still remain sufficient oxygen to permit a candle to burn in it, and even to permit continued respiration if the carbonic acid be removed. The presence of a certain amount of carbonic acid in the air prevents the exhalation of that gas from the blood. As we read the explanation, nothing can seem clearer, and we admire the skill with which the laws of the absorption of gases are brought to bear on the fact. But as we pursue our researches, various difficulties arise; and as we extend the inquiry from the respiration of warm-blooded to that of coldblooded animals, we learn that the fact so luminously explained is not at all true of the simpler organisms. Let us for a moment consider one striking contradiction in the theory; the air which has once passed through the lungs of a man, and which, in losing four or five per cent of its oxygen, has become charged with three or four per cent of carbonic acid, will yield but very little of its remaining oxygen when again passed through the lungs; and if this air be breathed over and over again, until the sense of suffocation forces a cessation, the air will still be found to contain ten per cent of oxygen-that is to say, nearly half its original quantity. In air thus vitiated, the respiratory process is impossible, but only impossible for warm blooded animals in health: frogs, reptiles, fish, and molluscs, instead of perishing when the air has lost about half its oxygen, continue to breathe, and to absorb oxygen, almost as long as there is any left. Spallanzani, Humboldt, and Matteucci, have placed this beyond a doubt by their experiments; and when we consider how long these experiments have been before the world, it is a matter of surprise that the contradiction they give to all the purely physical theories of Respiration has not been insisted on. If the process depends simply on the proportion of gases in the atmosphere, how is it that one animal can continue to breathe in an atmosphere irrespirable by another? If it be simply the interchange of oxygen and carbonic acid, and this interchange be frustrated whenever eleven per cent of oxygen has disappeared, the law must be absolute, and as applicable to reptiles and molluscs as to birds or mammals. Instead of this, we find that reptiles can continue to breathe long after such a limit has been passed; they continue to absorb oxygen as long as even only three per cent remains, in spite of the continually increasing proportion of carbonic acid. How is it that the physical laws of absorption frustrated the Respiration of one class of animals, and were powerless with another class? Why is it that, when a bird and a frog are confined in the same vessel, the frog will continue to absorb oxygen from the vitiated air in which the bird has long perished? Clearly the cause of this difference lies in the difference of the organisms; and we must no longer seek in the mere quantities of gases an explanation of interrupted respiration; we must no longer say that "breathing becomes impossible when the air is charged with a certain amount of carbonic acid, because that amount prevents the gaseous interchange ;" but we must say that such an amount prevents the gaseous interchange, because it interferes with the organic action of the pulmonary apparatus. The distinction becomes palpable when we have an organism which is not affected by this amount of carbonic acid, and is even more palpable when we see a warmblooded animal capable of breathing for a long period the air which, under a dif ferent condition, it would find irrespirable. We have seen how a bird, with its functions depressed, can continue to breathe for an hour in an atmosphere which immediately suffocated another bird of the same species; whereby it became clear that the lungs of one warm-blooded animal could absorb oxygen from an atmosphere in which there was such a proportion of carbonic acid, that sufficient oxygen could not be absorbed by a vigorous animal of the same species. The intervention of organic conditions, modifying the simple physical laws of gaseous exchange, is sufficiently evident from what has just been said; but we have as yet no clear insight into the nature of this intervention; we do not know why blood, charged with carbonic acid, can not in the one case exchange that gas for the oxygen, of which 10 per cent still remains, since in another case the same blood can effect the exchange when there is even less than 10 per cent of oxygen. Atmospheric air contains only 21 per cent of oxygen. But if 50 per cent of oxygen be mixed with 50 of carbonic acid, a warm-blooded animal is suffocated in it, in spite of there being more than double the amount of oxygen there is in ordinary atmosphere. Bernard, who made the experiment, thinks that the carbonic acid | found 20 and 17 of oxygen replaced by 3 of carbonic acid; in two others, 20 and 19 by 4 and 8 of carbonic acid; clearly showing that the exhalation had been one process, and the absorption another. in this mixture prevented the oxygen from entering the blood, not only because of its greater solubility, which gives it a tendency to displace the oxygen, but also be cause of the obstacle it presents to the exhalation of carbonic acid. On the other hand, the extensive and careful experiments of Regnault and Reiset show that Respiration will take place quite well in an atmosphere which contains as much as 23 per cent of carbonic acid, if at the same time it contains as much as 40 per cent of oxygen. How are we, on physical principles, to reconcile such facts as those just cited? In the one case we see that 50 per cent of oxygen is insufficient if the amount of carbonic acid be also 50 per cent; in another case we see that 40 per cent of oxygen suffices if the carbonic acid do not exceed 23 per cent; and we could explain both by saying, that