University of Virginia Library

4. CHAPTER IV.
TESTIMONY OF OXYGEN—Concluded.

BESIDES the two extreme conditions of oxygen, there exists still a third, in a measure intermediate between them, but still differing essentially from either,—a condition in which the element discharges functions, less brilliant it is true, but not less interesting and instructive, than those which we studied in the last chapter. The phenomena in which this condition of oxygen is chiefly active require, as a general rule, months, or even years, for their full manifestation. Moreover, they are so silent and unobtrusive, as frequently to be passed unnoticed; but nevertheless, when we have become acquainted with their magnitude and importance, I am sure you will agree with me that they far surpass in true grandeur those dazzling displays of power which the fire-element manifests when fully aroused. This third phase of the element can be best studied in its effects, and to two of these I now ask your attention.

Every one knows that, when wood or any other organized structure is exposed to the moist atmosphere, it gradually decays. It first becomes rotten,

and then slowly disappears. All may not know, however, that decay consists in a slow union of the organized structure with oxygen, and that the log of wood which is left to rot in the forest undergoes precisely the same change as one which is burnt on the hearth. The sole difference is, that, while the last is burned up in a few hours, the first entirely disappears only after the lapse of many years. Wood, like all organized vegetable structures, consists mainly of three elements, carbon, hydrogen, and oxygen. When heated on the hearth, in contact with the air, it takes fire and burns; that is, its combustible elements combine with oxygen, the carbon to form carbonic dioxide, and the hydrogen to form aqueous vapor, both of which escape by the chimney. But of these two ingredients of the wood, hydrogen is by far the most combustible; that is, it has the greatest tendency to combine with oxygen, and therefore burns first, leaving the less combustible carbon in the form of glowing coals. If at this point we take up one of these coals and quench it in water, it will be found to be common black charcoal; but if left on the hearth, the coal also burns, gradually smouldering away, and passing up the chimney as carbonic dioxide gas.

Quite a similar succession of phenomena is presented in the forest during the process of decay. In decay, as in burning, the oxygen of the air unites with the hydrogen of the wood more rapidly than with the carbon, and in consequence the rotten wood becomes darker and darker, from the excess of black charcoal, as the change advances. Moreover,

if the supply of air is insufficient, as when the wood is buried in swamps, it is finally reduced to coal, which corresponds to half-burnt wood. In the open air, however, the charcoal as well as the hydrogen is burnt, and the log of wood is resolved, as in ordinary combustion, into carbonic dioxide and water, leaving only a few handfuls of earth to mark the spot where it lay. This change requires years before it is fully consummated, and it is not therefore wonderful that its nature should not have been understood until a comparatively recent period. Thanks to modern chemistry, the subject is now less obscure. We may not be able to trace all the steps of the process, but this much we know. Decay and burning are essentially the same chemical change. The substances involved are the same, the results are the same, and we have even been able to prove that the amount of heat generated is the same, the only difference being, that, in burning, the whole amount of heat is set free in a few hours, producing phenomena of intense ignition; while in the process of decay the same quantity, slowly evolved during perhaps a century, escapes notice.

It has been observed, that, if wood be left in contact with dry oxygen, it may be kept indefinitely without undergoing change,—a fact sufficiently proved by the mummy cases of Egypt, which in that dry climate have been preserved for over three thousand years;—also, that if wood is impregnated with certain salts, as in the process of Kyanizing or Burnetizing, decay may be arrested, even in a damp situation, for a long time. In both cases the prevention

