are gradually converted into azotized principles; and that the fluid of the splenic lymphatics, assists in effecting this change. Thus it would seem that when, in any circumstances, the food of an animal does not contain the proportion of chemical elements proper for forming the blood, provision is made, in its complicated structure, for supplying what is deficient. The fact that blood contains more nitrogen than chyle, as proved by the experiments of Macaire and Marcet, further confirms these views. Whether the azote resulting from the decomposed tissues, be susceptible of being again employed, when the supply by other means is insufficient, we have not the means of determining; but it is not unreasonable to conjecture that such may be the case. 185. The air we breathe is the next possible source of nitrogen. This substance is not a chemical compound, but a mechanical mixture, principally of two gases,oxygen and nitrogen, in the proportion of 23 parts of the former, to 76 of the latter. The important agency of the oxygen, in reference to the blood, and the mode of its action, have already been pointed out (172); but what becomes of the nitrogen? Does it answer no useful purpose in the animal economy ? Is it, as some suppose, merely a diluent for the oxygen? Has nature, then, produced a gas nearly equal to four times the amount of the oxygen, for the simple purpose of moderating its effect on respiration, and of checking the progress of vitality? The supposition is unworthy of the wisdom exhibited to our view in every department of nature, where we invariably find two or three purposes accomplished by one arrangement. "In human works, though labour'd on with pain, Yet serves to second too some other use. 186. It needs, therefore, but little experimental evidence to prove, that this nitrogen of the atmosphere fulfils some great design of the Creator; and, among others, that of supplying this element to the animal system, when not derivable in sufficient abundance from other sources. 187. Experiment shows, that there is a continual absorption of nitrogen by the blood; and as continual an exhalation of it. Sometimes the quantity absorbed, exceeds the quantity exhaled; in which case the excess must have been, by some means, appropriated in the system; and if a chemical union takes place, in the capillaries, between the oxygen conveyed by the blood-globules, and the carbon of the decomposed tissues; and if a part of the oxygen enters into chemical union with the blood, -there is no reason why the nitrogen absorbed by the blood, should not (in the same locality) enter into combination with the other elements of the blood, if an additional quantity of it be required. 188. The weak affinity existing between nitrogen and the other elements, even at high temperatures, seems the principal objection to the belief, that this gas can be appropriated in the human system. But it is a known property of nitrogen, that when it meets with hydrogen in a nascent state, within an enclosed space, it readily unites with the latter, and forms ammonia; and as hydro * ESSAY ON MAN. Epis. I. L. 53. gen is developed, not only in the whole extent of the alimentary canal, but also in the capillaries,-where the disintegration of the worn-out tissues takes place,—we have all the conditions necessary for the combination of nitrogen with the other elements of protein. From the researches of M. M. Bouchardat and Sandras it appears, that the digestion and absorption of albuminous matters take place exclusively in the stomach by the veins; and although the solution of fecula (or starch) also occurs in the stomach, its absorption takes place there less exclusively; which fact accords with the special disposition and length of the intestines of animals not carnivorous. Majendie found little hydrogen in the stomach one hour after food had been taken, and none at the end of two hours; whereas, in the small intestines, upwards of fifty per cent of this gas was found; but at the expiration of four hours, only eight per cent. It is probable, therefore, that much of this element enters into combination with nitrogen in this locality, when the food consists principally of non-azotized principles.* 189. Dr. Prout thinks the azote of the tissues may, in some instances, be derived from the air; and Sir Humphry Davy states that, in his experiments, an absorption of nitrogen took place during respiration, to the amount of 1-17th of the volume of the oxygen which disappeared from the atmosphere; so that, in twenty-four hours, the quantity of nitrogen absorbed was as much as 2246 grains. Professor Pfaff, also, observed a diminution in the quantity of the nitrogen; and estimated it at from 1-107th to 1-80th of the volume of the air inspired. * Vide LIEBIG'S ANIMAL CHEMISTRY. P. 325. 190. Priestley, Cuvier, Dr. Henderson, Edwards, and others, obtained similar results; but Allen and Pepys, Berthollet, Jurine, Nysten, Dulong, and Despretz, detected an increase of nitrogen. "Allen and Pepys ascertained, that when guinea-pigs were made to breathe in a mixture of hydrogen and oxygen, nitrogen was exhaled; and in a quantity exceeding the volume of the whole body of the animal, which shows that it could not be derived from the air previously contained in the lungs. ”* 191. Dr. Bostock observes, that it is probable 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. The discrepancy in the results obtained by different experimenters upon this point, is also explained by M. Edwards in a similar way. He supposes that, in certain circumstances, the absorption of nitrogen is most active;-in others, the exhalation. These circumstances are probably dependent on the condition of the blood with respect to this necessary element ;-the absorption being greatest when the food and the alimentary organs have not supplied it in sufficient quantity for the requirements of the system. Absorption and exhalation of this gas seem also to take place by the skin; and Dr. Pereira has suggested, that the ammonia of the atmosphere may furnish nitrogen to the system; but there has been no experimental proof of this. The evidence already adduced is so much in favour of the * MULLER'S ELEMENTS OF PHYSIOLOGY. Vol. I. P. 327. opinion that the nitrogen contained in the tissues of man and the Herbivora, may be obtained independently of food, that there is scarcely any room for doubt upon the subject; it may be considered an established fact; and in our investigations respecting human diet, it is of great importance that we should never lose sight of it. 192. That azote in food is, to a certain extent, unnecessary to the support of man, we may infer from various well authenticated facts. Adanson asserts, that the Nomadic Moors have scarcely any other food than gum senegal. Hasselquist relates, that a caravan of Abyssinians, consisting of one thousand persons, subsisted for two months on a stock of gum arabic alone, which they found among their merchandise. Those who gather gum from the trees in Arabia and Senegal, live (for a time) almost entirely upon it; and six ounces in twenty-four hours, have proved sufficient for a man's support. Humboldt informs us, that he has frequently observed the mule-drivers who carried his luggage on the coast of Caraccas, giving the preference to unprepared sugar over fresh animal food; and it is well known that negroes, and individuals otherwise imperfectly fed, soon become fat and vigorous from masticating the sugar-cane. Now, gum and sugar contain little or no azote. Potatoes and rice, also, are universally acknowledged to be far from the bottom of the nutritive scale; and yet the quantity of azote present in them is extremely insignificant, compared with the amount in even the inferior qualities of wheat. "Maize is said to contain no gluten, and little (if any) ready-formed saccharine matter; whence |