Answer :
Mercury is used in the capillary tube because it has a higher cohesive force compared to the adhesive forces.
Higher cohesive forces of mercury result in the greater attraction towards its own molecules rather than the walls of the capillary tubes. The walls of the capillary tubes are not wet by the presence of mercury as it shows capillary depression. Since mercury does not wet the walls of the capillary tubes, the phenomenon of capillarity does not occur. Capillary action is observed when the adhesive forces are stronger than the cohesive forces. The higher cohesive forces are present in mercury which explains the the angle of contact between glass & mercury is more than 90∘
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