depends on destroying certain very unstable compounds which are present in all green wood, and which start the decay. These are termed by chemists albuminous substances, the chief of which, vegetable albumen, is almost identical with the white of an egg. The great bulk of all vegetable structures, as was stated above, consists of only three elements, carbon, hydrogen, and oxygen; but these albuminous substances—which, as a rule, are present only in very small quantities—contain, in addition to the three just mentioned, a fourth element, nitrogen, Partly because they contain nitrogen, and partly, unquestionably, in consequence of the complex manner in which the four elements are combined, the albuminous substances are vastly more unstable than the great mass of vegetable matter, and in the presence of moisture they soon undergo an internal change, called putrefaction, or fermentation, by which they are broken up into simpler compounds. The precise nature of the process is not understood, but nothing appears to be added to the substance, unless it be water, and the change seems to consist in the falling to pieces of a complex organic structure. At all events, oxygen gas is not essential to the process, but the oxygen of the air which happens to be in contact with the fermenting substances, in some mysterious way, undergoes a remarkable change. It becomes endowed with active properties even at the ordinary temperature, and, with its affinities thus exalted, slowly consumes the wood, together with all other organic compounds present. Moreover, the process, once started, sustains itself. As, in burning, the union of the combustible matter with oxygen engenders sufficient heat to maintain the surrounding gas in its highly active modification, so in like manner the process of decay seems to modify continually the neighboring oxygen, arousing its energies, and thus continuing the change when once begun.

While the plant is in great measure made up of non-nitrogenized substances, the animal, on the other hand, consists almost entirely of albuminous compounds. The flesh, the nerves, and the bones of our bodies all contain nitrogen, and, like the vegetable albumen, are prone to decay; and this change is constantly going on in our living members. In a most profound sense, "in the midst of life we are in death.'' The materials of our bodies are being constantly renewed, and the great mass of their structure changes in less than a year. [*] At every motion of your arm, and at every breath you draw, a portion of the muscles concerned is actually burnt up in the effort. During life, in some utterly mysterious manner, beyond the range of all human science, the various gases and vapors of the atmosphere, together with a small amount of a few earthy salts, are elaborated into various organized structures. They first pass into the organism of the plant, and

thence are transferred to the body of the animal; but no sooner are they firmly built into the animal tissues, than a destructive change begins, by which before long they are restored to the air or the soil, only to renew the same cycle of ceaseless change. Life, during its whole existence, is an untiring builder, the oxygen of the atmosphere a fell destroyer; and when at last the builders cease, then the spirit takes it heavenward flight, and leaves the frail tenement to its appointed end. Dust returns to the dust, and these mortal mists and vapors to the air.

I know that there are some who entertain a vague fear that these well-established facts of chemistry conflict with one of the most cherished doctrines of the Christian faith; but so far from this, I find that they elucidate and confirm it. I admit that they do disprove that interpretation frequently given to the doctrine of the resurrection, which assumes that these same material atoms will form parts of our celestial bodies; but then I find that this interpretation is as much opposed to Scripture as to science. The Saviour himself, in his reply to the incredulous Sadducees, severely rebuked such a material conception of his spiritual revelation, and the great Apostle to the Gentiles, in his vision of the glorified body, distinctly declares that this body is not the body that shall be; but that, as the grain sown in the furrow rises into the glory of the full-eared corn, "so when this corruptible shall have put on incorruption, and this mortal shall have put on immortality,'' our natural body, sown in dishonor and weak

ness, will be raised a spiritual body, clothed in glory and in power. "And as we have borne the image of the earthy, we shall also bear the image of the heavenly.''

The glorious doctrine of the resurrection here presented, modern scientific discoveries most fully confirm. They have shown that our only abiding substance is merely the passing shadow of our outward form, that these bones and muscles are dying within us every day, that our whole life is an unceasing metempsychosis, and that the final death is but one phase of the perpetual change. Thus the idea of a spiritual body becomes not only a possible conception, but, more than this, it harmonizes with the whole order of nature; and now that we can better trace the processes of growth in the organic world, and understand more of their hidden secrets, the inspired words of Paul have acquired fresh power, and convey to us a deeper meaning than they ever gave to the early Fathers of the Church. It is no wonder that, when men were less enlightened, the doctrine should have been misinterpreted; but now, when the truth has been illuminated by the study of nature, why longer harass the understanding and vex the spirit with these material clogs? Hear again the words of the Apostle: "This I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption.'' "For this corruptible must put on incorruption, and this mortal must put on immortality.'' And now, turning to the glorious truth as Christ revealed it and Paul preached it, how greatly is our faith

strengthened by these lights of nature! All philosophy assures us that the finite and limited can be manifested only under form.

