THE SURFACE OF A LIQUID 



29 



water to glass. Since we pull water away from water, 

 we see that the cohesion of water is not as strong as the 

 adhesion of water to glass. 



Cohesion in solids causes them to be rigid; that is, to 

 resist being strained or broken. 



31. The Surface of a Liquid. We know that large 

 liquid surfaces are flat (horizontal) ; this is because grav- 

 ity pulls down equally on all parts of the surface. But 

 when the body of liquid is very small (a drop), the effect 

 of gravity is also small, and cohesion is able to pull the 

 liquid into the shape of a sphere. This is exactly the 

 shape that would be produced if the liquid were enclosed 

 in a tightly stretched, elastic covering, say, of rubber. 



The shape of a drop of liquid depends on cohesion 

 alone, but a quantity of liquid in a vessel is acted upon 

 by three kinds of force, and the shape of its surface will 

 depend on all three. These are: (1) gravity, (2) cohesion 

 of the liquid, (3) adhe- 

 sion between the liquid 

 and the material of the 

 vessel. 



Fia. 23. 



Surfaces of Water (A) and Mercury (B). 



Let us take the case of mer- 

 cury in a glass vessel (Fig. 23, 

 B). Mercury does not wet 

 glass. This means that the 

 cohesion of mercury is greater 



than the adhesion between mercury and glass. Hence the surface 

 of the mercury curves outwards, or is convex, like the surface of a drop. 



The case of water in a glass vessel is different (Fig. 23, A) : water 

 wets glass. This means that the adhesion between water and glass is 

 greater than the cohesion of water. In such cases the liquid is drawn 



