Imagens da página

which we have seen united in Mr. Cooke; were obscurred and marred by unfortunate circumstances in the early portion of his life, and by long continued habits of indulging those debased propensities, which those unfortunate circumstances had generated. Though his talents as an actor were obscured and lowered by these causes, he still retained enough of the form impressed by the bountiful goddess nature," to stamp him in men's minds, the legitimate successor of Garrick : but these causes had made of him, as a man, a mass of 'contradictions, not merely opposite, but in the extremes of opposition. With manners the most urbane, polished, and refined, and a mind delighting in the society of wit and reason, a large portion of his life was passed in the haunts of vice, or in the solitude imposed by poverty, or sickness, the consequences of voluntary madness; and that benevolence, which opened his heart and hand, to relieve the distresses of his fellow creatures, was iconverted into the extremes of anxious parsimony, or indiscriminate profusion: the latter, as more congenial to the natural impulse, prevailing over the former, to the utter exclusion of common sense or justice. Vol. II. pp. 361, 362,

[ocr errors]

Art. XIII. Inguiries into the Changes induced on atmospheric Air by the Germination of Seeds, the Vegetation of Plants, and the Respiration of Animals. By Daniel Ellis.

( Concluded from page 491.) WHEN we direct our attention to the various orders of

animal existence, we find them distinguished from each other by a variety of structure and formation so remarkable, and by habits and modes of life so dissimilar, that we should hardly expect, while uninstructed by experience, to recognise in them all, the existence of any one peculiarity calculated to maintain a direct and perpetual connection between them and the air of our atmosphere." This, however, is universally true of all animals; from the meanest insect to man, the lord of the creation; and it may be stated as a fact to which there is no exception, that every animal, from the first moment of its existence to its close, requires a constant supply of air, the frequent renewal of which is indispensably necessary to the continuance of life.

The organization by. which this important relation is es-" tablished between the animal and the external atmosphere, must of necessity be such as shall be adapted to the circumstances of its existence. In man and the higher order of animals, the structure of the respiratory organs is complicated; and the actions by which they are rendered subservient to the purposes of the animal economy, being partly voluntary and partly involuntary, and dependant not only on muscular action,

but also on the elasticity of the cartilages which connect the ribs with the sternum, a provision is thus made for its continuance during sleep, when volition is suspended, and for its being performed with increased force and frequency, during the periods of unusual activity and exertion. "In fishes the gills supply the place of lungs, and by their numerous sub-divisions afford a very extensive surface on which the minutė ramifications of the pulmonary artery are spread in an inconceivably fine network : while insects breathe by spiracula or pores only, over which they do not appear to have any voluntary power, and which are consequently always open for the reception of air.

There is, however, a remarkable difference in the degree in which different species of animals can bear the privation of air. Some will continue to live in a confined atmosphere until every atom of its respirable portion is consumed. Others die long before the air has reached this point of deterioration, and while even a large proportion of the respirable air remains. It is impossible, perhaps, c in the present state of our knowledge' to assign the precise cause of this difference, but the fact itself has been observed of a great variety of animals... Thus bees, flies, snails,' &c. when living in a confined atmosphere are found to consume the whole of the respirable part of the air, and the amphibia in general (which are remarkably distinguished by their tenacity of life, and haye their temperature only a few degrees above that of the medium in which they are placed) possess the same poweto Birds, on the other hand, are remarkably delicate, with respect to the purity of the air which they breathe, and generally die before two thirds of its respirable portion is consumed. This observation may be extended with very little variation to all the superior animals ; but it is singular, that, with them, the power is in some measure under the inAuence of habit : hence pearl divers aequire the power of remaining longer under water without injury, than would be practicable to others without fatal consequences.

Although the atmospherie air, which is thus universally necessary to animal existence, is a compound or rather a mixture of apote and oxygene, yet all the experiments which have been made on this subject and their number is very considerable) go to prove that the necessity of 'a perpetual renewal of the air employed in respiration, is connected, principally if not entirely, with a change induced on its oxygene. This portion of the compound, as it gradually disappears, is replaced by a quantity of carbonic acid, equal or nearly so in volume, and when this change has taken place to a certain extent, the functions of life cease. Nor does the medium which the animal inhabits cause any exception to this general principle. It is equally true of fishes and various orders of beings which inhabit the seas, lakes, and rivers, as of those which live constantly in air. Water, whether salt or fresh, contributes to the support of its inhabitants, only as it contains a portion of that vital air which is necessary to every living being; and a fish confined in a vessel of water dies as certainly if the communication with the external air is cut off, as if it were placed in the exhausted receiver of an air-pump. The oxygene or vital air consumed by aquatic animals is not, therefore, derived from the decomposition of the water itself, but from a portion of atmospheric air which all water contains, and which is constantly renewed, so long as a free communication with the atmosphere is permitted. And it is worthy of remark, that the air expelled from water by boiling or otherwise, contains a larger proportion of oxygene than atmospheric air ; a circumstance which may be regarded as a compensation for the smallness of its volume, which (from the experiments of Humboldt and Provençal on the water of the Seine) may be estimated at about that of the water. This air when carefully analyzed was found to contain 31 per cent. of. oxygene, and from 5 to 11 of carbonic acid, the remainder being nitrogene ; nor was the proportion of the oxygene found to vary, except in a very small degree, during many months, and under considerable varieties of weather.

