The angle of contact depends on the relative strengths of interfacial tensions at the three interfaces: liquid-air (S_la), solid-air (S_sa), and solid-liquid (S_sl). At the line of contact, these surface forces must be in equilibrium, giving: S_la cos θ + S_sl = S_sa.

For mercury on glass, the molecules of mercury are attracted strongly to themselves and weakly to those of glass. This means S_sl > S_la, resulting in an obtuse angle of contact (θ > 90°). Mercury therefore forms a convex meniscus in a glass tube.

For water on glass, the molecules of water are strongly attracted to those of glass. This reduces S_sl, making S_sl < S_la, which results in an acute angle of contact (θ < 90°). Water therefore forms a concave meniscus in a glass tube.

When a liquid wets a solid surface, it tends to spread out. This happens when the molecules of the liquid are strongly attracted to those of the solid, which reduces S_sl and results in an acute angle of contact. Water on glass exhibits this behavior - it wets the glass surface and tends to spread out.

On the other hand, when a liquid does not wet a solid surface, it tends to form drops. This happens when the molecules of the liquid are attracted strongly to themselves and weakly to those of the solid. This makes it energetically costly to create a liquid-solid surface, resulting in an obtuse angle of contact. Mercury on glass exhibits this behavior - it does not wet the glass surface and tends to form drops.