xi ENERGY AVAILABLE FOR RAISING SAP 205 



concave tends to bring them into equilibrium with a lower 

 vapour pressure, and consequently reduces evaporation, 

 whilst the increased concavity of the lower menisci for the 

 same reason renders them more ready to condense water 

 vapour. 



When this model has been in action for some time it 

 will be noticed that the lower pot is at a higher temperature 

 than the upper one. This difference of temperature is 

 to be attributed to the " sorting demon " action progres- 

 sing at the two surfaces. The lower menisci form a trap 

 for more energetic water vapour molecules in the soil 

 spaces, the upper retain the less energetic, while those 

 which are more energetic escape into the surrounding 

 space. Hence there is a gain of heat to the lower menisci, 

 and a loss of heat by molecular convection from the 

 upper. This cooling of the upper menisci maintains a 

 regular flow of heat into the evaporating surfaces, which 

 is constantly being abstracted again by the escaping 

 molecules. As they escape, fresh molecules are drawn into 

 their places from the water beneath by the attractions 

 of those remaining in the menisci, and these mutual 

 attractions find expression in the tensile strength of the 

 liquid which joins the whole column to the evaporating 

 menisci. Thus the loss of molecules from the menisci, 

 kept up by the inflow of heat, is able, by calling into play 

 the mutual attractions of the water molecules, to set up 

 a stress in the water which may be transmitted to the 

 lower menisci. 



If we suppose, in order to imitate the conditions in the 

 plant more closely, the outer surfaces of the two pots in 

 the model to be covered with osmotic cells, we can readily 

 see that the conditions are not essentially altered. The 

 osmotic pressure in these cells need not change the gradients 

 of pressure in the water. The osmotic pressure is the 

 pressure which the dissolved substance exerts against the 

 membranes of the cells, while the tension is in the solvent 



