152 



SCIENCE 



[N. S. Vol. XXXIV. No. 866 



THE MOISTURE IN THE AIE WE BREATHE 



Dr. Gulick's letter about the air we breathe 

 in buildings, in the March number of Science, 

 calls attention to some difficulties that have 

 been troublesome to many of us for a long 

 time. 



During the winters of 1896-7 and 189T-8, I 

 made a series of observations in office, resi- 

 dential and school buildings in Milwaukee, 

 Wis., giving particular attention to the hu- 

 midity during the period of artificial heating. 

 The results of this preliminary study were 

 published in a condensed form in U. S. 

 Weather Bureau Bulletin, No. 24, in 1899. 

 Later observation and study tend to confirm 

 most of the conclusions reached at that time, 

 but have failed utterly to furnish a satisfac- 

 tory answer to that most important and all- 

 including question, why in-door living is less 

 healthy than out-door living. Certainly, there 

 is some condition of environment, inimical to 

 health, seemingly brought about by artificial 

 heating that, thus far, has escaped observa- 

 tion. 



The most obvious difPerence between inside 

 and outside air appears to be in the moisture 

 content, and, as Dr. Gulick asks a number of 

 pointed questions about this important con- 

 stituent of the atmosphere, a non-technical 

 statement of the generally accepted view may 

 be of interest. 



Unfortunately the terminology used to ex- 

 press various conditions of atmospheric mois- 

 ture was invented before we knew as much as 

 we now think we know about the several fac- 

 tors involved, and, therefore, instead of assist- 

 ing to a proper understanding tend to con- 

 fusion. 



1. The expression, capacity of air for mois- 

 ture, is misleading. A better expression is, 

 capacity of space or vapor for moisture, be- 

 cause the presence of air in space has nothing 

 whatever to do with the capacity of the space 

 for moisture, the only efl^ect of the presence of 

 air being to retard the diffusion of moisture 

 within the space. 



2. Likewise, the expression, saturation of 

 air, implies that the presence of air affects the 



amount of moisture required to saturate a 

 given space, which is not the case. It is a 

 rather curious fact that, although atmospheric 

 air is a mixture of nine or ten different gases, 

 each gas, including vapor of water, tends to 

 arrange itself according to its density and acts 

 in all respects as it would if no other gas was 

 present. In other words, each gas forms an 

 atmosphere about the earth independent of all 

 other gases. We, therefore, may eliminate 

 dry air from consideration because it is not a 

 factor in the problem. 



To assist in obtaining a definite view let us 

 imagine a cylinder of space 50 feet in diam- 

 eter extending upward from the surface of a 

 lake a distance of 10 miles, which is about as 

 high as vapor will rise to an appreciable ex- 

 tent in our atmosphere. We will assume that 

 the average temperature of the space within 

 the cylinder is 40° F., and that the tempera- 

 ture of the surface of the lake is the same. 

 How does the water in the form of vapor pass 

 from the lake into this space ? The molecular 

 theory of matter teaches that the molecules of 

 water are in a constant state of agitation; 

 that the velocity and amplitude of their vibra- 

 tions varies with the temperature, being 

 greater for high temperatures than for low 

 temperatures; and that some of the molecules 

 in darting about attain a velocity and direc- 

 tion that carry them beyond the attraction 

 they have for each other, and, hence, they fly 

 off into the space above the water. This is 

 our understanding of the process of evapora- 

 tion. But it so happens that these molecules 

 of water in the form of vapor do not escape 

 the control of gravity, which operates to pull 

 them down toward the earth in accordance 

 with their weight or density. As the mole- 

 cules continue to escape and a greater number 

 pass into the space, they impinge more and 

 more upon the surface of the water and in- 

 creasingly impede the escape of other mole- 

 cules from the surface, so that the process of 

 evaporation becomes slower and slower. It, 

 however, does not cease entirely, because the 

 molecules of vapor also are in a constant state 

 of agitation, and, in darting about and beat- 