Chemistry has shown us that it is the form alone of our mortal bodies which is permanent, and that we retain our personality under constant change; and lastly, in organic nature, the sprouting of the seed, the breaking of the bird from the egg, the bursting of the butterfly from the chrysalis, and ten thousand other transmutations not less wonderful, which we are daily witnessing around us, all unite their analogies to elucidate and confirm the glorious and comforting doctrine of a material resurrection in form.

Moreover, when we remember that our organs of vision and hearing are capable of receiving impressions either of light or sound only when the rapidity of the undulations which cause them is comprised within certain very narrow limits, and when we recall the facts stated in a previous chapter, that there are waves of light and sound of which our dull senses take no cognizance, that there is a great difference even in human perceptivity, and that some

men, more gifted than others, can see colors or hear sounds which are invisible or inaudible to the great bulk of mankind, you will appreciate how possible it is that there may be a world of spiritual existence around us—inhabiting this same globe, enjoying this same nature—of which we have no perception; that, in fact, the wonders of the New Jerusalem may be in our midst, and the song of the angelic hosts filling the air with celestial harmony, although unheard and unseen by us. Let me not be understood as implying that science has in any sense revealed to us a spiritual world, or that it gives the slightest shadow of support to those products of imposture, credulity, and superstition, which, under the name of witchcraft, mesmerism, or spiritualism, have in every age of the world deceived so many. The only revelation man has received of a spiritual existence is recorded in the Bible; but modern science has rendered the conception of such an existence possible, and in this way has removed a source of doubt. The materialist can no longer say that the spiritual world is inconceivable; for these discoveries show that it may be included in the very scheme of nature in which we live, and thus, although science may not remove the veil, it at least answers this cavil of materialism.

Returning now to the main subject, consider for a moment the importance of this ghostly office of oxygen in the scheme of organic nature. Reflect how soon this fair world would become a great charnel-house were it not for these provisions, by which its youth is constantly renewed. Remember

also that this process of decay furnishes the materials from which young life builds her fresh and blooming forms; that, although in the midst of life we are in death, it is equally true that death is only a phase of life. Then these changes of outward nature will assume a new aspect. It will be seen that they are the beneficent provisions of infinite wisdom, in themselves full of interest and beauty, and only sad and melancholy as they are associated with bereaved affections and disappointed hopes, or with that only real death, the moral death of the soul. "O death, where is thy sting? O grave, where is thy victory? The sting of death is sin; and the strength of sin is the law. But thanks be to God, which giveth us the victory through our Lord Jesus Christ.''

I might profitably occupy several hours in describing the various processes of slow combustion, for they are all rich in illustrations of skilful design; but I must content myself with only one other example, and from the many which crowd upon me I have chosen respiration, because it is so well understood and because it is so intimately associated with our own physical existence. Respiration is a true example of combustion. The seat of the combustion is the lungs. The substance burnt is sugar. The products are carbonic dioxide gas and water.

The materials of animal food may bc divided into three classes: non-nitrogenized substances, such as starch and sugar; nitrogenized substances, like lean meat and eggs; and, lastly, fatty substances, like butter. To these must be added a small proportion

of earthy salts, which, however, as they enter into the composition of almost all varieties of food, do not properly form a distinct class. All of the three classes of food are absolutely necessary to support the life of the higher animals, and especially of man, and they are all contained in those articles of diet which will of themselves alone sustain life. Milk may be regarded as the type of animal food.

Composition of Milk. [*]

               

	Natural State	Evaporated to Dryness
Water	87	..
Curd or casein	4 1/2	34 3/4
Butter or fat	3	23 3/4
Sugar (of milk)	4 3/4	37
Ash (nearly)	3/4	4 1/2
	—	—
	100	100

It contains, in the first place, a non-nitrogenized substance, sugar; in the second place, a nitrogenized substance, casein, which separated from milk forms cheese; and, lastly, a fatty substance, which when separated by churning forms butter.

