VENTILATION is the art of providing any confined air or apartment with an adequate supply of air, in a condition suitable to the purposes for which it may be required. The air is usually introduced in a stream which maintains a freshness and purity of atmosphere in the place that is ventilated, any contaminated air being withdrawn as pure air enters.
Numerous operations of nature and of art, and more especially during combustion, respiration, and the decay that attends the putrefaction of animal and vegetable matter, unwholesome products are evolved; and so deleterious are these to animal life, that death is the necessary consequence when they accumulate in a more concentrated form around the person. In a smaller proportion, they produce an endless variety of discomfort or disease, from the most trying sense of languor or debility to the more violent aplectic headache, for the time suppressing all attempts at exertion either of mind or body, while on other occasions a slow and insidious action gradually undermines the constitution, and induces a permanent loss of health. In extreme cases death is induced, more or less rapidly, according as the oxygen of the atmosphere is withdrawn, or an impregnation of poisonous gases communicated to it. The air of respiration is the great pabulum vite: we draw upon it no less than twelve hundred times an hour, or an average, for nourishment and support, during the whole period of our existence; we consume oxygen, and replace it by carbonic acid; and were it withdrawn from us, or changed in its qualities, death would inevitably ensue.
The air acts incessantly, not only on the blood as it passes through the lungs, but also on the surface of the body; and disease and death may ensue from an unwholesome atmosphere in contact with the skin, even when the lungs are supplied with pure air. Well attested cases are recorded where severe oppression has attended the action of an amount of impurity so small as from 1-15000th to 1-5000th part of sulphureted hydrogen gas; while the absolute amount of impurity in air tainted by miasma is so excessively minute, that its precise nature, as well as the minuteness of its weight in the most pestilential atmosphere, is unknown.
A supply of a fresh and wholesome atmosphere may accordingly be ranked among the first and most essential necessities of life. In the atmospheric ocean which rests on the surface of the terraqueous globe, a perpetual movement, or natural ventilation, is sustained on a great scale by numerous causes, more especially however by the unequal action of the rays of the sun on the equator and at the poles; the colder air moving along the surface from the poles to the equator, while the warmer air from the equator ascends and proceeds in a contrary direction towards the poles. These great and primary currents are modified in endless variety by the attraction of the sun and moon, the rotation of the earth upon its axis, the relative effect of the land and water, the ever-varying influence of local temperature, volcanic action, meteoric phenomena, more especially the evaporation and deposition of moisture, the electrical condition of the air and of the surface of the earth, and the innumerable changes that attend chemical action in the mineral kingdom, as well as in those that occur in the organic world. The animal and vegetable kingdoms not only contribute to the movement of the air, but are at once the great causes of the most important changes induced upon it, and of its restoration to its former composition; the animal kingdom consuming its oxygen and producing carbonic acid, while in the vegetable kingdom, the great tendency is to absorb carbonic acid and replace oxygen. But where these great movements are interrupted by local causes, or an undue accumulation of vegetable and animal debris takes place, there the due balance is not sustained, pestilential effluvia contaminate the air, and were it not for the wind, the rain, and the impetuous storms which from time to time visit such localities, and the operation of a peculiar diffusive power, in consequence of which no gas can accumulate permanently on the surface of the earth, whatever may be its specific gravity, they would in general become at last as fatal as the valley of death in Java, or the carbonic acid springs in Bavaria, in the grotto del Cano, and other places, which are so notorious for their destructive atmosphere.
In selecting a site either for a house, a city, or any establishment where persons are crowded together, too much attention cannot be paid to its natural ventilation; an aspect towards the south, a dry gravelly soil and moderate elevation, securing efficient drainage and freedom of access to the air, with protection from offensive currents, and an immunity from local impurities, are the great desiderata.
In the open air the temperature is generally under that of the body; and the air expired from the lungs, and also that in contact with the surface of the body, being expanded by the heat which it receives, escapes, from its levity, and a fresh stream is immediately supplied; but when the body is confined within a limited space, special means must be employed to sustain a constant renewal of fresh air.
This artificial ventilation, it is obvious, must be modified and adapted to the peculiar circumstances of each individual case, and the state of the external air which is supplied. It is a subject therefore of great extent, and as various in its details, as the climate in which man lives, the habitations in which he dwells, the occupations in
which he is engaged, the food upon which he lives, the means of protection against heat and cold which he can command, and the peculiarity of constitution which he may present. We shall therefore content ourselves, in the remaining space allotted to this article, by pointing out the leading facts connected with the more prominent objects which it includes in reference to the frame of man. It must not however be forgotten that this branch of science is as yet in its infancy, that the history of numerous points connected with the motion of aerial fluid, is only beginning to be minutely investigated; and that, in a practical point of view, it is as yet so imperfectly attended to, that it is impossible to turn to any city without seeing discomfort, disease, and even death, from time to time induced by ignorance of the laws of ventilation; and in assemblies of every variety, the whole audience is not only too frequently subjected to extreme uneasiness, but the tone of the mental faculties, and the capacity for exertion and attention are often affected by the state of the atmosphere.