called practically dry air—that is, air capable of containing much more moisture than it has. The amount of drying work done is according to the dryness, warmth, and speed of the air; with little of these qualities, a great bulk of air is required, where otherwise a small bulk might have sufficed. By warmth and dryness we have an advantage in requiring less rapid currents. I have said elsewhere that a climate has a certain advantage from being very rainy. To call dryness an advantage is not a contradiction. When rain falls and washes the air, we can feel the benefit; when the substances floating in the atmosphere are dried up, we can imagine the advantage; but when the air is kept loaded with moisture which does not fall as rain, and is not carried off by wind, we can easily understand why the results should be hurtful. That it is not the watery vapor itself that injures may be learned from dye-houses, where men spend their lives in all conditions of dampness, sometimes in steam dense enough to make it difficult to see to the distance of a yard. There they have not the heavily-laden moisture of hot, damp climates with rich vegetation, and they have abundant warmth, so that the moisture is not used for absorbing heat and producing colds.
The demands of ventilation would best be explained if we could reply to these questions: What is the smallest amount of carbonic acid which may be call injurious? and what is the smallest amount of organic matter?
The amount of carbonic acid in the air is under .04 per cent, in places that are healthy, but not above .032 in the most open and healthy places. About five times that amount affects a candle sensibly, a photometer being used, and it is extremely probable that less affects it also. Are human beings affected as readily? I rather avoid this question at present; we have not facts enough. We will now speak of the gas in conjunction with organic matter.
Let us take, the two together, and we then find that much depends on the temperature also. If the day be warm, we may pass from a room having .06 in a hundred of carbonic acid to the air with .03, and feel refreshed. If the day is not warm, we do not feel the difference; at least, such persons as I have examined do not. The conclusion is that, in the early stages of the want of ventilation, the organic exhalations are the most injurious. Now, these increase with the temperature, while the acid does not. For this reason we ventilate in warm weather for the organic matter far more than for the sake of the acid. As the former has not hitherto been estimated by weight, we may view the subject only in relation to the carbonic acid. I think it probable that we shall be able to view it also in relation to other substances. For example, so much temperature will represent so much organic exhalation, and the volume of air will differ accordingly.
When the ventilation is desired to be very good, the amount required when pure air is supplied is much less than with imperfect air.