             

	Wheaten Bread.	Lean Beef.
Water	45	78
Fibrin or gluten	6	19
Fat	1	3
Starch, etc.	48	..
	—	—
	100	100

Bread, again; consists of starch, a non-nitrogneized substance; of gluten, a nitrogenized substance,

and it also contains about two per cent. of a peculiar oil. No article of food which does not contain all three of these classes of substances can alone support life for any length of time. A man would starve to death on starch alone, on meat alone, or on butter alone. The relative proportion, however, in which these three classes of substances are required by man, depends on his outward circumstances, such as the climate, his physical activity, his occupation, or his peculiar temperament, and to the right balance of his food he is guided by experience.

The different classes of food serve different functions in the body. The nitrogenized and a portion of the fatty substances are used to supply the constant waste of the tissues which results from all the animal processes. They are in some unknown way vitalized in the system, and converted into new muscles, tendons, and nerves, which take the place of those that have been used up. On the other hand, the non-nitrogenized substances, such as starch, are supposed to take no part in the formation of new tissues, and to be merely the fuel by which the animal heat is maintained. Let us very briefly follow these substances through the body, and see when and how they are burnt.

By far the greater part of our daily food consists of varieties of starch or sugar. These two substances are almost identical in composition, and starch may be converted into sugar with the greatest ease. Leaving out of view the large amount of water which all our food contains, we find that of wheaten bread no less than 39 per cent. consists RESPIRATION. of starch or sugar; of potatoes fully 92 per cent. is made up of the same materials, and in general they form over four-fifths of the solid part of all our food. These substances when taken into the stomach are almost instantaneously converted by the saliva and the gastric juice into the variety of sugar known as grape-sugar, so called because it is the sweet principle of ripe grapes. The sweet principle of honey and molasses, and the incrustation which is so frequently seen on figs and raisins, are also essentially the same substance. Grape-sugar, being very soluble, dissolves in the water present, and the solution is absorbed by the veins which ramify on the surface of the intestinal canal, into which the digested food passes from the stomach. The blood, now containing sugar in solution, returns through the liver to the right side of the heart, and by this organ, which consists essentially of two ingeniously contrived force-pumps, arranged side by side, it is forced through the lungs, where the sugar is brought in contact with the air. Let us next examine for a moment this remarkable structure.

The lungs, as is well known, consist of two large organs, on either side of the chest, called the left and the right lung. The right lung is divided into three smaller lungs, called lobes, the left into but two. On examining any one of these lobes it will be found to be made up of an immense number of small membranous bags, all closely packed together. These small bags, called cells, connect by means of the bronchial tubes and windpipe with

the air, through the nose and mouth. They vary in size, but on an average are about 1/100 of an inch in diameter, and the total number of the cells in the lungs has been estimated at six hundred millions. Their walls are exceedingly thin, and the cells may therefore be easily compressed. The whole mass of the lungs is also exceedingly elastic, and by the action of a system of muscles their volume is alternately increased and diminished in the process of respiration. The amount of air which is thus drawn into the cells, and again expelled at each inspiration, differs in different individuals. The average quantity in the ordinary tranquil respiration of an adult is about a pint; but in a full respiration it may be as much as two and a half pints, and by an effort the lungs may be made to inhale from five to seven pints. As the average in health is about eighteen inspirations a minute, which corresponds to about eighteen pints of air inhaled and exhaled, it follows that three thousand gallons of air pass through the lungs of an adult man every day. Some estimate it as high as four thousand gallons a day for an average man in average circumstances, and as high as five thousand seven hundred gallons a day for an athletic man undergoing severe exertion. In order that you may form an idea of this quantity, I will add that four thousand gallons of air would fill a room measuring about eight and a half feet in each dimension.

