Now, therefore, the grand problem of sanitary science (almost untouched, almost unrecognized) proves to be no other and no less than this: 

What can be done to remedy the obstructive nature of an inclosure, so that its gaseous contents shall move off, and be replaced by pure air, as freely, as rapidly, and as incessantly, as in the open atmosphere?

It happens to be the most necessary preliminary in approaching this problem, to show how not to do it, for that, respectfully be it spoken, is what we have hitherto practiced, as results abundantly prove.  Fallacies, both vulgar and scientific, obstruct our way.  A fundamental fallacy respects the very nature of the work, which is supposed to be to get in fresh air.  In point of fact, this care is both unnecessary and comparatively useless.  Take care of the bad air, and the fresh air will take care of itself.  Only make room for it, and you cannot keep it out.  On the other hand, unless you first make room for it, you cannot keep it in; pump it in and blow it in as you may, you only blow it through, as the Jordan flows comparatively uncontaminated through the Dead Sea.  This is a law of fluids that must be kept in view.  The pure air is quite as ready to get out as to get in; while the air loaded with poisonous vapors is as sluggish as a gorged serpent, and will not budge but on compulsion.  Such compulsion the grand system of wind suction, actuated by the sun, supplies on the scale of the universe; and this we must imitate and adapt for our more limited purposes.

It would seem as if we need not pause to notice so shallow though common a notion as that which usually comes in right here, namely, that confined air will move off somehow of itself, if you give it liberty; being supposed to be much like a cat in a bag, wanting only a hole to make its escape.  Air is ponderable matter—­as much so as lead—­and equally requires force of some kind to set it or keep it in motion.  But applied philosophy itself relies on a fallacious, or, at best, inadequate source of motive power for ventilation.  It gravely prescribes ventilating flues and even holes, and promises us that the warmed air within the house will rise through these flues and holes, carrying its impurities away with it, from the pressure of the cooler and denser air without.  But we very well know that the best of flues and chimneys will draw only by favor of lively fires or clear weather.  They fail us utterly when most needed, in warm and murky weather, when the barometer is low, and the thin atmosphere drops, down its damp and dirty contents, burying us to the chimney tops in a pestilent congregation of vapors.