inch of surface; so that the body of a man of ordinary stature, the surface of which Haller estimates to be fifteen square feet, sustains a pressure of 32,400 pounds. Yet, as the elasticity of the air within the body exactly balances or counteracts the pressure from without, he is not sensible of any pressure.

The experiments of Davy, Dalton, Gay Lussac, Humboldt, Despretz, and others, have shown, that pure atmospheric air is composed essentially of two gases—oxygen and nitrogen; which exist in it, in the proportion of 21 of the former to 79 of the latter. Dr. Thomson, whose analysis is the most recent and satisfactory, says 20 of oxygen to 80 of azote or nitrogen; and these proportions have been found to prevail in the air whencesoever taken;—whether from the summit of Mont Blanc, the top of Chimborazo, the sandy plains of Egypt, or from an altitude of 21,735 feet in the air. Nor has chymical analysis been enabled to detect the presence of any emanation from the soil of the most insalubrious regions, or from the bodies of those labouring under the most contagious diseases,—malignant and material as such emanations unquestionably must be.

Julia, who published a work on marshy effluvia in 1823, affirms, that he, sixty times, subjected to examination the air of the marshes of Cercle, near Narbonne; of the pond of Pudre, near Sigéan; of Salces and Salanque, in Roussillon; of Capestang, not far from Béziers; and of the different marshes on the coast of Cette; yet in all cases he found only the same constituent principles, and the same quantity of those principles, as are contained in the purest atmospheric air. Seguin examined the infectious air of a hospital, the odour of which was intolerable; yet he could discover no appreciable deficiency of oxygen, or other peculiarity of composition; and Professor Woodhouse, of the University of Pennsylvania, on examining the air from the gallery of a crowded theatre, was not more successful. That these failures to detect the matter of miasmata must be solely ascribed to the imperfection of our means of analysis is obvious; for the results show, in the clearest manner, that such emanations exist in the air of these situations; and hence the time may yet come, when their nature may be discovered. In the meanwhile, the speculatist has ample space for hypothesis; and this is the cause why so many phantasies have been indulged regarding the nature of the malaria, or that emanation from soils, which gives rise to intermittents and other affections, termed malarious. One affirms it to be azote; another carbonic acid gas; and another hydrogen, carburetted hydrogen, or sulphuretted hydrogen. But the extreme ignorance, that prevails on this subject, will be most clearly exhibited by a reference to some of the ex professo essays on malaria, that have appeared within the last few years. In the year 1820, Dr. Ferguson read a paper on "marsh poison" to the Royal Society of Edinburgh, in which he states his conviction, that malaria does not arise from aqueous or vegeta

ble putrefaction, singly or combined. In the fifty-ninth number of the London Quarterly Review, the author of this work endeavoured to show the slight positive knowledge that then existed on the subject. Julia, in the work already referred to, ascribes the poison to a union of animal and vegetable putrefaction, but expresses his total ignorance of its nature. In the seventy-second number of the Edinburgh Review, an article appeared, which was ascribed to Dr. M'Culloch, and in which he maintains, that putrefaction, in the proper sense of the term, is not necessary to its production, but that the stage or mode of vegetable decomposition, required for the production of the malaria, is different from that which generates a fetid gas.

A writer in a recent number of the Southern Review, more bold in his asseverations, affirms it to be a doctrine, established by incontrovertible reasoning, "founded on a vast mass of accumulated facts," that the principal source of this deleterious agent is the decomposition of vegetable substances; whilst the author of an article on the subject, in the American Quarterly, attempts to prop what he properly terms the exploded opinion,—that miasma is animalcular; and we may add, that others, amongst whom was President Dwight, of Yale College, who was extremely fond of physical investigations, have maintained the opinion, that the diseases, commonly imputed to stagnant waters and marsh miasmata, are produced by animalcular putrefaction.

Lastly, Dr. Charles Caldwell of Transylvania University, in an Essay on Malaria, recently crowned by the Medical and Chirurgical Faculty of Maryland, has ascribed its origin to vegetable and animal matter in a state of "dissolution," by which term he means "the decomposition of dead organic substances, and the re-union of their elements."