Let us now turn to the blood, and examine the apparatus by which it is exposed to the air in the lungs. As we have already seen, the blood

charged with sugar is received into the heart, from whence it is pumped through a long tube, called the pulmonary artery, into the lungs. This artery divides again and again until it is reduced to very small capillary tubes, which ramify on the surfaces of the air-cells. The walls of these capillaries are formed of the thinnest conceivable membrane, so as to bring the blood into as close contact as possible with the air. Here oxygen gas is absorbed in large quantities, and carbonic dioxide gas evolved. The blood now holds in solution at the same time oxygen gas and sugar, and, thus charged, it returns, by a series of veins to the left side of the heart, when by the second of the two force-pumps it is again forced through the general circulation of the body. In the meantime the oxygen absorbed by the blood while in the lungs burns up the sugar. Sugar, like wood, consists of carbon, hydrogen, and oxygen. The last two are present in the proportions to form water, so that sugar may be said to be composed of charcoal and water. Of these two substances the charcoal only is combustible. This, during the circulation of the blood, is slowly burnt up by the dissolved oxygen, and converted into carbonic dioxide, which remains in solution until it is discharged, when the blood returns again to the lungs, or else escapes through the skin.

Thus it appears that respiration is a process of combustion, in which the fuel is sugar, and the smoke carbonic dioxide and aqueous vapor. I need not dwell on a fact so universally known as the

presence of carbonic dioxide in the breath. All, however, may not know how large is the volume of this gas which they daily exhale. It varies with age, sex, food, health, and a variety of other circumstances. In a full-grown man the weight of carbonic dioxide evolved from the lungs varies from one to three pounds in twenty-four hours, which is equivalent to from nine to twenty-seven cubic feet. During the present lecture the amount of carbonic dioxide which has been exhaled into this room by the audience is equal to at least seven hundred and fifty cubic feet,[16] and would fill a room measuring about nine feet in each direction. From the quantity of carbonic dioxide gas exhaled we can very readily calculate the amount of charcoal burnt, which in a full-grown man will vary from five to fifteen ounces in twenty-four hours. Hence, the amount of charcoal which, in the form of sugar, has been burnt up in the lungs of the audience during the last hour, is equal to at least fifteen pounds, [*] which I have had weighed out and placed on the lecture table, in order to give you an idea of the quantity. Moreover, it has been proved that the quantity of heat evolved by a given amount of charcoal in burning is absolutely the same, whether the combustion be rapid or slow, so that the same amount of heat has been generated in our bodies during the last hour by the slow process of respiration as would have been set free by burning this basketful of charcoal. It is no wonder, then, that the temperature of the body is always so much above that of the air, and that even in the coldest climate the heat of the blood is maintained as high as ninety-six degrees. In regulating the temperature of his body, man follows instinctively the same rules of common-sense which he applies in warming his dwellings. In proportion as the climate is cold, he supplies the loss of heat by burning more fuel in his lungs, and hence the statements of arctic voyagers, who have told us that twelve pounds of tallow-candles make only an average meal for an Esquimaux, are not inconsistent with the deductions of science.

Respiration, then, like decay, is a process of slow combustion, in which the oxygen of the air attacks and consumes, even at the ordinary temperature, the sugar in the blood. Let us now compare with it the rapid combustion of the same substance. During this lecture every robust man present has, on an average, burnt up the equivalent of about one ounce of sugar. This combustion has taken place so quietly, and has set free the requisite amount of heat so gradually, that we have not been conscious of it. In the blood, where the burning has been going on, sugar and oxygen, as we have seen, are in close contact. In this crucible I have mixed together just one ounce of sugar and one and one-eighth ounces of solidified oxygen, solidified by the force of chemical affinity and bound up in a white salt called chlorate of potash. The oxygen and sugar are therefore here lying side by side, as in the blood, but the conditions of slow combustion which exist in the body not being

present in the crucible, they will remain in contact indefinitely, until some external agency is applied. The oxygen is now in its passive condition, but a single drop of sulphuric acid will arouse its dormant energies, and you have instantly one of the most dazzling displays of combustive energy which can be produced by art. The only difference between this brilliant deflagration and the combustion which, during the last hour, has taken place in each of our bodies, is simply this: the heat which in the blood has been imperceptibly evolved during an hour, was here concentrated into a few moments, and therefore produced phenomena of intense ignition. All the other conditions,—the material burnt, the quantity of material employed, the products generated, and the amount of heat evolved,—are in both cases essentially the same.

