Why does the mercury level fall in a capillary tube? Explain.

(N/A) The height of a liquid column in a capillary tube is given by the formula $h = \frac{2S \cos \theta}{r \rho g}$,where $S$ is the surface tension,$\theta$ is the angle of contact,$r$ is the radius of the capillary,$\rho$ is the density of the liquid,and $g$ is the acceleration due to gravity.
For mercury and glass,the angle of contact $\theta$ is obtuse (approximately $135^{\circ}$),which is greater than $90^{\circ}$.
Since $\cos \theta$ is negative for angles greater than $90^{\circ}$,the value of $h$ becomes negative.
$A$ negative height indicates that the level of mercury in the capillary tube is lower than the level of the mercury in the surrounding container. This phenomenon is known as capillary depression.