EXPERIMENT STATION BULLETINS. 



551 



following table shows the great influence that temperature has upon 

 the vapor pressure. 



It will be seen that the vapor pressure increases very regularly and 

 rather markedly with rise in temperature. At the temperature of 0° C. 

 the pressure is 4.5687 millimeters, while at 50° C. it is 91.982 milli- 

 meters. From these figures it can be easily conceived why the volume of air 

 expelled from the moist soils, and the pressure exerted by the air of the 

 same soils, do not follow the laws of Charles, but instead, they increase 

 differently at the various temperatures, and that the actual values are 

 far greater than the theoretical. To obtain, therefore, the real increase 

 in volume and pressure of the air alone, containing no moisture, the 

 increase in volume and pressure due to the aqueous vapor must be de- 

 ducted from the foregoing experimental data. This can be done by 

 the use of suitable formulae but it would be of no practical value be- 

 cause the data then would not show the complete influence of tempera- 

 ture upon soil ventilation. The expansion or pressure of this aqueous 

 vapor is dependent upon the temperature and upon the nature of the 

 liquid, but it is not affected by the pressure of other gases nor by the 

 amount of the liquid present. If a closed space, such as the tube here used 

 is filled with a mixture of gases, each gas will exert a definite pressure 

 which is the same whether it exists alone in the space or whether other 

 gases are present. The total pressure of the gas mixture is the sum 

 of the partial pressure of the various gases. The partial pressure of the 

 aqueous vapor can be determined by the formula already given, pro- 

 vided the gas is saturated. In the moist soil the air was saturated, 

 but in the dry soils it was not, and some irregularities observed in the 

 latter .soils, are attributed to this fact. 



As already stated it has long been known that absoption of gases 

 by solids decreases with temperature. This fact was verified in the 

 present investigation. In order to ascertain to what extent absorption 

 influenced the foregoing abnormal results, on soil aeration, a series of 

 experiments Avas conducted upon the subject. The problem was in- 

 vestigated by filling the usual tube with air dry soil and drying the 

 latter in an oven for several hours at a temperature of 105° C. By 

 liolding the tube horizontally the solid rubber stopper and the rubber 

 stopper which contained the bent tube, were in.serted at the opposite 

 ends. To the bent tube was attached also a tube containing CaCL. 

 As the soil cooled if drew air through the CaClo, the latter absorbed 

 the water vapor and thus dried the air as it entered the soil tube. The 

 soil was cooled to 0° C. and allowed to remain at that temperature for 

 a few hours to absorb gas and come to an equilibrium. The temperature 

 was then raised to different degrees and the gas expelled measured in 