unless the amount of oxygen nearly doubles that of carbonic acid, respiration is impossible, were it not for the irresistible objection that reptiles breathe in an atmosphere which has become charged with carbonic acid, and has gradually lost all but 3 per cent of its oxygen. We have raised difficulties which we can not pretend to remove. It is enough to have called attention to the physiological problem involved, as a justification of our skepticism in presence of the physical explanations. Respiration is not a simple interchange of gases, but an organic function, which chiefly consists in exhaling carbonic acid and absorbing oxygen; whatever interferes with the exhalation or the absorption, checks Respiration, no matter what may be the condition of the atmosphere. As a final proof of the correctness of this conception, we will add that oxide of carbon, by preventing the exhalations of carbonic acid from the blood, prevents all Respiration, whatever amount of oxygen may be in the air. Moreover, experimenters are now agreed that there is no accurate correspondence between the amounts of oxygen absorbed and carbonic acid exhaled, as there ought to be were the process one of simple exchange. Spallanzani placed four couples of snails in four separate vessels containing atmospheric air; he found that two of these couples absorbed 20, one 191, and the fourth only 17 of oxygen, but that the amount of carbonic acid exhaled was strikingly at variance. In two vessels he VOL XLV.-NO. IIL If we have gained some idea of Respiration, we shall be able to understand what Suffocation is, and why carbonic acid in the air is so injurious. Carbonic acid is. not a poison, as was formerly maintained. Its accumulation in the blood is only fatal when there is such an accumulation in the atmosphere as prevents its exhalation; its mere presence seems to be quite harmless even in large quantities, provided always that it be not retained there. Carbonic acid, when absorbed into the blood, which is alkaline, can not there exert its irritant action as an acid, because it will either be transformed into a carbonate or be dissolved. Bernard has injected large quantities into the veins and arteries and under the skin of rabbits, and found no noxious effect ensue. The more carbonic acid there is in the blood, the more will be exhaled, provided always that the air be not already so charged with it as to prevent this exhalation. Oxide of carbon seems, however, to be truly a poison. The blue flame which rises from the coals or lighted wood is this same oxide, the product of an imperfect combustion; and being notoriously poisonous, it has by some writers been selected as the real agent in those numerous deaths by asphyxia, occurring from voluntary and involuntary exposure to the fumes of charcoal in closed chambers. Carbonic acid was said to be innocent, and oxide of carbon had to bear the whole infamy. There is no doubt, however, that although carbonic acid is not a poison, it will produce asphyxia, and deaths from charcoal-fumes may occur either from this asphyxia or from poisoning by oxide of carbon, or from a conjunction of the two. Oxide of carbon is truly called a poison, because its action is deleterious even in slight doses, no matter what may be the state of the atmosphere; but carbonic acid is only deleterious when the quantity in the atmosphere is such that the absorp tion of oxygen is frustrated. But how does this oxide of carbon act? If venous blood be exposed to it, we see at once the change into scarlet blood take place. acts on the blood like oxygen? you will ask. Not precisely; for if venous blood be exposed to oxygen, it becomes 24 scarlet; but when left to itself, it becomes | mint was now placed, where it flourished, black again, (except at the surface,) proba- and so completely revivified the air, by bly because the oxygen has gradually absorbing its carbonic acid and giving formed carbonic acid. After exposure to out oxygen, that mice could again breathe oxide of carbon, the blood remains scarlet in the one, and a candle burn in the other. for days and days-nay, even for weeks, In these experiments we seem to have a according to Bernard. Prussic acid acts demonstration of the identity of Combusin a similar manner. Poisoning by prussic tion and Respiration-and this, indeed, acid or oxide of carbon, mag be detected was the conclusion drawn; but that the by this scarlet color of the venous blood. conclusion is erroneous, appears from the experiments of Claude Bernard, who takes a bell-glass containing an atmosphere of 15 per cent of oxygen, and 2 per cent of carbonic acid-the rest of the oxygen having disappeared to form water with the hydrogen of the candle which has just gone out. In this atmosphere, in which a candle will not burn, a linnet will breathe at ease for some time. He reverses the experiment, and makes an atmosphere in which a candle will burn, but in which an animal instantaneously perishes an at The effect of oxide of carbon is to render the blood-discs incapable of that process of exhalation, on which, as we have seen, the activity of the organism depends. The blood, to all appearance, preserves its vitality, for neither the form nor the color of its discs is altered; but the blood is really dead, because its restless changes are arrested. Ever wonderful is the fact constantly obtruding itself upon us, that Life is inseparably linked with Change, and that every arrest is Death. Only through incessant destruction and recon-mosphere composed half of oxygen and struction can vital phenomena emerge, an half of carbonic acid, in which a candle ebb and flow of being. The moment we will burn better than in the air, because preserve organic matter from destruction, of the greater amount of oxygen; but in we have rendered it incapable of the