............is the phenomena of rise or fall of a liquid in a capillary tube.

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the name of the phenomenon that describes the rise or fall of a liquid in a capillary tube.

2. **Defining Capillarity**: The phenomenon is known as "Capillarity" or "Capillary Action." This occurs when a liquid moves up or down in a narrow space, such as a capillary tube, due to the interplay of cohesive and adhesive forces.

3. **Explaining the Mechanism**:
- When a capillary tube is placed in a liquid (like water), the adhesive forces between the liquid molecules and the walls of the tube are stronger than the cohesive forces among the liquid molecules themselves. This leads to the liquid rising in the tube.
- Conversely, in the case of mercury, the cohesive forces are stronger than the adhesive forces, causing the mercury to fall in the tube.

4. **Observing Meniscus Formation**:
- In the case of water, a concave meniscus is formed due to the rise of the liquid, indicating that the liquid is climbing up the walls of the tube.
- In the case of mercury, a convex meniscus is formed, indicating that the liquid is falling down the tube.

5. **Conclusion**: The overall phenomenon of the rise or fall of a liquid in a capillary tube is referred to as "Capillarity" or "Capillary Action."

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