464 WATER SUPPLY AND SEWAGE DISPOSAL 



varies somewhat from hour to hour. At the time of storms the 

 pressure is often considerably less than on the day before or 

 upon the day following. This variation frequently amounts to 

 J^o f the entire pressure of the atmosphere. Generally, how- 

 ever, the change is less than this. 



Now, if the soil-air is relieved of J-^o of the pressure which it 

 has been sustaining, it will expand J^o m volume, according to 

 Boyle's Law. This means that J-^o of the soil-air will be ex- 

 pelled from the soil. After the storm has passed and the at- 

 mospheric pressure has again increased, fresh air again enters 

 the soil. Since on the average, fairly well-marked storms pass 

 over central and eastern United States about once every four 

 or five days, it follows that the earth takes an additional 

 breath, or rather an extra breath, about once in four or five 

 days. 



545. Another Cause of Change in Soil-air. While it is true 

 that both the changes in soil temperature and the changes in 

 atmospheric pressure cause the changes in soil-air as shown, it is 

 also true that they are not the only, nor even the chief causes of 

 the exchanges of soil-air which take place. Experiments have 

 shown that a still more important cause of this exchange is that 



OF DIFFUSION. 



Exercise 94. Diffusion of Gases 



Close all the windows and doors of the room. Open a bottle of 

 the oil of peppermint or musk (camphor or ammonia will do) in 

 one corner of the room and let a few drops fall upon the floor. Let 

 another person be at the opposite corner of the room. Let him notice 

 carefully to see how long it is before he can detect the odor of the liquid 

 you are using. It will not be very long before he can do so. How 

 does it happen that he can thus detect the presence of the liquid so 

 distant from him? 



EXPLANATION. Evidently the liquid does not reach him 

 It must be that it first changed to a vapor, or gas, and that the 

 gas by some means reaches him. The truth is that all gases 

 are made up of many rapidly moving particles. The particles 

 of air ; for instance, under the usual conditions of temperature 



