i8o 



The Living Plant 



ure 61, b), the surface layers of molecules are drawn strongly 

 inward so that collectively they press on the liciuid as if they were 

 tightly stretched rubber, — a phenomenon known as surface 

 tension. Now surfaces that are fiat press inward with a definite 

 force, but those which are concave, being partially buried, as it 

 were (figure 01, c), within the body of the liquid, and therefore 

 having the inward attractions of the molecules a little com- 



a b c 



Fig. 61.- -A diagram to illustrate the operation of forces concerned in capillarity, repre- 

 senting sections through convex, flat, and concave water surfaces. The small circles, 

 open and solid, are water molecules, and the larger circles are the areas within which 

 given molecules, represented black, are cohesively attracted by others. Where these 

 areas lie wholly within the liquid, as shown in the lower part of b, the attractions 

 balance one another, and no effect is produced; but where the areas fall partly outside 

 of the liquid, the inward attractions are not resisted by equivalent outward ones, 

 though the exact degree thereof depends on the form of the surface. 



pensated by partial attractions outward, press inwards with less 

 force, while those which are convex, projecting as it were outside 

 of the liquid, have their molecules drawn in with an even stronger 

 attraction than have those of a flat surface (figure 61, a). There- 

 fore it follows that the very mobile water will always be pressed 

 away from fiat or convex surfaces towards those which are con- 

 cave. Now it happens, furthermore, that water adheres both to 

 glass and to wood, and hence in a tube of either of these substances 



