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membered, however, that we cannot, by any muscular effort whatever, reduce the dimensions of the chest so far, as to empty the lungs entirely of their contents. After the most violent expiration, a considerable quantity of air still remains within their cells ;-nay, it is found extremely difficult to expel this residual air altogether, even by subjecting the lungs to very great compression, after they have been removed from the body. That, after an extreme expiration, they still retain, on an average, about 40 cubic inches, seems probable, from considering both the structure of the lungs, and the extent to which the thorax seems capable of being diminished by muscular action, as well as the result of an experiment of Mr Davy.

These two processes, of inspiration and expiration, generally alternate with each other, while the body is at rest, about 20 times in a minute. If, therefore, we adopt 40 cubic inches, as the average bulk of air inhaled and exhaled, it will follow, that a full grown person respires 48,000 cubic inches in an hour, or 1,152,000 cubic inches in the course of a day; a quantity equal to about 79 hogsheads.

It has been long ascertained, however, that the air which is emitted by expiration, does not possess the same properties as that which has been inspired. Now, the only gaseous substances which chemists have hitherto found existing, permanently and uniformly, in the atmosphere, are oxygen, nitrogen, and carbonic acid; the relative properties of which may be judged of from the analysis of one cubic-inch, which gives nearly as of oxygen, and so of nitrogen, with a quantity scarcely perceptible of the acid gas. What, then, is the precise nature of the changes which this atmospherical air undergoes, when it is received into the lungs?

That air which has been breathed is loaded with moisture, seems at all times to have been generally known. Upwards of fifty years ago, the celebrated Dr Black demonstrated, that it was also combined with much more carbonic acid ; and Dr Priestley proved, in 1776, that it contained much less oxygen than the air inhaled. Yet, at the publication of Mr Ellis's Inquiry, physiologists had not established, either the proportion of these gases existing in it, or the composition and quantity of the vapour with which it is united; nor had they ascertained what relation its nitrogen bore to that of the surrounding atmosphere. It appears, indeed, to have been the prevalent opinion, that a given quantity of atmospheric air, in passing once through the lungs, lost about 35th part of its bulk of nitrogen, about tőrth of oxygen, and gained nearly 'th of carbonic acid; 100 cubic inches, for example, losing 1.47 cubic inches of nitrogen, and 9:117 cubic inches of oxygen ; while they acquired 7.647 cubic inches of carbonic acid, by a single respiration. This conclusion was deduced chiefly from experiments performed by Mr Davy; in which he found, that when he applied his mouth to a tube connected with a mercurial air-holder containing atmospheric air, and made a single inspiration and expiration from and into this vessel, as much in the manner of ordinary breathing as possible, the contents of the airholder were diminished in bulk, and contained less nitrogen and oxygen, and more carbonic acid,

-nearly in the proportions just stated. To the deductions which had been made from such experiments, in as far as they related to the disappearance of nitrogen, Mr Ellis had objected, that they were such as the results obtained did not warrant; since there was no proof, that the chest was reduced exactly to the same capacity after as before the experiments ; and we could not therefore infer, that the nitrogen gas which had disappeared from the airholder was not to be found in the lungs. We wish he had extended this obvious and substantial objection, to the inferences which had been drawn from the same experiments regarding the proportions of oxygen and carbonic acid; for it appears in all respecis equally applicable to them. If a small quantity of the nitrogen of the inspired air remained in the lungs, merely because the thorax was of larger dimensions after than before the experiment, for the same reason a portion of oxygen, or carbonic acid, which otherwise would have been found in the airholder, might have been retained in these organs.

Two memoirs on this subject, the joint production of Seguin and Lavoisier, were read to the Academy of Sciences of Paris, in 1789 and 1790; and Laplace has preserved the results of those experiments, in prosecution of the same inquiry, with which the philosopher last named was engaged when he was dragged to the guillotine ;--experiments which he himself would have communicated to the world in detail, had not the short respite of a few days, which he requested for that purpose alone, been with such unprecedented barbarity denied. It would have been agreeable to us, if we could have attached any value to these investigations of two chemists so celebrated. But that caution which ought to be inseparable from every philosophical pursuit, precludes our placing the least reliance on results of experiments, when they are not detailed with the most circumstantial minuteness. We honour the memory of Lavoisier, and respect the talents of bis surviving coadjutor. But their memoirs to which we have alluded, will hereafter be read, chiefly because they are arpong the last labours of one of the greatest

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philosophers of the eighteenth century. And, indeed, it is impossible to observe the tone of enthusiasm which these essays have in some parts received, from the prevailing spirit of the times in which they were written, without melancholy reflections without feeling again awakened in us those emotions of deep regret with which the recollection of that period of disappointment must ever be contemplated.

