suddenly through the intervening stratum of air to the nearest con ducting substance, producing the well known phenomena of lightning and thunder. It is known that the Leyden vial cannot be charged, nor the charge retained, in a vacuum; and it is very certain that but for the nonconducting power of the air, the phenomena of lightning and thunder would be entirely unknown, nor could any of the benefits resulting therefrom, at present indeed in a measure concealed from our view, ever be experienced. Thus far we have confined our remarks to the mechanical effects of the atmosphere, but in its chemical composition and relations we shall find not less to admire. This fluid is composed of two gases, oxygen and nitrogen, in the ratio of about twenty parts of the former to eighty of the latter. Of these two simple substances, there are no less than five other well determined compounds, differing in the relative proportion of the ingredients, all of which are essentially different in their nature from the one under consideration. All of them are dis. tinguished for their energetic and even destructive action upon the animal system. There has been some difference of opinion whether air is to be considered as a chemical compound of these two substances, or only as a mixture, but the latter seems more generally to prevail. It matters not to us for our present purpose which is correct. 8. The first important particular that strikes our attention in examining the chemical constitution of the atmosphere is the peculiar ingredients of which it is composed. These, we have already remarked, are oxygen and nitrogen; and they are the only substances known in nature that would answer the purpose. Oxygen appears to perform by far the most important offices, the nitrogen, so far as we at present know, serving only to dilute it; but as the latter substance, as well as oxygen, enters largely into the composition of nearly all animal and some vegetable substances, it is highly probable it serves some important purposes not yet discovered. It is by means of the oxygen contained in the air that both respira. tion and combustion are supported; and though a kind of combustion would in some cases be produced if the oxygen in the atmosphere was replaced by chlorine, or some other gas, yet no other gas known will support respiration. The blood in its perpetual course through the system, yielding to every part that support, whatever it may be, with out which the vital principle cannot be retained, undergoes a most important change, and becomes absolutely destructive of life unless thoroughly renovated by coming in contact with the oxygen of the atmosphere in the lungs. This renovation is constantly going on, though the precise changes that take place are not fully understood. By the motion of the chest the air is constantly inhaled and exhaled, a man in health and at rest usually taking in, it is said, at each full inspiration, about forty cubic inches. By examining the air that is exhaled from the lungs, it is found that it contains less oxygen than before, and has become charged with carbonic acid gas, and the color of the blood has at the same time changed from a dark purple to a livid red. If by any means the supply of fresh air is cut off, as is the case when a person is drowned or strangled, the blood passing on through the lungs without the necessary renovation, produces in a few minutes a suspension of most of the powers of life, which in a short time results in death. If within a very limited period the supply of air is restored, and the proper means are used, the functions of life may often be made to resume their wonted regularity. Now no gas except oxygen will serve for a moment the important purposes of respiration; but infinite wisdom has provided that this very gas shall always be present as a portion of the atmosphere; and by an involuntary motion of the chest, the supply is constantly kept up at the precise point where it is needed. It has already been remarked that the nitrogen of the atmosphere, so far as is yet known, seems chiefly designed merely to dilute the oxygen, but it is worthy of remark that it is the only gas in nature that could be made use of for this purpose! Pure nitrogen seems to exert no direct influence upon the system when taken into the lungs, but the individual experiences a sense of suffocation for want of the supply of oxygen; any other gas would produce at once injurious, if not fatal, symptoms. 9. But oxygen is no less necessary to combustion than respiration. Without it we should be destitute of the means of producing artificial heat, except the slight elevation of temperature that may be produced by mechanical or chemical action. Many of the disastrous consequences that would ensue, were we at once deprived of the means of producing fire, or other equivalent elevation of temperature, are evident at first view, but others become more obvious by a little consideration. To say nothing of the impossibility of inhabiting a large portion of the earth's surface without fire a part of the year, it is evident that without it many of the arts must cease at once, and all of them ultimately; and man, should he find himself capable of existing, would soon return to his former state of barbarism! We see here also another evidence of design in the expansibility of air by the elevation of its temperature. When the air is heated by coming in contact with a burning body, it is expanded, and notwithstanding its actual increase of weight by the union of the oxygen with the combustible matter, it becomes specifically lighter, and rises, giving place to a new supply of fresh air to yield its oxygen to the burning body. Precisely the same remark might be made with reference to respiration. Did air not possess this property, that portion surrounding a burning body would soon be deprived of its oxygen and become charged with carbonic acid; and there being no means to remove it, and furnish a fresh supply, the combustion would soon be checked and at length entirely cease. In respiration the same effects would follow, unless animals should keep constantly in motion, which however, if perpetual, might perhaps obviate the difficulty. 