rest- which the animal perishes, because, in less strivings of Life. A spirit like that spite of the amount of oxygen, that oxyof Faust seems ranging through all mat- gen can not be absorbed. The bird, when ter; and if ever it should say to the pass- about to expire in vitiated air, will be reing moment, "Stay! thou art fair," its ca- called to life if the carbonic acid be rereer will be at an end. moved by the introduction of potashshowing that it is owing to the presence of this carbonic acid that Respiration is impeded; but we can not thus restore the expiring flame of the candle by removing the carbonic acid. Take two bell-glasses, and as soon as the combustion grows feeble, introduce into one glass some potash to remove the carbonic acid; you will, nevertheless, find that the candles in both glasses will go out at the same instant. The experiment is very simple, and its significance is plain. By it we see the difference between Combustion, which is only oxidation, and Respiration, which is not oxidation but exchange. In the combustion of the candle the oxidation is every thing, and no process of exchange takes place. In the breathing of an animal the exchange is every thing. The candle expires because there is not enough oxygen in the air; the animal expires because there is too much carbonic acid in the air. The reader has doubtless often heard, with surprise, that the rusting of iron, the burning of a candle, and the breathing of an animal, are only three forms of the same process, three names for Combustion, or Oxidation. There is a certain fascination in such generalizations, and one always regrets to find them not correct. The rusting of iron and the burning of a candle are indeed two forms of one oxidizing process; but Respiration can no longer be considered as in any sense a process of combustion-it is a twofold process of exhalation and absorption. The interesting experiments of Priestley will enable us to set forth the differences between Respiration and Combustion. He placed mice in a bell-glass, where in due time they were suffocated by the air which they had vitiated; other mice were introduced, and they expired immediately. In another bell-glass a candle went out, after having in its combustion absorbed a part of the oxygen; another burning candle was introduced, and it was at once extinguished by this vitiated air. In both of these vessels some Further, to prove that Respiration is an exchange of gases in the lungs, and not a process of oxidation, we need only refer to the experiments of Spallanzani and W. Edwards-experiments so celebrated, that one is amazed to find one's self citing them in this discussion, which they ought long ago to have closed. These physiologists found that cold-blooded animals will breathe in an atmosphere of pure hydrogen, almost if not quite as easily as in ordinary air; the carbonic acid is exhaled, and hydrogen absorbed.* This proves that carbonic acid preexists in the blood, and is not formed during respiration by the oxygen as it enters; and proves, likewise, that the respiratory process is one of exhalation and absorption, which can take place as well with hydrogen as with oxygen: and we are thus forced to exclude the idea of oxidation altogether. Although Respiration can take place without oxygen, life will not long continue without it; for, as before stated, oxygen is the power which burns organic matter into life. mountain and valley-subsist on the carbonic acid which is exhaled from the lungs and bodies of animals. Plants take up this carbonic acid from the atmosphere, mould the carbon into their own substance, and set free the oxygen, once more returning it to the atmosphere. Animals reverse the process, taking up the oxygen, and giving out carbonic acid for the nourishment of plants. This beautiful rhythmus of organic life has been so often described, that it has almost become a commonplace, without, however, losing its charm for the contemplative mind. The dependence of plant on animal, and of animal on plant, united in one mystery, and ever acting each for the advantage of the other, is not an idea to lose its charm by becoming familiar; but it sometimes leads to misconceptions. What, for instance, seems more natural than that the influence of trees planted in our cities Why is death inevitable when the ac- should be very beneficial? If trees can cess of fresh oxygen is excluded? The thus withdraw the noxious carbonic acid fact we know of the reason we are ignor- from the vitiated air of cities, would it ant. There still remains a large quanti- not be desirable-nay, ought it not pety of oxygen in the blood of the expiring remptorily to be demanded that as animal; nor will death be sensibly re- many trees should be planted in our tarded if fresh oxygen is injected into the streets as we can find room for? Such veins and arteries. How it this? The conclusions are soon reached by swift process of Respiration brings oxygen to logicians. But nature is apt to elude the the blood; yet, if the oxygen be brought grasp of swift logicians, and she repeatedthere through a more direct channel ly declines to fall into the most symmetriwhile respiration is impeded, the animal cal of their formulas. Not that Nature will die as quickly as if left to itself. is capricious or illogical; but logicians Bernard tied a dog's head in a bag, which would in a certain time produce suffocation, and he found that period by no means retarded when he injected oxygen into the arteries. Quitting for a moment this labyrinth of difficulty and doubt, which alternately fascinates and disheartens us when we strive to gain some explanation of the myriad processes of Life, let us stand apart and contemplate the marvel of respiratory interchange no longer as an animal function, but rather as a planetary phenomenon; let