Such discrepancies prove incontestably, that there are no fixed. ideas on the subject. Our ignorance is not, however, confined to the aerial impregnation, which constitutes malaria. We know no more of the immediate cause of epidemics,--of the influenza, for example, which frequently visits us,—than we do of the cause of the incessant vicissitudes, which occur in the atmosphere itself; nor have we the slightest acquaintance with the character or constituents of any emanation from the subject of any one of the numerous contagious disorders,—small-pox, measles, &c.,—active as such emanations unquestionably are. The influence of air on mankind is most interesting and important in its hygienic relations, and has accordingly been the topic of study since the days of Hippocrates; but unfortunately not with the effect of materially improving our knowledge.

The uniformity of the proportion of the oxygen to the nitrogen in the atmosphere has led to the conclusion, that as there are many processes, which consume the oxygen, there must be some natural agency, by which a quantity of oxygen is produced equal to that

consumed. The only source, however, by which oxygen is known to be supplied is in the process of vegetation. A healthy plant absorbs carbonic acid during the day; appropriates the carbon to its own necessities, and gives off the oxygen with which it was combined. During the night, however, an opposite effect is produced. The oxygen is then taken from the air, and carbonic acid given off; but the experiments of Davy and Priestley show, that plants, during the twenty-four hours, yield more oxygen than they consume. It is impossible to look to this as the great cause of equilibrium between the oxygen and azote. Its influence can extend to a small distance only; and yet the uniformity has been found to prevail, as we have seen, in the most elevated regions, and in countries, whose arid sands never admit of vegetation.

In addition to the oxygen and azote,—the principal constituents of atmospheric air,—another gas exists in a very small proportion; but yet is always present. This is carbonic acid. It was found by De Saussure on Mont Blanc, and by Humboldt in air brought down by Garnerin the aeronaut, from the height of several thousand feet. The proportion is estimated by Dalton not to exceed the 7th or Tth of its bulk.

1000

These, then, may be looked upon as the constituents of atmospheric air. There are certain substances, however, which are adventitiously present in variable proportions; and which, with the constitution of the atmosphere as to density and temperature, are the causes of general or local salubrity, or the contrary. Water is one of these. The quantity, according to De Saussure, in a cubic foot of air, charged with moisture at 65° Fahr., is 11 grains. Its amount in the atmosphere is very variable, owing to the continual change of temperature to which the air is subject; and even when the temperature is the same, the quantity of vapour is found to vary, as the air is very rarely in a state of saturation. The varying condition, as to moisture, is indicated by the hygrometer. From a comparison of numerous observations, Gay Lussac affirms, that the mean hygrometric state of the atmosphere is such, that the air holds just one-half the moisture necessary for its saturation. In his celebrated aerial voyage, he found the air to contain but one-eighth of the moisture necessary for saturation. This is the greatest degree of dryness ever noticed.

It has been presumed, that the hygrometric condition of the atmospheric has more agency in the production of disease than either the barometric or thermometric. It is not easy to say which exerts the greatest influence: probably all are equally concerned, and when we have a union of particular barometric, thermometric, and hygrometric conditions, we have certain epidemics existing, which do not prevail under any other combination. When the air is dry, we feel a degree of elasticity and buoyancy; whilst if it be saturated with moisture, especially during the heat of summer,—languor and lassi

tude, and indisposition to mental or corporeal exertion are excited. This is the cause why we suffer little more during the hot summers of this climate than in those of Great Britain, where the air is always more loaded with humidity; although the thermometer may be 15° or 20° higher here than there. When the air contains much moisture the perspirable matter does not readily evaporate, and is, consequently, apparent upon the surface; hence, we appear to perspire more than in a hotter and dryer atmosphere, although we are really exhaling less; and as the evaporation is a cooling process, if it cannot take place, we experience a feeling of heat greater than the state of the thermometer will explain. It is asserted by SCHMIDTMEYER, that in the climate of Chili, notwithstanding the very high temperature in summer, the perspiration passes off so entirely in the insensible form, that, during the most violent exercise, it might be doubted whether any perspiration exists at all.

