Capillary depression in mercury occurs due to the greater cohesion between mercury molecules compared to the adhesion between mercury and the capillary walls.

Explanation:
1. Capillary action: Capillary action is the phenomenon in which a liquid rises or falls in a narrow tube, known as a capillary tube. It occurs due to the balance between cohesive forces (forces between similar molecules) and adhesive forces (forces between different molecules) in the liquid.

2. Surface tension: Surface tension is the property of a liquid that enables it to resist external forces and minimize its surface area. It is caused by the cohesive forces between the liquid molecules.

3. Capillary depression in mercury: When a capillary tube is dipped into a container of mercury, the liquid level inside the tube is lower than the liquid level outside the tube. This is known as capillary depression.

4. Cohesion and adhesion in mercury: Mercury is a cohesive liquid, meaning that the cohesive forces between mercury molecules are stronger than the adhesive forces between mercury and the capillary walls. This results in capillary depression.

5. Cohesion vs. adhesion: Cohesion is the force of attraction between molecules of the same substance, while adhesion is the force of attraction between molecules of different substances. In the case of mercury, the cohesive forces between mercury molecules are greater than the adhesive forces between mercury and the capillary walls.

6. Capillary depression mechanism: The greater cohesion in mercury causes the liquid to pull back and minimize its surface area inside the capillary tube. The cohesive forces between mercury molecules overcome the adhesive forces between mercury and the capillary walls, leading to the depression of the mercury level inside the tube.

In conclusion, the capillary depression in mercury is due to the greater cohesion between mercury molecules compared to the adhesion between mercury and the capillary walls. This difference in forces allows mercury to minimize its surface area inside the capillary tube, resulting in a lower liquid level inside the tube compared to outside.