101. If no material influence is produced upon the sum of oxygen absorbed, or of carbonic acid disengaged, by the different quantities and nature of our aliments, it follows, that the conversion of chyle into blood is produced by some other agent than that of the atmospheric air. At the same time, ample experience has taught us, that the nature of our ingesta is not a matter of indifference to the respiratory organs: diseased lungs are exasperated by a certain diet, and pacified by one of an opposite kind. The celebrated diver, Mr. Spalding, observed, that whenever he used a diet of animal food, or drank spirituous liquors, he consumed in a much shorter period the oxygen of the atmospheric air in his diving-bell; and he, therefore, had learnt from experience to confine himself upon such occasions to vegetable diet. He also found the same effect to arise from the use of fermented liquors; and he accordingly restricted himself to the potation of simple water. The truth of these results is confirmed by the habits of the Indian pearl-divers, who always abstain from every alimentary stimulus previous to their descent into the ocean. Those physicians who have witnessed the ravages of pulmonary disease will readily concur in the justness of these views.

The experiments of Dr. Prout would lead us to the conclusion, that less carbonic acid is given off from the lungs during the influence of an alcoholic stimulant: but he justly observes, that this may arise from the specific action upon the nerves; and, indeed, it appears probable, that the evolution of carbon from the blood is determined by nervous energy1. The principal changes which the chyle undergoes during its passage through the respiratory organs appear to consist in the more perfect elaboration of some of its principles; for instance, the albumen is converted into fibrin, and the colouring matter acquires its more decided characteristics. But these changes may be in a great measure produced by the action of the pulmonary vessels. It has been estimated, that about eleven ounces of carbon and twenty ounces of water are given off by the lungs during the twenty-four hours; but what portion of these products are to be placed to the account of the aliment has not been ascertained. It does not even appear, that the useless carbon is always evolved from the blood during its passage through these organs; it may be retained for want of sufficient nervous energy, and thus produce a morbid change upon the body.

1 Linnaeus considered the great object of respiration to be the extraction of the electric fluid from the air by the lungs, and then to transmit it to the Medulla, becoming, as it were, the Spirittts Ani-malis. Linnseus's Diary, in Maton's edition of Pultney's " Life and writings," p. 353.

2 Annals of Philosophy, vol. ii. p. 328.

1The experiments of Drs. Prout and Fyfe have shown, that whatever depresses the nervous energy, diminishes the quantity of carbonic acid expired. The depressing passions, violent and long continued exercise, low diet, mercurial irritation, and spirituous liquors, uniformly produce this effect. The quantity is also, for the same reason, diminished during sleep. Sir A. Cooper has very lately shown, by what may be regarded as an Experimentum Crucis, that "the changes of the blood are not chemical alterations merely, but dependent also upon the vital agency of the nerves and bloodvessels." On the effects of placing ligatures on the Pneumo-gastric Nerves, etc. by Sir A. Cooper, Bart. Published in the Guy's Hospital Reports, No. III.

102. The quantity of pulmonary transpiration is also influenced by various circumstances, especially the liquid nature of the food, and the quantity of fluids taken into the stomach. I have paid some attention to this circumstance, for it suggests many important links in the treatment of disease1.

103. The only safe conclusions at which we can arrive upon this intricate subject, may be embodied in the following canons: viz. 1st, That animal food proves more stimulant to the lungs than vegetable aliment. 2d, That for merited liquors are injurious to these organs, both on account of their general effects upon the circulation, and their specific action upon the nervous system; increasing, on the one hand, the necessity of respiratory changes, and on the other, diminishing the energies of the organs by which they are accomplished. 3d, That moderate exercise, hilarity of mind, free ventilation, and abstinence from fermented liquors, are essentially necessary in that stage of the digestive process at which the chyle is poured into the blood-vessels, in order to promote the free evolution of carbonic acid.

See Pharmacologia, p. 150.

104. The office of the kidneys in secreting urine may, in reference to the present subject, be considered in a double point of view; as removing from the body, generally, certain principles whose presence must be noxious; and in carrying off some portion of the aliment which cannot be assimilated with the blood, or such useless products as may arise during the progress of its elaboration. In examining the composition of urine, we shall find certain ingredients which existed in the blood, and which have therefore passed through the urinary organs without change; we shall, at the same time, discover peculiar compounds which owe their existence to the acidifying action of the kidneys upon certain substances contained in the blood. The principles derived from the blood by filtration are water, lactic acid, with its accompanying animal matters, the fixed alkalies, and lime. The new compounds formed are sulphuric and phosphoric acids, from the sulphur and phosphorus in the blood; urea, probably derived from albumen; and lithic acid.

Dr. Prout observes, that in certain forms of disease the acidifying tendency of the kidneys is carried to an excess, and that nitric acid, oxalic acid, etc, are produced; whereas, on the other hand, it is occasionally suspended, diminished, or altogether subverted; and unchanged albumen or blood, neutral substances, as sugar, or even alkaline substances, as ammonia, etc, are separated in abundance; while the phosphorus and sulphur, at the same time, pass without being acidified.

105. With respect to the character of the diseases attending these states of the urine, it will, says Dr. Prout, be generally found, that when acids are generated in excess, the urine is commonly small in quantity, and high-coloured, and the disease imflammatory; when neutral or alkaline substances, the urine, on the contrary, is generally pale-coloured, and larger in quantity, and the diseases are those of irritation and debility. In examining, however, the state of the urine, with reference to its pathognomonic indications, we must carefully distinguish between that which is voided after abstinence, and which is termed urina potus, and that which is voided five or six hours after a meal, and which is distinguished by the appellation of urina sanguinis. The temperature and hygrometric state of the atmosphere ought also to be taken into consideration; for these circumstances, by modifying the quantity of water given off by the skin, will exert a considerable influence upon the urinary secretion. Nothing, however, produces a greater effect upon the character of the urine than the state of the digestive functions; and by carefully inspecting its appearances, many valuable indications may be obtained.

It is true, that the practice of examining the urine has fallen into discredit, from the abuse which it has suffered in the hands of empiricism and imposture 1; but the new views which modern chemistry has unfolded, and the valuable practical purposes to which they have been made subservient by the labours of Wollaston, Marcet, and Prout, ought to restore it to its merited reputation.