CONSTITUENTS OF ATMOSPHERIC AIR.

67

the physiological effects produced by it on the blood, its influence in the production of the animal heat, its effect in carrying off a large quantity of carbon from the body, and the influence which interference with these great processes on the one hand, or their promotion on the other, exercises on the health of man, on his expectation of life, and on the causes and prevention or removal of disease.

The importance of ventilation,-of a sufficiently frequent change of the air of our towns and our dwellings, to preserve it in an adequately pure state for the purposes of life, and the maintenance of health, and the prevention of disease,-is probably second to that of no single subject included in the general elements of hygiene; and it is as true, that no subject has been proved to have been so little attended to, and that on so single question has neglect been followed by such marked and injurious consequences to the physical well-being, and indirectly to the moral well-being, of the human race.

Atmospheric air consists of oxygen and nitrogen. These elementary gases are not, in all probability, in a state of chemical combination, but are simply mixed together. The probable uses of this simple mixture of these gases, in promoting the rapid and sanatory diffusion of any excess of either gas, or of any foreign gaseous substance, throughout the atmosphere, without reference to rarity, or density, or temperature, deserve to be noticed. There is, besides the oxygen and hydrogen, a small proportion of carbonic acid, always present in atmospheric air,

100

computed to amount to but Tooth part by weight. This proportion of carbonic acid is due to the mutual action and relation of animal and vegetable life; the animal organism throwing off carbonic acid, and the vegetable organism consuming it; their antagonistic influence maintaining the proportion of carbonic acid at the same point; and the rapid diffusion of uncombined and mixed gases, causing the air that travels over large tracts of uninhabited country, covered with a dense and gigantic vegetation, to be inappreciably different from the air that has passed over a fully peopled district. It should be added, although it seems to be of no practical importance, that the air of the atmosphere appears to contain a uniform proportion of ammonia: this proportion is even less than that of the carbonic acid. The chemical properties of the air are chiefly those of the oxygen which it contains, so far diluted by four times its bulk of nitrogen: that is, one hundred volumes of the air contain eighty parts of nitrogen and twenty parts of oxygen. By weight, the proportions are nearly two equivalents of nitrogen to one equivalent of oxygen. The air is a supporter of combustion and of respiration by virtue of the oxygen it contains; in both these important processes converting carbon into carbonic acid, with the attendant effect of a proportionate elevation of temperature. The effect of the nitrogen as to combustion and respiration, seems to be merely the dilution of the oxygen, and to diminish accordingly the rapidity of these great processes. The elasticity of the air; its density-100 cubic inches

weighing about 31 grains; its general pressure on the surface of the earth-a column of air with a base equal to a square inch, and of the whole weight of the atmosphere, weighing 14lbs., and pressing on the surface to that amount; the diminution of the density of the air in the more elevated parts of earth surface, and the effect of this on its character and influences-are some of the more prominent of the physical characters, uses, and effects of the atmospheric air.

In respiration, the air inspired, consisting, as has been said, of two equivalents of nitrogen and one equivalent of oxygen, undergoes a great and important change. A larger or smaller proportion of the oxygen disappears, and is replaced usually by an equal volume of carbonic acid, which is expired along with the unaltered nitrogen, and a considerable quantity of watery vapour. The effect of this upon the condition of the circulating blood is very great. The dark-coloured venous blood, returned from the system to the lungs, undergoes a great change: its colour is brightened; its globules have absorbed the oxygen of the inspired air, and parted with the carbonic acid they had been previously charged with, which has been removed from the economy by the act of respiration. The absorption of the oxygen by means of the globules, gives to arterial blood its florid colour, which it may be made to lose artificially, out of the body, by the action of carbonic acid; and this may be again separated from the blood, and its bright colour restored, by the action of oxygen, which is

likewise attended, as in the lungs, with the separation of the carbonic acid. It seems to be ascertained that this remarkable property of absorbing oxygen possessed by the blood, is due to its globular constitution; and it is probable that the absorbent power of the blood-globules for oxygen, is owing to the presence of a certain proportion of iron, which becomes thus more or less oxydised. As carbonic acid appears to contain its own volume of oxygen, the fact that the proportion of carbonic acid evolved is exactly equal in volume to that of the oxygen absorbed, seems to admit of being readily explained. The globules of the blood, thus conveying oxygen through the arterial system to all the parts of the animal economy, yield it up, in passing through the capillary vessels, to certain of the bodily constituents. In the case of certain of the fully organised particles, this expenditure may determine their separation from the living structure, and solution in the blood, preparatory to their removal from the system; in the case of recently organised alimentary matters not yet developed into vital tissue, it may interfere with this further development, and make immediate use of them, for the consumption of the oxygen and consequent evolution of temperature, or for the ulterior purpose of using the oxydised compound in some of the processes of secretion. However obtained, the oxygen of the arterial globules finds carbon on which to act, either in the blood itself, or in the tissues it permeates, and thus promotes the expenditure of effete matters that are no longer wanted in

the economy, aids in the great immediate or secondary purposes of secretion, and maintains the body at the elevated temperature which is essential to life.

A temperature that is more or less elevated above that of the surrounding atmosphere, seems to be essential to life in all animals. In birds, the temperature is higher than in any other class of animals. According to Tiedemann, the temperature of birds ranges from 100° to upwards of 111 degrees. In the mammalia, on the same authority, it is from 97° or 98° in the horse, to 104° in the sheep, and 105° or 106° in the bat. In the cetacea, the temperature is much the same as in the other mammalia,―ranging in the different species from between 95° and 96° in the porpoise, to 104° in the Greenland whale. The temperature of the amphibia does not differ so much from that of the surrounding medium, being from 4° to 10° higher than that of the medium lived in,—the temperature of the animals being greater in warmer than in colder air. Even in what are peculiarly considered to be cold-blooded animals, as fishes, the animal temperature seems to be from 3° to 7° above that of the surrounding water.

The temperature of the animal system appears to be intimately connected with the number of the bloodcorpuscles, as well as with the rapidity of the circulation, although more closely with the latter than with the former. Whether the theory of Baron Liebig, that the elevated temperature of the animal system. depends upon the corpuscles of the blood conveying the oxygen to the capillaries, by means of the