Adhesion and cohesion
The force of attraction between molecules of the same substance is called cohesion, distinct from adhesion or the force between molecules of different substances. The adhesion of water to glass is stronger than the cohesion of water. Hence, . ‘hen water is spilled on a clean glass surface it wets the glass and spreads out in a thin film. On the other hand, the cohesion of mercury is greater than its adhesion to glass. Therefore, when mercury is spilled on glass it forms small spherical droplets or larger flattened drops. The difference between the adhesive and cohesive properties of water and mercury explains why the meniscus of water curves upwards and that of mercury curves downwards when these liquids are poured into clean glass vessels (Fig. 13.9).