part of it should be secreted from the blood itself as it passes through the lungs, this must be regarded as only indirectly connected with the process of respira tion.

It would appear, therefore, that by far the most important change which the blood experiences, at least so far as quantity of effect is concerned, consists in the removal of a portion of its carbon. Some attempts have been made to prove that venous actually contains more carbon than arterial blood, and the results are said to correspond with the hypothesis, but the evidence of its truth must principally rest upon a knowledge of the changes which take place in the air. The air certainly acquires carbon by being brought into proximity with the blood; there is no assignable source whence the carbon can be acquired except the blood; the blood obviously undergoes some change from the action of the air upon it, and the crassamentum, when removed from the vessels, affects the air in the same manner with respiration.

But although the fact be thus established, the manner in which this change is effected is much less easy to comprehend than the nature of the change. Two hypotheses have been formed to account for the operation, each of which is supported by the authority of great names, and by many ingenious arguments, as well as by direct experiment. According to one hypothesis, which may be regarded as that originally proposed by Black, and adopted by Priest

3 Abildguard, in Ann, Chim. t. xxxvi. p. 91. et seq.

ley, Lavoisier, and Crawford, the oxygen of the inspired air immediately attracts carbon from the venous blood, the carbonic acid being directly generated by their union. According to the other, the oxygen is absorbed by the blood, is mixed with it, and unites with a portion of its carbon; when the blood, in the course of the circulation, again arrives at the lungs, the carbonic acid that had been formed is discharged, while a fresh portion of oxygen is absorbed. The essential difference between the two hypotheses may be expressed in the following query ; are the changes induced by respiration entirely effected in the lungs, or are they brought about in the body at large, the lungs serving merely as the organ by which the substances are absorbed or discharged ? The first of these hypotheses has the recommendation of being the most simple, but several objections were urged against it, which gave rise to the more complicated hypothesis that was proposed by La Grange.

Priestley originally took this view of the subject, but he afterwards thought it more probable that the oxygen is ab sorbed by the blood; an opinion which he appears to have adopted in consequence of his supposing that the quantity of oxygen which disappears is greater than what is necessary to form the carbonic acid; Phil. Trans. for 1790. p. 106 et seq.

s It is not intended by this expression to signify, that, at this period, Black, Priestley, and Crawford, had a correct conception of carbonic acid, as consisting of carbon and oxygen, which was a subsequent discovery of Lavoisier; Black announced the actual formation of carbonic acid, while Craw

In order to form a judgment of their respective merits, as well as to complete the theory of respiration generally, it is necessary to inquire into the source of the carbon which is removed from the lungs, and to consider the probable effect which would result from its union with oxygen, according as it may take place in the lungs only or in the course of the circulation. The first attempt to explain the mode in which the blood acquires its inflammable matter was made by Crawford. He observes that the particles of which the body is composed have a tendency to change, the old ones are perpetually removed, while fresh matter is continually deposited in their room. This gradual interchange of particles is effected by the capillary vessels; the arterial blood conveys nutritious matter to all parts of the body,

ford and Priestley supposed that an inflammable matter was discharged from the blood, which converted part of the inspired air into carbonic acid; See Mem. Acad. pour 1775. p. 520..6; also Black's Lect. by Robison, v. i. p. 99, where he fully admits of Lavoisier's claim. The successive steps by which we arrived at a correct opinion respecting the constitution of carbonic acid, and the share which Lavoisier had in the discovery, are well pointed out by Mr. Aikin, in the article "Lavoisier," Gen. Biog., v. vi. p. 162. Lavoisier's paper referred to above was first read to the Academy in 1775, read a second time in 1778, and published in the same year. In his memoir of 1777, p. 191. he clearly states the two hypotheses of the absorption of oxygen and of its direct conversion into carbonic acid, and thinks it probable that both the operations may take place, although, as we have seen, he afterwards determined exclusively in favour of the latter opinion. See Dr. Edwards, De l'Influence &c. p. 437, 8.

and employs it in repairing the waste that is necessarily going on, while, at the same time that the blood loses its nutritive particles, it receives the effete or putrescent matter, which is now become useless or even noxious to the system; this is carried by the veins to the lungs, and is there discharged, after being united to oxygen." It is to this change of particles that the difference between arterial and venous blood is ascribed, and it follows, according to this view of the subject, that the matter which is received into the systemic veins contains more carbon than that which is carried off by the arteries, and is employed in the growth and nutrition of the body.

Crawford's hypothesis possesses much ingenuity; it accords with some well established facts, and seems to afford a simple and natural explanation of them, yet, upon a closer inspection, it will be found to be inadmissible. We have no evidence of the existence of any set of vessels or other apparatus, by which the carbon can enter the veins at their capillary extremities, while there is an obvious source of this matter in the chyle which is poured into them, near their termination in the right side of the heart,

6 On Animal Heat, p. 150, 1. He brings forward a direct experiment of Hamilton's, in order to prove that blood is venalized by the addition of the basis of hydrogen, p. 149, 0; but the experiment is not of that nature which can enable us to draw any important consequences from it. The same experi ment, as well as some of Priestley's, on the action of hydrogen

on the blood, is also referred to by Seguin, in order to prove the absorption of hydrogen as stated above; Ann. Chim. t. v. p. 266, 7. See also Crawford, p. 147.

immediately previous to the passage of the blood through the lungs. The properties and uses of the chyle will be fully considered hereafter; but I may remark in this place, that there can be no doubt that it is the substance destined for the support of the system, by which the waste of the body is repaired, and materials are furnished for its growth and increase. Hence we are led to the conclusion, that arterial blood becomes venalized, not in consequence of any thing which it receives while it is passing into the veins, but from what it loses in forming the various secretions, or in contributing to the growth and nutrition of the body.

Mr. Ellis's hypothesis, respecting the origin of the carbon which is employed in respiration, differs essentially from the opinion that is generally adopted on this subject, in supposing that the carbon does not proceed immediately from the blood, but that it is an excretion, produced by the action of the exhalent vessels of the lungs. These vessels, he conceives, possess the power of discharging both carbon and water; the former, as it may be presumed, in a gaseous, although uncombined state, and that it then meets with the oxygen of the air in the vesicles, and forms carbonic acid. The principal arguments which he adduces in favour of his hypothesis are, that a gas cannot penetrate the cellular or vascular structure which separates the blood from the air in the cells of the lungs; that we have no proof of the existence of any gas existing in the blood, while he insists much upon the analogy between the functions of the