and the most visionary hypotheses had been indulged. On opening the thorax of a living animal, he observed the precise point of the circulation at which the change of colour takes place, and he showed, that it is not in the heart, since the blood continues to be purple, when it leaves the right ventricle. He then kept the lungs artificially distended; first with a regular supply of fresh air, and afterwards with the same portion of air without renewing it. In the former case, the blood experienced the usual change of colour. In the second, it was returned to the left side of the heart unchanged.

Experiments, more or less resembling those of Lower, have been performed by Goodwyn, Cigna, Bichat, Wilson Philip, and numerous others, with precisely similar results.

The direct experiments of Priestley more clearly showed, that the change, effected on the blood, was to be ascribed to the air. He found, that the clot of venous blood, when confined in a small quantity of air, assumed a scarlet colour, and that the air experienced the same change as by respiration. He afterwards examined the effect produced on the blood by the gaseous elements of the atmosphere separately, as well as by the other gaseous fluids, that had been discovered. The clot was reddened more rapidly by oxygen than by the air of the atmosphere; whilst it was reduced to the dark purple by nitrogen, hydrogen, and carbonic acid.

Since Priestley's time, the effect of different gases on the colour of venous blood has been investigated by numerous individuals. The following is the result of their observations as given by Thenard. It must be remarked, however, that all experiments have been made on the blood, when out of the body; and that it by no means follows, that precisely the same changes would be accomplished if the fluid were circulating in the vessels.

It is sufficiently manifest, then, from the disappearance of a part of the oxygen from the inspired air, and from the effects of that gas on venous blood out of the body, that it forms an essential part in the function of sanguification. But we have seen, that the expired air contains an unusual proportion of carbonic acid. Hence carbon, either in its simple state or united with oxygen, must have been given off from the blood in the vessels of the lungs.

To account for these changes on chymical principles has been a great object with chymical physiologists at all times. Priestley supposed the conversion of venous into arterial blood, to be a kind of combustion; and, according to the notion of combustion then prevalent, it was presumed to consist in the disengagement of phlogiston; in other words, the abstraction or addition of a portion of phlogiston made the blood, he conceived, arterial or venous; and the removal of phlogiston he looked upon as the principal use of respiration. This view was modified by Lavoisier, who conceived, that both carbon and hydrogen are given off from the lungs, and that they unite with the oxygen of the air by a kind of combustion; a part of the oxygen uniting with the carbon and forming carbonic acid, another portion uniting with the hydrogen and forming water. The presence of hydrogen was, however, found to be entirely ideal; and, subsequently, the general opinion was, that the most important change experienced by the blood in respiration, consists in the removal of its carbon.

Two chief chymical hypotheses have been formed to explain the mode in which this carbon is given off. The first is that of Black, Priestley, Lavoisier, and Crawford;—that the oxygen of the inspired air attracts carbon from the venous blood, and that the carbonic acid is generated by their union. The second, which has been supported by LAGRANGE, HASSENFRATZ, EDWARDS, BosTOCK, and others,—that the carbonic acid is generated in the course of the circulation, and is given off from the venous blood in the lungs, whilst oxygen gas is absorbed.

The former of these views is still maintained by a number of physiologists. It is conceived, that the oxygen, derived from the air, unites with certain parts of the venous blood,—the carbon and the hydrogen, the result of which union is that carbonic acid and water are found in the expired air; and the venous blood, thus depurated of its carbon and hydrogen, becomes arterialized; and, in consequence of these various combinations, heat enough is disengaged to keep the body always at the due temperature. According to this theory, as we have seen in the views of Priestley, Lavoisier, &c. respiration is assimilated to combustion.

The resemblance, indeed, between the two processes is, at first sight, considerable. The presence of air is absolutely necessary for respiration; in every variety of respiration the air is robbed of its oxygen; and hence a fresh supply is continually needed; and it is always arrested before the whole of the oxygen of the air is ex

hausted, and this partly on account of the carbonic gas given off. during expiration. Lastly, it can be continued much longer when an animal is confined in pure oxygen gas than in atmospheric air.

All these circumstances likewise prevail in combustion. Every kind of combustion requires the presence of air. A part of the oxygen of the air is consumed; and, unless the air be renewed, combustion is impossible. It is arrested, too, before the whole of the oxygen is consumed, owing to the carbonic acid formed; and it can be longer maintained in pure oxygen. Moreover, when the air has been respired, it becomes unfit for combustion,—and conversely. From these analogies, respiration has been assimilated to combustion.

Again, the oxygen of the air, in which combustion is taking place, combines with the carbon and hydrogen of the burning body; hence the formation of carbonic acid and water; and as, in this combination, the oxygen passes from the state of a very rare gas, or one containing a considerable quantity of caloric between its molecules, to the condition of a much denser gas, or even of a liquid, the whole of the caloric, which the oxygen contained in its former state, can no longer be held in the latter, and it is accordingly disengaged; hence the heat which is given off. In like manner, in respiration, the oxygen of the inspired air combines with the carbon and hydrogen of the venous blood; giving rise to the formation of carbonic acid and water; and, as in these combinations, the oxygen passes, also, from the state of a very rare to that of a denser gas, or of a liquid, there is a considerable disengagement of caloric, which becomes the source of the high temperature, maintained by the human body.

M. Thenard, admits a modification of this view,—sanguification being owing, he conceives, to the combustion of the carbonaceous parts of the venous blood, and probably of its colouring matter, by the oxygen of the air.