In addition to aqueous vapour it is manifest, that numerous emanations from animal and vegetable substances must be generally present, especially in the lower strata of the atmosphere; by which the salubrity of the air may be more or less affected. All living bodies, when crowded together, deteriorate the air so much as to render it unfit for the maintenance of the healthy function. If animals be kept crowded together in ill-ventilated apartments, they speedily sicken. The horse becomes attacked with glanders; fowls with pep, and sheep with a disease peculiar to them if they be too closely folded. This is probably a principal cause of the insalubrity of cities compared with the country. In them, the air must necessarily be deteriorated by the impracticability of due ventilation; and be

"Sated with exhalations rank and fell,

The spoil of dunghills, and the putrid thaw of Nature,"

and perhaps one of the most signal blessings, that ever befel the metropolis of England was the great fire of 1666; which destroyed the narrow streets; gave occasion to a better system of ventilation and medical police; and was thus a main agent in preventing the recurrence of the plague, which has not visited London since that period.

One of the greatest evidences we possess of the positive insalubrity of towns is in the case of the young. In London, the proportion of those that die annually under five years of age, is as much as thirty-eight per cent., and under two years, twenty-eight per cent.; .in Paris, under two years of age, twenty-five per cent.; and in Philadelphia, rather less than a third. These estimates may be considered approximations; the proportions varying somewhat, according to the precise year in which they have been taken.

Manifest, however, as is the existence of some deleterious principle in these cases, it has always escaped the researches of the chymist.

Lastly. Air is indispensably requisite for organic existence. No being, animal or vegetable,—can continue to live without a due supply of it; nor can any other gas be substituted for it. This is proved by the fact, that all organized bodies cease to exist, if placed in vacuo. They require, likewise, renovation of the air, otherwise they die; and if the residual air be examined, it is found to be diminished in quantity, to have lost a part of its oxygen, and to have received, in its place, a gas, which is totally unfit for life,carbonic acid. The experiments of Hales prove this as regards vegetables; whilst Spallanzani, and Vauquelin have confirmed it in the case of the lower animals. The necessity for the presence of air, and its due renewal,—as regards man and the upper classes of animals,is sufficiently obvious. The sufferings in the Black Hole at Calcutta are a melancholy, but impressive, memento of the dreadful evils attendant upon its privation. Not less necessary is a due supply of air to aquatic animals. They can be readily drowned if prevented from coming to the surface; if the fluid be put under the receiver of an air-pump, and the air be withdrawn, or if the vessel be placed so that the air cannot be renewed, the same changes are found to have been produced in the air; and hence the necessity of making holes through the ice, where small fish-ponds are frozen over, if we are desirous of preserving the fish alive.

The necessity for the renewal of air is not, however, alike imperative in all animals. Whilst the mammalia, birds, fishes, &c. will speedily expire, when placed under the receiver of an air-pump, if the air be exhausted, the frog is but slightly incommoded. swells up, almost to bursting, but retains its position, and when the air is admitted, seems to have sustained no injury. This exception afforded by the amphibious animal to the ordinary effect of destructive agents we have already had occasion to refer to more than once; and it is strikingly exemplified in the fact, now indisputable, that the toad has been found alive in the substance of trees and rocks, where no access of air appeared practicable.

Physiology of Respiration.

Within certain limits, the function of respiration is under the influence of volition. The muscles, belonging to it, have consequently been termed mixed, as we can at pleasure increase or diminish their action, but cannot arrest it altogether, or for any great length of time. If, by a forced inspiration, we take air into the chest in large quantity, we find it impossible to keep the chest in this condition beyond a certain time. Expiration irresistibly succeeds, and the chest resumes its pristine situation.

The same occurs if we expel the air as much as possible from the lungs. The expiratory effort cannot be indefinitely prolonged, and the chest expands in spite of the effort of the will.

These facts have given rise to two curious and deeply interesting