result of secretion or transudation, than as a proper effect of respiration. 6. The blood, in passing through the lungs, absorbs a portion of oxygen, and this appears to be more than what is necessary for the formation of the carbonic acid which is discharged. 7. It is probable that the blood, as it passes through the lungs, both absorbs and exhales nitrogen, the proportion which these operations bear to each other being very variable, and depending upon certain states of the system, or upon the operation of external agents. 8. It appears, upon the whole, probable, that the atmospheric air is absorbed by the blood in its whole substance, and that certain proportions of each of its ingredients are discharged or retained according to the demands of the system. 9. We have no proof that hydrogen is discharged from the blood." § 5. On the Respiration of the different Gases. It may be presumed that no gaseous body, except the compound of oxygen and nitrogen which constitutes the atmosphere, is adapted to the permanent support of life. Of the other gases, there are some which are properly speaking unrespirable, which, on account of the irritation they produce in the upper part of the trachea, it is impossible to take into the lungs; but there are others, which may be received 7 I have not entered upon the question respecting the change of its capacity for heat, which the blood has been supposed to experience in its passage through the lungs, because it will fall under our notice, with more propriety, in the next chapter. into the pulmonary cavities, although their employment is followed sooner or later by some derangement of the system, or even by the extinction of life. The gases upon which experiments of this kind have been made are oxygen, nitrous oxide, nitrogen, hydrogen, and carburetted hydrogen. 8 The first account which we have of the effect of oxygen, when respired in its unmixed state, is given us by Priestley, who almost immediately upon his discovery of this substance, perceived its remarkable capacity of supporting life, and tried the effect of it upon his own person: he informs us that he felt an agreeable lightness in his chest; but nothing particular appears to have resulted from the trial, and it may be fairly questioned, whether the sensation which he described is not to be attributed rather to a mental, than to a physical impression. Some individuals who respired oxygen, conceived that it even produced exhilarating effects, while others describe it as giving rise to pain and uneasiness in the thorax. But it can scarcely be doubted, that much of what was described depended upon the imagination, while something was probably owing to the impurity of the gas that was employed, or to the unusual efforts which were made to take it into the lungs. Priestley was also the first who tried the effect of the respiration of oxygen upon animals that were immersed in it; but his experiments went no farther than to prove the superior power which it possesses of sup On Air, v. ii. p. 162. porting life. He remarks indeed that after an animal had expired in a portion of oxygen, a second animal was able to live for some time in the same air, and it has been inferred from this circumstance, that there must have been something noxious in the gas, which the powers of the constitution were unable to resist for more than a certain length of time. He himself supposed that the death of the mice, the animals which he employed in his experiments, was owing to cold, in consequence of passing through the water with which the gas was confined; he accordingly found that by keeping up the temperature, he was able considerably to prolong the life of the animal, and he remarks, that this experiment completely convinced him that there was nothing in the nature of the gas itself which prevented the mice from living in it." But another and a more efficacious cause may be assigned for the death of the first animal, that the carbonic acid which was generated, and of which a small portion only would be absorbed by the water, must have acted more powerfully upon the system exhausted by having been for some time exposed to its influence, than upon a fresh and vigorous animal, who would be able for a short period to bear the effect with impunity. Exactly the same occurrence has been observed by Morozzo and others' to take place in the respiration of a limited 9 On Air, v. ii. p. 165. Jurine, Enc. Meth. "Medecine," t. i. p. 496; Chaptal's Chem. v. i. p. 131, 2; the experiments of Morozzo on this subject appear to have been made with sufficient accuracy; Journ. quantity of atmospheric air, and, on the contrary, it has been found by Lavoisier and others, that when the carbonic acid was removed by potash, as fast as it was produced, this effect did not take place.1 The next account that we have of the respiration of oxygen is by Lavoisier. In his first experiments on this subject he examined the state of the internal organs of an animal, after having been for some time confined in this gas, and he conceived that their appearance indicated that there had been an increased action of the sanguiferous system produced, or something which indicated an approach to the inflammatory state. We may, however, presume that these appearances must have been the consequence of some accidental cause, for the same philosopher, in his de Phys. t. xxv. p. 102. et seq. ; in one set of experiments a bird lived six hours and a half in a certain quantity of oxygen, and a second lived two hours and five minutes in the same gas; in another set of experiments, the first lived five hours and twenty-three minutes, and others lived in succession in the same gas, as far as the tenth, which lived twenty-one minutes in the same air that had proved fatal to the nine birds that had been previously immersed in it. We are informed by Dr. Edwards, contrary perhaps to the opinion which is generally adopted, that when warm-blooded animals are confined in a limited quantity of air, they always deoxidate it to the same degree, and that a second animal introduced into the same air expires immediately; De l'Influence &c. p. 184. But although the air is ultimately all reduced to the same standard, the effect is produced in very different intervals of time, depending upon various circumstances connected with the constitution of the individuals. 2 Mem. Soc. Roy. Med. pour 1782, 3, p, 576. subsequent experiments, which were performed with a more perfect apparatus, and with every appearance of great accuracy, and where the respiration was continued for a much greater length of time, informs us that neither the circulation nor the temperature were affected by it, and in short that no perceptible, change was produced by it upon the animal. This conclusion is the more worthy of our attention and the more to be confided in, as it not only indicates a change in Lavoisier's opinion, but is unfavourable to the analogy which he wished to establish, between the effects of respiration and combustion. 4 We have some experiments on the respiration of oxygen by Higgins; he informs us that the pulse was quickened, and that a sensation of warmth was experienced at the chest; but it may be reasonably inferred that these effects were as much owing to the mechanical method in which the gas was inspired, as to any specific operation produced by the nature of the gas. M. Dumas relates a series of experiments,. which he performed on this subject, that were attended with very different effects. He had formed an opiniou that the lungs possessed a great degree of irritability, and in order to put it to the test, entered upon the investigation, expecting that the irritability would be rendered more peculiarly obvious by the action of oxygen upon them. A dog was accordingly confined in this gas, and the apparatus was so eon 3 Mem. Acad. Scienc. pour 1789, p. 573. |