STEPHEN HALES 441
Mankind did not arrive apparently by the aid of "the light of nature*' alone at a knowledge of the supreme importance of ventilation. To some results of great practical importance, purely natural instincts have guided mankind ; for there are certain things known to be poison- ous when eaten, certain waters are declared non-potable; but, as regards the quality of the air to be breathed and what constitutes impure air, the natural teachings are exceedingly ambiguous. The natural man is all right so long as he remains under the open heaven, but as soon as he surrounds himself with four walls he seems not to know that he must constantly keep changing the invisible air around him. No doubt it is because it is out of sight that air is also out of mind : certain it is that at the present moment there are vast multitudes of people who never conceive of air as a real thing, as real as their meat and drink and just as necessary to be kept fresh. Cave-man had no trouble with ventila- tion, nor had those in " the tents of Shem *' ; but from the day that man began to sleep inside stone and lime he had to face the problem, although he was not in the least conscious of it, how could the foul air be re- moved and the pure air brought in without producing a chilling draught. To do this is to ventilate. The unpleasantness and even danger to health of this movement of the air was, doubtless, much of the reason why he was so long in grappling with the problem even when he had awakened to its existence. Not that he has even now by any means consciously solved the problem. There axe millions of houses over the length and breadth of the earth entirely unprovided with the means of ventilation; heated they may be, ventilated they are not. With their closed stoves and their windows shut, such rooms are as devoid of the means of changing the air in them as is an oven. The least fresh of rooms in England with their open chimney, even when no fire is burn- ing, and with "sash'* windows capable of being easily opened from the top without causing a draught, niay be said to be exceedingly well ventilated as compared with the typical room one finds on the Continent of Europe. All living things vitiate air on breathing it even once; all living things subsist by means of the absorption of oxygen, that is, of fresh air : this was what Hales grasped, and he saw how very far many members of the community were from being in a position to command at all times a supply of this absolutely necessary though perfectly in- visible material. Hales in England and Leeuwenhoek in Holland, neither of them medical men, were, about the year 1720, probably the two persons who saw more clearly than any one else in Europe the prime necessity for ventilation, that is, the constant change of the air in the neighborhood of living beings.
Hales did a very great deal else in science besides devising ventila- tors; he was a pioneer in the experimental method in both vegetable and animal physiology quite a hundred years before physiology as an experimental science existed in England. But in regard to the public
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