Lower endeavoured to prove by experiment, indepen dent of any theory, that the scarlet colour which the blood acquires in the lungs is due to the action of the air. By opening the thorax of a living animal, he discovered the precise point in the course of the circulation where the change of colour takes place, and proved that it was not effected in the heart, as had been previously supposed, but when the blood passes along the capillaries of the lesser circulation. Ile kept the lungs distended, but without renewing the air, and he observed that the blood in this case returned to the heart without any change of colour ; but when there was a continued supply of fresh air introduced into the lungs, the purple was converted into scarlet blood, exactly as in natural respiration. Lower enforced his opinion respecting the action of the air upon the blood by observing the change which it produces in the crassamentum out of the body; and he proved that the redness of the upper part of the clot is owing to its being exposed to the atmosphere, not, as was imagined, to the red particles subsiding to the bottom. Notwithstand ing the direct nature of Lower's experiments, they made but little impression upon the minds of his contempora ries ; the mathematicians thought that the change of lour might be referred, with more plausibility, to some mechanical operation ; and even Haller decidedly opposed the doctrine of Lower.
After a long interval, Lower's opinion was revived by Cigna, and supported by a train of experiments which may be regarded sufficient to establish the point ; but they seem to have produced little conviction, until Priestley took up the subject, confirmed Cigna's experiments, add ed many of his own, and finally disclosed a series of facts which has served as a basis for all that has since been dis covered upon the subject. After having unequivocally proved the action of the air in converting purple into scarlet blood, as in natural respiration, he proceeded to examine the effects of the constituents of the atmosphere, taken separately, and also of the other gases which had been lately discovered. He found that purple coagulum was reddened more rapidly by oxygen than by the air of the atmosphere, while azote, hydrogen, and carbonic acid, gave to scarlet crassamentum the purple colour of venous blood. The conclusions from these experiments were direct and highly important ; they showed that the altera tion of colour which the blood experiences in the lungs depends upon the oxygen in the atmosphere, and con versely, that the change which the air undergoes in the lungs depends upon the action of the blood in the pulmo nary vessels. As we have stated above, Priestley con ceived that change to consist in the addition of what he termed phlogiston to the air, and the consequent abstrac tion of this substance from the blood.
We have already given an account of the modifications which Lavoisier introduced into the doctrine ol Priest ley, partly in consequence of his more correct views of the nature of what had been styled phtogistic pro cesses, and partly from the theory he adopted of the for mation of the water which is exhaled from the lungs. His idea was, that the air, in passing through the pulmo nary vessels, acquires carbon and hydrogen; and conse quently he concluded, that these elements were given out by the blood, and constituted the difference between the arterial and the venous state of this fluid, thus converting the phlogiston of Priestley into hydro-carbon. So great was the authorit) of that his opinion was almost universally received by his contemporaries ; although, as we have seen above, the proof of the dischat ge of hydrogen was quite detective, and the opinion is now generally abandoned. We are there.ore reduced to the conclusion, that the venous blood, in passing ,hrough the lungs, loses a portion of its carbon, which is carried off by the air, in the form of carbonic acid.
We may therefore consider the fact, that the venous blood loses carbon in passing through the lungs, as fully established but there is some difficulty in explaining the manner in which this change is effected. On this subject two hypotheses have been proposed, each of which has been supported by the authority of great names, and by a variety of ingenious and plausible at guments. According to the original idea of Black, which was the one adopted by Priestley, L,0 oisier, and Crawford, the oxygen simply attracts carbon from the blood, as it passes through the lungs : while, according to the hypothesis of Lagrange, which was adopted by many physiologists both in this country and in France, the operation is more complicated. Here it is supposed that the oxygen is absorbed by the blood, is mixed with it, and gradually unites with a portion of the carbon. When, in the course ol the circulation, the compound of oxygen and carbon is again brought to the lungs, it is discharged in the form of carbonic acid, while at the same time a fresh portion of oxygen is all, sorbed. The essential difference between the two hypo• theses is, that, according to the first, the change induced by respiration is entirely completed in the lungs, while in the other the change is effected in the body at large, the lungs serving merely as the organ where the ingredients are absorbed or discharged. Our judgment respecting them must be decided, partly from investigating the source of the carbon that is emitted from the lungs, and partly from considering the effects which must be produced by the union of the oxygen and carbon.