10. The proportion of these two gases contained in atmospheric air may also be adduced in proof of the divine wisdom. Though oxygen is absolutely necessary to support respiration, it has been proved by experiment that in a state of purity its action upon the system is too powerful to be long continued without producing injurious effects. The present exact ratio of their quantities is not perhaps essential to animal existence, but it seems well estab lished, as the result of much investigation, that the present uniform supply of vital air is best adapted to promote continued health and vigor. Should the relative proportion of oxygen be increased, the effects at first would be stimulating and agreeable, but in the end it would produce languor and debility. If the present relative proportion of oxygen was diminished, the reverse effects would be produced. The blood not being sufficiently renovated as it passed through the lungs, the system would soon lose its elasticity and vigor, and man would become unfit to attend to the duties of life; and if the diminution of oxygen was but slight and very gradual, a long continued decline would terminate in death. If the relative quantity of oxygen should be considerably diminished, the effects would of course be more rapid. The present relative proportion of these gases is also, all things considered, best adapted for purposes of combustion. Was the proportion of oxygen in creased, combustion would in many cases be too violent, and even dangerous. What, for instance, would be the result if a building in a large city should take fire was the atmosphere composed of pure oxygen? Utter destruction would be the almost certain consequence to the whole city. Indeed, it is not probable, if the atmosphere was composed of pure oxygen, that any combustible matter would long be preserved upon the face of the earth. Nor would this remark be confined to those substances usually considered combustible: many of the metals burn freely in oxygen gas; and when once heated by any means to ignition, combustion would not cease until they were entirely consumed. On the other hand, if the quantity of oxygen gas was less than it is, combustion would be too languid; and, in many cases, could not be kept up without difficulty; nor could the elevation of temperature desirable in many cases be obtained. 11. Besides these two gases and watery vapor which have been spoken of as composing the atmosphere, a small per centage of carbonic acid is always found to be present. We have before alluded to this gas, and the question may have occurred, if combustion and respiration, which are perpetually going on, both require a constant supply of oxygen from the air, and both constantly yield carbonic acid, may we not naturally expect the proportion of the latter will gradually increase, while that of the former will diminish? And may not all the oxygen at length entirely disappear? These are important and interesting questions, and have been most satisfactorily answered by numerous and skilful experimenters. Carbonic acid gas does not seem to form a necessary ingredient of the atmosphere, but is considered as a foreign body, though it is said to be always present. Portions of air have been obtained from the most elevated regions of the atmosphere that man has been able to attain, and from almost every other imaginable situation, in all of which the oxygen and hydrogen are found in the same relative proportion, with carbonic acid gas as a never failing attendant. The relative quantity of this latter gas, however, is found to vary considerably in different places, and at different times in the same place. Experiments generally indicate a larger proportion in the same place in summer than in winter, in the night than in the day time. So also more is usually found in the atmosphere of large cities than in that of the open country. By a long course of experiments made by Sausure in the year 1828, in the city of Geneva. in Switzerland and vicinity, it appears the proportion of this gas there is seldom less than three parts in ten thousand of air, nor greater than six. Probably the result would not be essentially different were exact experiments made in other places. The means, so far as they have been determined, by which nature preserves constantly the necessary proportion of oxygen in the atmosphere, without either increase or diminution, are no less wonderful than the fact. Carbonic acid gas, even when much diluted with air, is extremely injurious, when taken into the lungs of animals, but in the minute proportion in which it is found in the atmosphere, it is probably rather beneficial, and to promote healthy vegetation, it is absolutely essential. The presence of oxygen seems to be one of the essential requisites to produce the healthy germination of the seeds of vegetables, but without a supply of carbonic acid they can never come to maturity. In plants, as in animals, a kind of respiration is ever going on in the leaves, which seem to serve the purposes of lungs, the sap answering to the blood. The sap, as it passes through the leaves, absorbs carbonic acid and gives out the oxygen, while it retains the carbon* for the nourishment of the plant; thus producing a change in the atmosphere precisely the reverse of that produced by the respiration of animals, and by combustion. The effect, however, seems to be in some degree dependent upon the influence of light, as in the dark, plants absorb oxygen and give out carbonic acid, but the quantity of oxygen given out in the day is much greater than that of carbonic acid given off in the night. * Carbonic acid is composed of carbon and oxygen, in the ratio of six parts of the former to sixteen of the latter. How complicated and wonderful these changes, and yet how admirably adjusted to each other! We are accustomed to look with admiration at the nice adjustments in the various motions of the heavenly bodies, and adore that wisdom which could originate and perpetuate so great and complicated a system; but the means by which animals and vegetables are reciprocally made to perpetuate the existence of each other, each supplying to the atmosphere that without which the other could not exist, but an over supply of which would necessarily cause the destruction of both, are scarcely less worthy of our regard. How beautiful, in view of this subject, appears the language of the poet when comparing the divine and human agencies: "In human works though labored on with pain, Yet serves to second too some other use!" 12. Nor is it an unimportant fact that the nature of the atmosphere is such as usually to prevent the carbonic acid it contains from collecting