us endeavor to picture to ourselves the silent creative activity every where dependent on this interchange. The forests, the prairies, the meadows, the corn-fields, and gardens the mighty expanse of plant-life covering are apt to draw inferences before they have collected sufficient data. Nature, in the present case, point-blank declares that the influence of vegetation on the atmosphere is totally inappreciable, unless the atmosphere be in a closed chamber or vessel, and then the influence is striking. Human wit has discovered no test delicate enough to appreciate the influence of plants on the free atmosphere in which we live. The depth and compass of this air-ocean are too vast, and the amount of oxygen absorbed by animals too trivial in comparison, for any effect to be appreciable; moreover, the mixture of the gases in the air, and their mutual diffusion, is so rapid, that no difference has yet been detected in the proportions of oxygen and carbonic acid in the air of crowded towns or wooded valleys. The air of cities will hold more noxious exhalations suspended in it, but its gaseous composition will be the same as that of the country. To give an idea of the in significant part played by animals as vitiators of the great air-ocean, we may mention the calculation made by the distinguished chemist Dumas, that all the oxygen consumed by all the animals on the surface of the globe during one hundred years would not amount to more than the of the quantity in our atmosphere; and even supposing all vegetation to be annihilated, consequently no oxygen to be returned to the air by the incessant reduction of the carbonic acid, there would still need a period of ten thousand years before the diminution of the oxygen could become appreciable by any instruments we have hitherto invented. After having thus described the essential characters and conditions of the respiratory process, it will be interesting to glance at the results obtained by various investigators respecting the variations among different animals, and in different states of the same animal. We learn, for example, without surprise, that animals of large bulk consume more air than the smaller animals; horses and oxen more than men; men more than dogs and cats. But, to use an Eastern figure, it raises the eyebrow of astonishment when we learn that the proportion of carbonic acid exhaled by a man and a horse bears no sort of correspondence to the differences in their relative bulk-the proportion being one hundred and eighty-seven to sixteen. We are, in like manner, puzzled to find that a full-grown cat only exhales one and two thirds of carbonic acid, where a rabbit produces more than two. How is this to be explained? Is there not a streak of light trembling on this question when we bring forward the fact previously mentioned, that the vegetable feeders uniformly exhale more carbonic acid than the animal feeders, and that carnivorous animals exhale more than their usual quantity if they are fed on vegetables? Some light may fall from this source, but it does not suffice to clear up the obscurity. Another interesting problem also arises here. Although the larger the animal the greater is the absolute amount of carbonic acid it produces,* yet the smaller the animal the greater is the relative amount it produces. Thus, supposing the production of carbonic acid be estimated according to each pound weight of the animal, then we shall find that the smaller the animal the greater will be its proportion. But it is not size and weight alone which determine the differences in the amount of air consumed; far greater differences will arise from the varieties of organization. We may accept it as an axiom in physiology, that the activity of Respiration is inseparably connected with vital activity-not simply muscular activity, as some writers maintain, but all processes whatever involving chemical change within the body. The most striking confirmation of this axiom is perhaps to be seen in the phenomena of hybernation or winter-sleep. No sooner are the vital functions reduced to this extremely feeble condition, in which we may almost say life is suspended, than these hybernating animals are so incapable of ordinary respiration that they may be placed in an atmosphere of pure carbonic acid and remain there unhurt for four hours; whereas if they were placed in such an atmosphere when their breathing was going on, they would instantly perish. One would imagine, on hearing this, that our ordinary sleep would also bring with it a diminution of the quantity of air consumed. And in as far as sleep may be considered a diminution of the vital activity, such a conclusion must be correct. But in how far is sleep a diminution ? That is a question not hitherto asked, consequently without as yet an answer. In sleep there is very obvious diminution of some forms of vital activity, but we are by no means sure that the organic changes are so much less rapid on the whole. We are led to this by the experiments of Moleschott and Böcker, which establish that the chief cause of the difference noticed between the amount of carbonic acid produced during the day and night is the influence of sunlight, and that a man lying quietly awake will produce less instead of more than a man asleep, if the conditions of light and temperature are the same. Sleep, as sleep, is not therefore a diminution of the vital activity; although the sleep which we take at night dans le même tems, qu'un amphibie mille fois plus volumineux qu'elle."-Mémoires sur la Resp.. p. 69. This is because the insect lives so much more rapid * This applies, of course, only to animals of the same kind. "Vous serez étonné," says Spallanzani, "quand je vous dirai qu'une larve du poids de quelques grains s'approprie presqu'autant d'oxygènely than the reptile. |