358 THE POPULAR SCIENCE MONTHLY. 



called practically dry air that is, air capable of containing much 

 more moisture than it has. The amount of drying work done is ac- 

 cording 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 advan- 

 tage in requiring less rapid currents. I have said elsewhere that a 

 climate has a certain advantage from being very rainy. To call dry- 

 ness 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 wdiich 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 sensi- 

 bly, 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 ex- 

 ample, so much temperature w T ill represent so much organic exhalation, 

 and the volume of air will differ accordingly. 



When the ventilation is desired to be very good, the amount re- 

 quired when pure air is supplied is much less than with imperfect air. 



