conversion of the iron they contain into the state of peroxyde, or that of M. Mulder, that this absorption of oxygen depends on the oxydation of the protein compounds introduced into the system by the processes of nutrition,-be or be not the correct explanation of the great fact, can signify very little, further than to show the difficulty and intricacy besetting such questions as these. It is enough to have it admitted, that oxygen is absorbed by the blood in its passage through the lungs,—that it is conveyed to the capillary vessels of the different parts of the body, that there it acts upon the compounds of carbon,-that in consequence of their combination, heat is evolved, and at all the different parts of the body, and that the carbonic acid thus formed, is conveyed to the lungs, and disengaged by expiration, unless what may have escaped, combined or uncombined, in the different excretions, or by the various means of egress afforded to it through the tissues of the body.

The effect of the nitrogen in the atmospheric air, appears then to be simply negative; acting as a diluent to the oxygen, and diminishing its power as a disintegrant and stimulant to the system. As has been said, moreover, the change effected in the blood by the respiratory process, appears to be almost entirely confined to the corpuscles. The respiration is effected in man and the mammalia, by the dilatation and contraction of the cavity of the thorax. The capacity of the chest being enlarged by the flattening of the diaphragm and elevation of the ribs, the air

EFFECTS OF RESPIRATION ON THE AIR. 73

rushes in to restore the atmospheric equilibrium,-the volume of the lungs is by so much enlarged,—and the air is freely distributed throughout the pulmonary parenchyma, by the countless and minute ramifications of the air-tubes, which terminate in minute vesicular dilatations, the walls of which are covered with a capillary net-work of blood-vessels. It has been

calculated, that the whole internal surface of this membrane, through which the air and the blood are thus brought into such extensive and intimate relation, amounts to 1400 square feet,—that the lungs contain, after the strongest expiration 35 cubic inches of air, after an ordinary expiration 108 cubic inches, and after a very full inspiration 240 cubic inches, that the amount of air changed at every respiration, amounts to from 10 to 13 cubic inches, being probably the half, are rather more than the half, of the whole of the air admitted at every inspiration, which is calculated to be, according to individual stature, from 15 to 25 cubic inches,-that the quantity of air respired in the 24 hours, may be from 400,000 to 500,000 cubic inches,that women expire less carbonic acid than men,—that the quantity of carbonic acid expired, is greater in proportion to the greater vigour of the system, unless under some peculiar circumstances of diseased action, -that it increases with age to the 40th or 45th year, and then diminishes,—that the quantity of carbon expired is diminished by hunger and rest, and increased by the opposite conditions,—and that it is greater in the day than in the night. The quantity

of carbon expired is variously stated by different experimenters, but may probably be about 150 grains per hour, allowing for variations in the same individual at different times, and, in different individuals, for age, size, and other circumstances.

The inspired air is first warmed, then saturated with moisture, then exposed to the large surface of the moistened air-cells at the extremities of the airtubes. The nitrogen of the inspired air remaining essentially unchanged, the oxygen being to some extent exchanged for carbonic acid, as it would seem by a simple membranous diffusion,—the blood corpuscles absorb the oxygen; and as by its means, in the course of its circulation through the system, carbon becomes converted into carbonic acid, the blood at length returns to the lungs surcharged with this compound of carbon and oxygen. The moistened membrane of the air-cells offers little or no impediment to the action of the air on the blood: the permeability of membrane is well known; gases are found to permeate even dead membrane with considerable readiness; and blood is said to be susceptible to atmospheric influences, even when removed from the system, and enclosed in a moistened bladder.

The expired air contains a considerable proportion of watery vapour, which may possibly be formed during the more perfect organisation of the alimentary atoms, which may be attended with the disengagement of a definite proportion of the component water. Or it may be, in whole or in part, a consequence of

SANATORY EFFECTS OF RESPIRATION.

75

the exposure of so large a surface of moistened membrane to the action of the air. The quantity of water thus conveyed out of the system is too variously estimated by different experimenters, to afford any satisfactory conclusion respecting it. The lowest calculation gives it at nearly 3000 grains of water in the 24 hours.

All these facts only serve to show how important a share in the economy of the system, is performed by the process of respiration,-how vast must be the influence thus exerted, on the life and well-being of mankind,—and how important an agent the respiration may be, in conducing to the health, or in undermining the powers and shortening the duration of the vital processes. And, accordingly, not only is it quite true, that diseases of the lungs constitute a large proportion of the general mortality, but it is likewise true that, by breathing tainted air, by accidental, or occasional, or habitual exposure to an impure atmosphere,―as evidenced in the important class of diseases admitted to be infectious, and in the immediate influence frequently consequent on exposure to concentrated and noxious effluvia, or in the eventual consequence of breathing an atmosphere characterised by the modified impurity caused by a concentrated population, or defective ventilation, drainage, or sewerage,-a far larger number of people are prematurely destroyed, a far larger number of years are subtracted from the just duration of human life, than can be laid to the charge of all the vice, immorality, and several wrongs to the

individual organisations, that affect the life, degrade the physical strength and character, or derange the physical usefulness, of the human race.

On this view of this question, the subjects of drainage and sewerage rise into a degree of sanatory importance, which is greater than could have been readily preconceived, and which I believe can hardly be exaggerated. This importance, which may be less appreciable in degree when a dwelling-house is detached, and remote from other habitations, must rise immensely in extent and character when several houses come to be placed side by side, and still more when acres or miles are covered, and perhaps thickly covered, with densely-peopled dwellings. How far this has been attended to or otherwise, Parliamentary Reports have of late years only too fully demonstrated; and the fact that by these means the attention of the public has been in some degree aroused, and some few steps taken in different towns and districts in the right direction, for the removal of such abuses of the life-probability and health of the people, has only served to show how much remains to be done, and how difficult it is to move people to put themselves to temporary inconvenience, or to undergo any considerable amount of expense, or to subject themselves to the necessity of some immediate exertion, when the question is not one that immediately and sensibly affects the individuals, and does not interfere at once with the appliances and means of existence. What can be said to the facts, that one half of the mortality of England is directly.