzling brilliancy. The result is magnesia, so much used as a medicine. Unite magnesia to silica, and we have, according to the proportions, hornblende or augite, two minerals which abound in many varieties of rock. Add water to the composition, and we get also serpentine or soapstone, with several other allied mineral species.

I might multiply these illustrations indefinitely, but I will limit myself to only one other example. Here is a metallic element called potassium, so light and combustible that it swims and burns on water. Burning in water may seem, at first sight, very paradoxical; but in studying chemistry we must be ready to give up old prejudices. Water is almost pure oxygen, containing in the same volume more than one hundred times as much of the fire-element as air, and all combustibles would burn in water were it not that the oxygen is imprisoned in the liquid by an immensely strong force. Potassium, however, has such intense chemical affinities that it will break through all bars and bolts in order to unite with oxygen, and it therefore burns thus brilliantly even in the midst of water. [*] The final result is a

white solid called potash, which dissolves in the liquid. Melt together, now, potash, lime, and silicious sand, and we have glass. Unite silica, alumina, and potash, and we get feldspar; combine them in different proportions, and we have mica; varying again the proportions, we obtain garnet. Lastly, mix quartz and feldspar together with mica or hornblende, in an indiscriminate jumble, and we have the several varieties of granitic rocks.

Such, then, are some of the steps in the process of world-building. I do not mean to imply that we can reproduce all these substances in our laboratories, although even this is true in almost every case. My object is only to show what must have been in general the process of nature, and to make evident the fact that oxygen has been the chief world-builder.

But why call oxygen the world-builder more than the other elements? This diagram answers the question, and it illustrates one of the most

remarkable facts to which the study of this function of oxygen has led. Of the seventy known elements, more or less, thirteen alone make up at least 99/100 of the whole known mass of the earth. Of this, oxygen forms about 1/2 , silicon about 1/4 ; then we have aluminum, magnesium, calcium, potassium (K), sodium (Na), iron (Fe), carbon (C), sulphur (S), hydrogen (H), chlorine (C1), and nitrogen (N) filling up nearly the other fourth, while the remaining elements —including all the useful metals except iron— do not constitute altogether more than 1/100 . The diagram, however, only represents the relative proportions very rudely, as the subdivisions are necessarily based on very rough estimates and imperfect data.

Evidently, then, so far as our knowledge extends, oxygen, silicon, and carbon, together with a few metals, have been the chief building-materials employed by the Great Architect, and oxygen has been, as it were, the universal cement by which the other elements have been joined together to form that grand and diversified whole we call our earth.

One more remark in regard to this subject, and I will close this chapter. It is probable that there was a time, anterior to the earliest geological records, when the elements were in a free state; when the oxygen now solidified was a gas, and when, at the appointed time, the union of the elements began. Then our earth was a bright, burning star, radiating heat and light into space. Indeed, if we accept the nebular hypothesis of Laplace, the earth was formerly a part of the sun, was thrown off by the centrifugal force from his

burning mass, and, like a spark from a forge, soon burnt out, although after this lapse of time the great central fire is burning still. But whether Laplace be right or not, this much is certain;—the crust of the earth, so far as we can examine it, is like a burnt cinder, and the atmosphere of oxygen which surrounds it is merely the residuum left after the general conflagration,—left because there was nothing more to burn. Unmeasured ages have passed away since then; the earth's crust has cooled and solidified; the waters have been condensed and gathered into the great ocean-basins; the dry land has been covered with verdure and peopled with all kinds of four-footed beasts, winged fowls, and creeping things; the waters have been tenanted with countless forms of swimming creatures; and, last of all, man has come to live in this fair creation, and study the wonders of his dwelling-place. He finds on the earth's burnt crust an abundant supply of combustible material for all his wants. But if the world was once burnt up, and the elements glowed with fervent heat, how is it that these combustibles have been left unconsumed? Modern science has been able to answer this question. It has discovered that during the long geological periods a silent agency has been slowly recovering a small amount of combustible material from the wreck of the first conflagration. The sunbeam has partly undone the work of the fire, and whatever now exists on the earth unburnt, wood, coal, or metal, we owe to that wonderful agent the solar light. How the result has been accomplished, I propose to consider in a future chapter.