On comparing these two phenomena together, reflect for a moment on the false estimate which we are apt to make of the phenomena of nature. The splendid displays of combustion arrest our attention by their very brilliancy, while we overlook the silent yet ceaseless processes of respiration and decay, before which, in importance and magnitude, the grandest conflagrations sink into insignificance. These fire but the spasmodic efforts of nature; those, the appointed means by which the harmony and order of creation are preserved. Those of us who have merely studied the brilliant phenomena of nature appreciate but imperfectly the grandeur of its forces, and "those of us who limit our appreciation of the powers of oxygen to the energies displayed by this

element in its fully active state, form but a very inadequate idea of the aggregate results accomplished by it in the economy of the world.''[17] Contemplate the amount of oxygen employed in the function of respiration alone. Faraday has roughly estimated that the amount of oxygen required daily to supply the lungs of the human race is at least one thousand millions of pounds; that required for the respiration of the lower animals is at least twice as much as this, while the always active processes of decay require certainly no less than four thousand millions of pounds more, making a total aggregate of seven thousand millions of pounds required to carry on these processes of nature alone. Compared with this, the one thousand millions of pounds which, as Faraday estimates, are sufficient to sustain all the artificial fires lighted by man, from the camp-fire of the savage to the roaring blaze of the blast-furnace or the raging flames of a grand conflagration, seem small indeed.

Amount of Oxygen required Daily. [*]

           

Whole population	1,000,000,000
Animals	2,000,000,000
Combustion and fermentation	1,000,000,000
Decay and other processes	4,000,000,000
	—
Oxygen required daily	= 8,000,000,000 lbs.

       

Tons.

3,571,428 in a day.

1,304,642,357 in a year.

Whole quantity, 1,178,158,000,000,000.

How utterly inconceivable are these numbers, which measure the magnitude of nature's processes, —eight thousand millions of pounds of oxygen consumed in a single day! When reduced to tons, the numbers are equally beyond our grasp, for it corresponds to no less than 3,571,428 tons. If such be the daily requisition of this gas, will not the oxygen of the atmosphere be in time exhausted? It is not difficult to calculate approximately the whole amount of oxygen in the atmosphere. It is equal to about 1,178,158 thousand millions of tons; a supply which, at the present rate of consumption, would last about nine hundred thousand years. We need not, therefore, fear that the amount of oxygen in the atmosphere will be sensibly diminished in our day or generation; but then this period, immense as it is, is not to be compared with the ages of geological time. The time which has elapsed since the coal we are now burning was deposited in its beds is to be counted by many millions of years, so that since the coal epoch the oxygen of the atmosphere must have been all consumed again and again. Why, then, has it not all been removed from the atmosphere? Simply because, in the beautiful balance of creation, there is always some recuperative process for every such loss. In the case before us, it is, as we have seen, the vegetation. As fast as our

breath, our fires, and the process of decay around us are removing the life-giving oxygen, just so fast it is restored by every green leaf which waves in the sunshine, and by every blade of grass which sprouts under our feet. What the animal removes, the plant restores.