As we never can be assured that the capacity of the lungs is the same after as before the experiment, we cannot decide, with sertainty, how far the relative proportions of the gases in the air expired, may not have been influenced by this cause. But it is obvious, that if the diminution which may have been ob served in the bulk of air by one inspiration, depended on any natural and constant process in the animal economy, by whicli air is continually abstracted from the cells of the langs, the re-. duction in volume which would take place, in breathing a larg* quantity of air, would be directly proportional to the number of respirations necessary to transmit the whole of this air through the lungs. Whereas, were it dependent on any such accidental circumstances es we have now alluded to, no sucha increase ougha to be observed: the diminution might even be least, when the quantity of air inspired was largest; and, at all events, we dould not expect to find it, in any instance, exceeding 20 cu* bic inches, or half the bulk of an ordinary inspiration. This point has been determined, in the most satisfactory manner, by the experiments of Messrs Allen and Pepys. About three years ago, they constructed an apparatus, by which from 3000 to nearly 10,000 cubic inches of atmospheric air could be transmitted once through the lungs, by easy respirations, beginning and ending with a forced or extreme expiration : And, of thirteen experiments of this kind which they performed, the greatest dem ficiency in the expired air appeared in one where 3360 cubic inches had been inspired, in which it anounted to 62 cubie inches; in another, where 3620 cubic inches had been breath ed, the diminution was only 4 cubic inches; and, in a third, 9890 cnbic inches lost only 18. But, although it is shus established, that there is one cause to which the clininution is not owing, we do not feel ourselves entitled to assert, positively, that, in all these instances, it ought to be ascribed solely to the difference between the extent of the expiration immediately preceding, and of that closing the experiments. It nay, hereafter, be shown to be dependent on circumstarices entirely different. In the mean time, it may be remarked, that even the greatest deficiency in these experiments was not equal to half the differonge between an ordinary and an extremo expiration.

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When, too, a large volume of air is breathed only once, and in the manner practised by Messrs Allen and Pepys, any slight variety in the proportions of the gases composing the small quantity of air which may be retained in the chest, can very little affect the composition of the much greater balk which has been exhaled. The analysis of this expired air may therefore be regarded as affording a very satisfactory illustration of the changes which air in general undergoes, in its passage through the lungs during natural respiration, Accordingly, Messrs Allen and Pepys have denonstrated, by experiments which seem to be unexceptionable, that, by its transmission through the lungs in ordinary breathing, the air loses about 8 per cent. of oxygen, and acquires an exactly equal bulk of carbonic acid, while its nitrogen remains unaltered. They conclude, that 39,534 cubic inches of carbonic acid are emitted daily from the lungs of a full grown person. But we perfectly agree with Mr Ellis in his ob jections to this calculation, and are inclined to estimate the average quantity at about 92,160 cubic inches.

Not being acquainted with any attempt, since the imperfect trial made by Mr Abernethy, to ascertain the composition of the fluid which is brought off by the expired air in the state of vapour, we lately subjected a small quantity of this fluid to accurate analysis. For this purpose, we made a full grown person, in perfect health, expire through a thin glass tube, about three feet in length and a quarter of an inch in diameter, kept at a low temperature, by the evaporation from a slip of muslin moistened in spirits of wine, which was wrapped round it. In four hours, an ounce measure of a perfectly transparent, colourless, and insipid fluid, of the consistence of water, was col-. leeted in drops from the extremity of the tube. This fluid did not produce the least alteration on the colours of litmus or tur. meric paper. It suffered no change on the addition of corrosive muriate of mercury, tannin, or nitrate of silver.'s And. when a small glass, containing half an ounces of it, was con- : nected, by filaments of moistened cotton, with other two glasses containing each about two drams of pure water, and these were then attached to the opposite extremities of a galvanic battery, consisting of 24 four-inch double plates of copper and zinc, charged with diluted muriatic acid, at the end of four hours, we could not detect the slightest indication of the presence of albumen in the negative, or any saline substance in the positive glass. Hence we have been led to conclude, that the fluid dissolved in the exhaled air is pure water.

All those animals which suckle their young, constituting the : class Mammalia,--for example, the ape, the horse, the dog, the

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mouse, the seal, and the whale ; Reptiles, such as the turtle, the lizard, the frog, and the snake,--and the whole class of Birds, are provided with organs resembling the lungs of man, into which they are constantly, during life, receiving fresh air. Analogy alone, therefore, would lead us to suppose, that these animals produce similar changes on it by their respiration. But this inference is in a manner confirmed, by the appeal which Mr Ellis has made to the experiments of various physiologists on rabbits, guinea pigs, rats, mice, sparrows, vipers, tortoises and lizards; and by some very neat experiments of his own on toads and frogs. In all these it appeared, that the nitrogen of the air respired suffered no change, but that a quantity of oxygen was removed,--and a volume, nearly or exactly equal, of carbonic acid substituted in its place.

The opinion which seems to have prevailed almost univera sally of late years, is, that, during respiration, a portion of the oxygen or nitrogen of the atmosphere, or a quantity of the compound atmospherié air itself, actually passes out of the lungs into the bloodvessels of these organs, and was combined with, or absorbed by the blood; and respiration has been familiarly spoken of as a process analogous, if not absolutely identical, with that of combustion. This combination being assumed as a fact, a variety of other phenomena, more or less at variance with actual experience, have been supposed to succeed or accompany it. According to one hypothesis, the carbonic acid exhaled might have existed ready formed in the blood of the pulmonary arteries į and the blood might have a stronger attraction for oxygen gas than for carbonic acid, and, combining consequently with the oxygen of the air, might part with the carbonic acid, which would as easily pass from the vessels into the cells, as the oxygen from the cells into the vessels. - Another doctrine supposed, that a portion of oxygen being attracted by the blood in the lungs, was combined, during the circulation of that fluid through the other parts of the body, with a portion of carbon, so as to form an oxide of carbon, which, on being brought back to the pulmonary vessels, was there united with an additional quantity of oxygen, assumed the state of carbonic acid, and was discharged." In like manner, it was conceived by some, that the water emitted in the state of vapour might be formed, by the union of a portion of absorbed oxygen with hydrogen existing in the blood, so as to constitute an oxide of hydrogen, which, on passing into the pulmonary arteries, com: bined with another portion of oxygen, and was then exhaled, And, finally, with respect to the nitrogen, some were of opi- * nion, that while the blood actually attracted a large volume of VOL. XIX. NO. 37.

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