In the next place I come to consider, how much a certain Quantity of Air is requisite to preserve a single Animal Body, and the Knowledge of that, is what I account one of the chief Preservatives of Health. I have often been concern'd to find a Family of six or seven pinn'd up in a Room, that has not contain'd Air enough for the Maintenance of Health in one single Person; but such is the Hardship of our Poor in many Places, and is frequently the Occasion of their Death. We may easily conceive how this happens, if we examine the Case of the Diving Tub, how short a while a Man can live it, without a Supply of fresh Air; the occasion of which is, that when he has drawn in with his Breath, all the Grosser Parts from the Air enclos'd in the Tub, the rest grows hot and suffocating, by being too much rarified. From whence I suppose, a Room of Nine or Ten Foot Cube, will contain Air enough to keep a single Man alive for one Day, but if two were to inhabit that Space for the same time, each would receive but half his Nourishment, and so both would be Sufferers; but a Room, perhaps, containing twice that Space, might well enough serve five People for a Day, supposing that all External Air was kept from Communication with such a Room, during the time the People were in it; for, as I have observ'd, that Air has certain Nourishing Qualities in it, for the Maintenance of Human Life; so when those Nourishing Parts are imbibed, and drawn in by the Lungs, the Air is return'd and flung out as invalid, and cannot be of Use a second Time to the same Person; an Example of which, we find very curiously demonstrated by Mr. _Newyentyte_; he tells us, that in making this Experiment, he discover'd that the same Nourishing Quality in the Air, which is necessary to maintain Human Life, is also necessary to maintain Flame, which he proves thus: A lighted Candle being set under a Bell, closely fix'd upon a Table, will burn perhaps a Minute or two in Proportion to the Quantity of Air pent up with the Candle in the Bell; but as soon as the Quality in that Air, which is necessary to feed the Flame, is exhausted, the Candle goes out; this has been often try'd with the same Success; and we find, that by letting into the Bell some fresh Air, a little before the Candle should have gone out, it will still continue burning: And then to shew that this Quality in the Air is the same which feeds the Life in Humane Bodies, it was try'd, whether the Air, returning from the Lungs, would not have the same Effect upon the Candle, as the External Air had before, but it had not, the Candle went out at its usual Time: Thus, it seems, when we suck in Air for Breath, the Lungs takes what is necessary for the Nourishment of our Bodies, and returns back the rest.