2. Without examining closely the composition of atmospheric air, it may be accepted that it is a mechanical mixture of gasess - viz. oxygen and nitrogen - approximately in the proportions of one volume of oxygen to four of nitrogen, together with a varying percentage of carbonic acid; in addition to which ozone and, more recently, argon have been found in it. Vapour of water also, in varying quantities - principally regulated by the temperature of the air and its proximity to moisture - is always present. What may be the exact composition of air most suitable for sustaining healthy life we have no means at the present time of ascertaining, and the probability is that different constitutions demand for their healthy development different states of the atmosphere, and that these demands vary at different stages in life.

All investigators agree that in the composition of atmospheric air the most essential and active agent is oxygen; but alone, that gas would be too powerful in its action upon the blood, and would quickly consume life.

Nitrogen acts as a diluent, and many consider that it is limited to this office, but there are reasons for assuming that nitrogen has active although comparatively obscure functions to perform in connection with the support of the human frame.

Carbonic acid (a chemical composition formed by the combination of two parts of oxygen with one part of carbon) exists in the atmosphere in a gaseous state, and, where it does not exceed 1/2000 part in volume, is not regarded as an impurity; but air known to be impure has almost iuvariably been found to contain larger quantities of this gas. Consequently its relative proportion has been considered a test of atmospheric purity. The most recent investigations, however, tend to show that, within buildings, it must only be taken as an index, and not that this gas itself is the actual contamination.

The definite functions of other gases which have been found in the atmosphere are so far unascertained as regards the support of healthy life; it may therefore be generally accepted that air containing normal proportions of oxygen and nitrogen, with a small percentage of carbonic acid, as above stated, together with a varying amount of vapour of water proportionate to the temperature, is capable of fulfilling all its functions in connection with the maintenance of life, and in that state may be considered pure and wholesome to breathe. This may be regarded as the neutral side of the question in respect to ventilation.

Contamination of atmospheric air by the presence of other products is the posituve side of the question, and will claim more particular attention hereafter. Evidence is constantly forthcoming to prove that, if air be greatly contaminated, death results from breathing it, and that, even when the impurities in the air are not sufficient to cause death quickly, they may in the course of time so impair the respiratory organs, or lower the general tone of the body, that illness or untimely death results.

4. Contamination of the outer air may be the result of natural processes quite apart from human interference, not only in countries universally recognized as being unhealthy, but also in lesser areas even in our own country, so that mere change of one portion of such air within a building for another porrtion from the outside similarly contaminated will not secure good ventilation. In fact. there are districts in which at times it is more healthy to live within doors than to remain in the open.

5. The outer atmosphere is more liable to contamination by human agencies, which may affect whole districts, or be simply produced in or around a single dwelling or factory. In either case, it is unreasonable to expect that good ventilation can be secured by simply changing the air within a building for that from without, when the latter is contaminated.

6. Although atmospheric air is, even in the open, frequently contaminated by the presence of other gases, by vapours, and even by solid substances, nature has fortunately provided many processes of purification, both simple and complex, by means of which the air in most localities, after contamination, again becomes pure and healthy to breathe.

Movement is perhaps the most necessary for bringing about this desirable end, for where there is stagnation some impurities increase, and often become dangerous to health. Sunlight also plays a very active part Most vegetable growth assists by absorbing the excess of carbonic acid, and even the mineral kingdom exerts some influence. Water, in the raging storm, the soft rain and gentle dew, in many a tiny stream and brooklet, in the slowly gliding as well as in the rushing river, in the placid lake and mighty ocean, is almost everywhere assisting in maintaining the atmosphere in an uniform and healthy state. Heat and cold exert a powerful influence in the same direction.

Yet it must be remembered that, wherever impure matter or source of contamination exists, these purifying agencies may be neutralized, and some may even be made to assist in developing and spreading the impurities. In fact, so numerous and so complex are the agencies and processes which affect the atmosphere, either favourably or adversely as regards human existence, that the whole realm of natural science might be brought to bear upon the subject ere a complete knowledge could be gained respecting all the requirements necessary to secure the best ventilation.

7. When natural agencies are excluded from exercising their purifying effects upon atmospheric air within buildings, it becomes more and more contaminated, and impurities quickly increase by respiration and exhalations from living creatures, by combustion, by putrefaction of animal and vegetable matter, by fermentation, by volatilization, by particles of innumerable substances in the form of dust, by the presence of living organisms resulting from disease, and perhaps in other ways as yet undetected. The purification of the air in buildings is best secured by efficient ventilation, but where infectious disease has made its appearance in the building, antiseptic and more decided methods must be employed.