together in places, so as to become injurious or destructive to life. This gas is much heavier than atmospheric air, and we might therefore expect it would all settle down to the lower regions of the air, but contrary to this, nature has provided that it shall be uniformly diffused, or nearly so, through every part. This property, however, it possesses in common with all gaseous bodies, as it is found that if any two gases, however different may be their specific gravities, and even when the heavier is placed lowest, communicate together, each will gradually diffuse itself through the whole mass of the other. Thus, if two tall jars be filled, one with hydrogen and the other with chlorine, which is thirty-five times as heavy as the former, and the jar containing the hydrogen be suddenly inverted over the other, the contents of the two being allowed to come in contact by a narrow opening, it is found that in a very short time each of the gases will be uniformly diffused through the whole space; the chlorine having risen and the hydrogen fallen, contrary to the law of specific gravity, that universally prevails in solids and liquids. But the principle is not the less important for being universal with reference to the gases, as in all probability, did the gases follow the usual law of specific gravities, the accumulations of carbonic acid gas that would shortly be formed in places a little below the general level, would be destructive to life, if indeed the whole earth did not soon become uninhabitable. Limited accumulations of this gas are now sometimes formed, but always in consequence of some local cause, and then not by deposition from the atmosphere. The Grotto del Cane* in Italy is a familiar instance of the kind, that will instantly occur to every one. Here the gas is formed in the earth, probably by volcanic agency, and is constantly issuing from an aperture and collecting in a little valley that has received this name from the fact that a dog thrown into it is instantly suffocated, though a man may walk through it with safety. The reason is because the gas constantly issuing collects only just at the surface; and should the supply fail, the whole would shortly be diffused through the atmosphere. In wells and caves, too, carbonic acid is often found to accumulate, in consequence of decaying animal and vegetable matter in the vicinity from which it is supplied faster than it can diffuse itself abroad. In these instances, likewise, the collection is protected more or less from the influence of the wind. Besides the gases mentioned, other substances are occasionally present, as the effluvia of odoriferous substances and gases given off during spontaneous and other changes, but, in most countries, seldom in any considerable quantities, and only when supplied by some cause entirely local. This naturally suggests, 13. The last topic upon which I intend to remark in reference to this subject, viz., the general absence from the atmosphere of deleterious substances. This is a point of importance, for we can examine the substances * For a description of the Grotto del Cane, see Fisk's Travels, page 214. we eat and drink, but the air must be taken into the lungs, those delicate and vital organs, usually with little or no examination. Was it therefore liable to be charged with deadly effluvia, we should be subjected to constant danger and fear, as analyses of air for ordinary purposes of breathing would evidently be impossible. There are indeed some well known facts that at first might seem to be against my last proposition, as in many new countries, and sometimes in others, the atmosphere does seem to be charged with a miasma that gradually affects those who have not become accustomed to its influence, producing intermittent and other fevers, and gradually sapping the foundations of life. In all countries, indeed, contagious diseases seem to be propagated by means of effluvia conveyed in the atmosphere. Some have positively affirmed they have succeeded in collecting small portions of the matter in the air, but others seem to doubt with regard to it. It must, however, be admitted, that the atmosphere is sometimes made the vehicle of conveying far and wide the seeds of disease and death. So also, when put in violent motion, constituting the hurricane or tornado, it sometimes sweeps every thing before it, desolating in a few moments perhaps some of the finest portions of the earth's surface. And in various other ways the atmosphere certainly appears to us to be made the source of evil as well as of good. But this is no more than can be said of every blessing with which the Creator has favored us. Indeed, nothing is more evident than that the present system, with all the evidences it affords us of the wisdom and benevolence of its Author, is not a system of optimism; or it does not so appear to us with our present means of judging. Amid all the blessings which a kind Providence has so profusely lavished upon us, we are subjected to numerous evils from which we cannot free ourselves. We are exposed even to death itself, and constantly "Fierce diseases wait around To hurry mortals home." Constituted as man is, it might not perhaps be difficult, even in view of enlightened science alone, to vindicate the divine wisdom and benevolence in permitting the occurrence of all the natural ills (and we have nothing to do here with moral evil) which "flesh is heir to," by showing that taking into consideration the great whole, with reference to which the Creator acts, he is ever "From seeming evil, still educing good;" but we leave the task for the professed moralist. It has been our object to show the admirable adaptation of atmospheric air to serve the purposes for which it is designed in the various parts of creation, and particularly in promoting the convenience and happiness of man, and thence infer the wisdom and benevolence of its great Author. And though in doing this we may have found evidence that "this is not our home," that here we have no abiding place, yet we see indications, even in this respect, of a perfect conformity to other parts of the great system of nature; and in view of the great whole, while our love for the study of this system cannot but be increased, shall we not in the beautiful language of the poet, though perhaps in a modified sense, be led to exclaim, "Flee, flee, ye mists! let earth depart; Thou art the source, the guide, the goal; |