I have before stated that, in the process of decay, the oxygen of the atmosphere, which is active in producing the change, is undoubtedly in a peculiarly modified condition, a condition in which its affinities are highly exalted even at the ordinary temperature of the air; and I also stated that this active condition of the element is apparently maintained by the process of decay itself. This subject has been greatly elucidated by modern discoveries. Of all the known processes of slow combustion, the simplest and the most active is the slow combustion of phosphorus. This familiar substance, used to tip the ends of lucifer matches, if exposed to the moist air, slowly combines with oxygen, shining at the same time in the dark with a peculiar phosphorescent light, whence the name of the substance, from two Greek words, signifying light-bearer. The process is therefore entirely analogous to decay and respiration; but since phosphorus is a chemical element, the change is far simpler, and can be more readily studied, and for this reason it may serve to elucidate those more complex processes of nature.

Some years since, Professor Schönbein, a distinguished Swiss chemist, discovered that, while a stick of phosphorus was slowly burning in a jar of moist air, a portion of the oxygen present underwent a

most remarkable change. Without entering into the details of these experiments, I will simply state that, when thus modified, ordinary oxygen seems entirely transformed. The great mass of the oxygen of the air, as you will remember, is wholly devoid of odor, and without action on the most delicate organic structures or the most fleeting vegetable colors; but when thus treated it acquires a very strong and pungent odor, rapidly rusts polished metals, excites decay in organized tissues, and at once bleaches the most permanent dyes. Could there be a more complete inversion of properties? One of the most striking characteristics of this new modification of oxygen is its peculiar odor, and hence Schönbein calls it ozone, from a Greek verb signifying to smell. It frequently happens that a great discovery supplies the wanting links between a number of obscure facts, and thus adds quite as much to our knowledge by its indirect bearings as by the positive additions it makes to the general stock. So it has been with the discovery of ozone. Every one who has used an electrical machine must have noticed the peculiar smell which follows the electrical discharge. This was formerly supposed to be the odor of the electrical fluid itself; but as soon as ozone was discovered, the odor was recognized at once as belonging to this new agent, and it was soon ascertained that electricity is one of the most efficient means of modifying the oxygen of the air.

Returning now to the fact that the slow combustion of phosphorus throws a portion of the surrounding oxygen into a peculiar condition, in which it is

highly active in producing decay and other processes of oxidation,—it certainly seems probable that decay and respiration, which are also examples of slow combustion, may act on the air in the same way. Moreover, the inference that ozone is the active agent in these processes is also supported by the fact that it is always present, to a greater or less extent, in the atmosphere, although, at most, in exceedingly minute quantities. Ozone, being so highly corrosive, cannot be present in the atmosphere in perceptible quantities without producing important effects, and some persons have thought not only to refer to it the various processes of slow combustion, but also to trace a connection between the prevalence of various contagious diseases and the excess or deficiency of this agent in the air of the infected district; but these speculations are not as yet based on sufficient evidence, and are not worthy of serious attention.

Without, however, introducing any theories not yet fully established into the line of our argument, this much is clear. Oxygen gas appears in nature in three conditions, or under three manifestations:— first, entirely passive, as in the great mass of the air; secondly, partially active, in the processes of decay and respiration; thirdly, highly active, in the phenomena of combustion. In each of these conditions its properties have been adjusted with infinite skill and delicacy, on the one hand to the thermal and electrical conditions of the globe, and on the other, to the constitution of man and of all organic nature.

Here I must conclude my brief sketch of this wonderful element. If I have succeeded in impressing on your minds some of its more characteristic qualities, if, above all, you have become aware how exactly and delicately these qualities have been adjusted in the scheme of creation, and if you have seen how the smallest permanent change would disturb the result,—this is all that I could hope. It might be expected that the element with which creative power built up the greater part of the crust of our globe, leaving only a small excess to constitute its atmosphere, would furnish abundant evidence of design, and how fully is this expectation realized! Would that I might present to you the evidence more forcibly! But it is possible in a popular lecture only to touch at some of the more striking points, and I have felt all the time like a schoolboy at play, in spring, in some garden rich in flowers, snatching here and there a few of the more gaudy tulips, which had fully bloomed, but leaving the beautiful and delicate buds all unnoticed. But then these buds of knowledge will blossom, and, when the summer comes, will bear a still sweeter testimony of goodness and of love.