The disappearance, then, of a part or the whole of the oxygene, and its replacement by a corresponding proportion of carbonic acid is a constant and universal change produced by respiration, under whatever circumstances it may be carried on.

It is, too, the most obvious effect of this function. But, as all truth is progressive, and as in all physical inquiries the discovery of one fact naturally leads to further investigations, it becomes important to determine the precise relation which the disappearance of oxygene and the production of carbonic acid bear to each other, and also to ascertain how far the azote, which constitutes so large a proportion of the atmosphere, may be active or passive in this important process. It is, however, by no

means easy to determine these points, from the imperfection of the data on which our conclusions must rest. In the experiments which have been made on the inferior animals the phenomena being more immediately under our observation, and the sources of error more correctly appreciated) the results have

been sufficiently ụniform, and the conclusion appears to be fully established, by comparing the results of a great number of experiments, that, in their respiration, the oxygene which disappears is replaced by an exactly equal volume of carbonic acid; the whole volume of the air employed, therefore, suffers no alteration either of increase or diminution; and it is strictly philosophical to infer, that the azote undergoes no change in the respiration of the lower orders of animals.

In all our inquiries into the functions of the animal economy, however, the physiology of man must be the ultimate object; and the experiments above noticed are chiefly important as a foundation for analogical reasoning. With regard to those performed on the higher order of animals, though they have been generally conducted with extreme caution, and by the most able experimentalists, yet it is often difficult to reconcile their conclusions with each other, or to separate in every instance fact from hypothesis.

The first discovery of the formation of carbonic acid in human respiration was made by the illustrious Dr. Black, in the year 1757. At that time the composition of atmospheric air was unknown, and it remained an insulated and unconnected fact, until the discovery of oxygene by Dr. Priestley, in the year 1774, enabled that distinguished philosopher to propose the first consistent theory of respiration, From this period our knowledge of the subject has been gradually increasing, and acquiring greater accuracy. Respiration in all animals has been found to produce a change in the chemical composition of the air, which sustains a diminution of its oxygene, and receives an addition of carbonic acid. With respect to all the inferior classes of animals it has been clearly proved that there is a very close and almost an exact correspondence between these changes; the carbonic acid being found to be so nearly equal in volume to the oxygene, which has disappeared, as to replace it, and leave the whole of the air nearly of its. original bulk. In many of the experiments, indeed, a small diminution has been observed to exist, but least in those conducted with the greatest

care. It is the object of the physiologist, however, to acquire a knowledge of the phenomena which attend the process of natural respiration : and notwithstanding the importance of the facts which experiment has disclosed to view, still the circumstances have been too remote to allow us to apply the knowledge thus acquired to explain the natural function, except in a very cautious manner, and to a very limited degree. To obtain more complete knowledge, therefore, of the effects VOL. X.

3 H

produced by natural respiration in the more perfect animals, numerous experiments have been conducted on man, and a reference to the principal facts which have been established by this means, becomes indispensably necessary to our obtaining a tolerably complete view of the subject.

In all these experiments, the two principal circumstances which have attracted the notice of those who have conducted them, have been the changes already noticed in reference to the inferior animals, namely, a diminution of volume in the air employed in the experiment, and the substitution of a quantity of carbonic acid, for a portion of oxygene which had disappeared. The diminution of buik in the expired air, appears to have been noticed by the earliest inquirers, and was estimated by Hales at from 's to 36, and by Boyle at about o, while more recently it has been estimated by Lavoisier and Goodwyn at from to . It is evident however that these estimates must be liable to great uncertainty, until the quantity of air employed in an ordinary inspiration and expiration shall be known. To determine this, however, is by no means easy.

Indeed a mere comparison of the results obtained by perusing different modes of investigation proves, that even a near approximation to perfect accuracy is not often to be found: since the estimates formed by different experimentalists vary from 12 or 15 to 40 cubic inches. The largest of these quantities is supported by the respectable authorities of Jurin, Hales, Haller, and Sauvages, and farther by that of Dr. Menzies, whose method of determining the question appears to have been as simple and satisfactory as possible. He found the average of fifty six natural inspirations, to give 42. 8 cubic inches for each ; and his results were almost exactly coincident, whether they were deduced from the means which he adopted of measuring the air, or from computing the alternate dilatation and contraction of the chest when the body was immersed in a vessel filled with water. It is obvious to remark, however, that this estimate though it appears to be perfectly unexceptionable, as applied to the respiration of a middle sized man, can, after all, be only an approximation to truth, in any individual instance; since the capacity of the chest must vary in every individual, and with that, the quantity of air inspired in every natural effort of the respiratory organs. And this, again, is far from being the whole of the air contained in the lungs under ordinary circumstances. It has been ascertained by Dr. Menzies that, after a common expiration, many persons could still expel 70 cubic inches again, by a forcible effort; and it is well known that the lungs, after death, at which period the respiratory organs are in a state of expiration, contain a quan

« AnteriorContinuar »