This chymical theory, which originated chiefly with Lavoisier and La Place, and Seguin, was adopted by Crawford, Gren, Girtanner, and others, with but little modification. Of these modifications it may be well to refer to one or two. Crawford was of opinion, that venous blood contains a peculiar compound of carbon and hydrogen, called hydro-carbon, the elements of which unite in the lungs with the oxygen of the air, forming water with the one, and carbonic acid with the other; and that the blood, purified in this manner, assumes the scarlet hue, and becomes adapted to the necessities of the economy. It is only necessary to say, that this supposed hydro-carbon is entirely conjectural.

Mr. Ellis imagined, that the carbon is separated from the venous blood by a secretory process; and that, then, coming into direct contact with oxygen, it is converted into carbonic acid. The circumstance that led him to this opinion, was his disbelief in the possibility of oxygen being able to act upon the blood through the

animal membrane or coat of the vessel, in which it is confined. It is obvious, however, that if the oxygen penetrates to the blood circulating in the lungs, it must in all cases pass through the coats of the pulmonary vessels. These coats, indeed, offer little or no obstacle, and, consequently, there is no necessity for the vital or secretory action suggested by Mr. ELLIS. PRIESTLEY and HASSENFRATZ exposed venous blood to atmospheric air and to oxygen in a bladder. In all cases, the parts of the blood, in contact with the gases, became of a florid colour. The experiments of Dr. J. K. Mitchell are, in this aspect, pregnant with interest. They prove the great facility with which the tissues are penetrated by the gases, and confirm the facts developed by the experiments of Priestley and HASSEN

FRATZ.

The second hypothesis,—that the carbonic acid is generated in the course of the circulation,—was proposed by Lagrange, in consequence of the objection he saw to the former hypothesis,that the lung ought to be consumed by the perpetual disengagement of caloric taking place within it; or if not so, that its temperature ought, at least, to be superior to that of other parts. He accordingly suggested, that, in the lungs, the oxygen is simply absorbed; passing into the venous blood, circulating with it, and uniting, in its course, with the carbon and hydrogen, so as to form carbonic acid and water, which circulate with the blood, and are finally exhaled from the lungs. The objection of Lagrange was, however, ingeniously attempted to be obviated by assuming, that arterial blood has a greater capacity for caloric than venous blood, and, consequently, that when the combustion, under the former theory, takes place in the lungs, the disengaged caloric is taken up by, and becomes latent in, the arterial blood, so that no sensible influence can be exerted by it on the lungs; whilst it is disengaged in the capillary vessels, when the blood again becomes venous and acquires a less capacity for caloric; thus giving rise to the phenomena of animal heat, which will have to be considered hereafter.

The ingenious and apparently accurate experiments of Dr. EDWARDS prove convincingly, not only that oxygen is absorbed by the pulmonary vessels, but that carbonic acid is exhaled from them." When he confined a small animal in a large quantity of air, and continued the experiment sufficiently long, he found, that the rate of absorption was greater at the commencement than towards the termination of the experiment; and, at the former period, there must have been an excess of oxygen present, and at the latter an excess of carbonic acid. This proved to him that the diminution was dependent upon the absorption of oxygen, not of carbonic acid. His experiments, in proof of the exhalation of carbonic acid, ready formed by the lungs, are very decisive. Spallanzani had asserted, that when certain of the lower animals are confined in gases, containing no oxygen, the production of carbonic acid is uninterrupted. Upon the strength of this assertion, Edwards confined frogs

in pure hydrogen, for a length of time. The result indicated, that carbonic acid was produced, and, in such quantity as to show, that it could not have been derived from the residual air in the lungs, as it was, in some cases, equal to the bulk of the animal. The same results, although to a less degree, were obtained with fishes and snails, the animals on which Spallanzani's observations were made.

The experiments of Edwards were extended to the mammalia. Kittens, two or three days old, were immersed in hydrogen. They remained in this situation for nearly twenty minutes, without dying. On examining the air of the vessel after death, it was found that they had given off a quantity of carbonic acid greater than could possibly have been contained in their lungs at the commencement of the experiment. The conclusion, deduced by Dr. Edwards, from his experiments, is, "that the carbonic acid expired is an exhalation proceeding wholly or in part from the carbonic acid contained in the mass of blood." Several experiments have been recently made by M. Collard de Martigny, who substituted azote for hydrogen; and in all cases, carbonic acid gas was given out in considerable quantity.

These experiments, then, would seem to show, that carbonic acid. is exhaled in the lungs, and that oxygen is absorbed; but it is by no means proved, that the latter goes to the formation of the former in the lungs or elsewhere. They would, also, prove the existence of carbonic acid in the venous blood, respecting which so much dissidence has existed amongst chymists; and certain experiments by Girtanner are cited as proofs, that free oxygen is contained in arterial blood. He placed the arterial blood of the sheep under a bell-glass filled with azote. In thirty hours, enough oxygen was disengaged to admit a candle to burn for two minutes under the glass.

Chaussier and Adelon, again, regard the whole process of hæmatosis as essentially organic and vital. They think, that an action of selection and elaboration is demanded, both as regards the reception of the oxygen and the elimination of the carbonic acid. But their arguments on this point are unsatisfactory, and are negatived by the facility with which oxygen can be imbibed, and with which carbonic acid transudes through animal membranes. In their view, the whole process is effected in the lungs, as soon as the air comes in contact with the vessel containing the venous blood. The imbibition of oxygen they look upon as a case of ordinary absorption; the transudation of carbonic acid as one of exhalation; both of which they conceive to be, in all cases, vital actions, and not to be likened to any physical or chymical operation.

Admitting, that the oxygen and a portion of nitrogen absolutely enter the pulmonary vessels, of which we appear to have the most direct proof, are they, it has been asked, separated from the air in the air-cells, and then absorbed; or does the air enter, undecomposed, into the vessels, and then furnish the proportion of each of its con