The angle of contact at the interface of water glass is `0^(@)` ethylalcohol-glass is `0^(@)` mercury glass is `140^(@)` and methyliodide-glass is `30^(@)` A glass capillary is put in a through containing one of these four liquids. It is observed that the meniscus is convex. The liquid in the through is

Options

A. water
B. ethyalcohol
C. mercury
D. methyliodide

Explanation

To solve the problem, we need to analyze the given information about the angle of contact for different liquids with glass and determine which liquid produces a convex meniscus when placed in a glass capillary.

### Step-by-Step Solution:

1. **Understand the Concept of Angle of Contact**:
- The angle of contact (or wetting angle) is the angle formed between the tangent to the liquid surface and the solid surface at the point of contact.
- If the angle is acute (less than 90 degrees), the liquid tends to wet the surface, resulting in a concave meniscus. Conversely, if the angle is obtuse (greater than 90 degrees), the liquid does not wet the surface well, leading to a convex meniscus.

2. **List the Angles of Contact for Each Liquid**:
- Water-glass: \(0^\circ\)
- Ethyl alcohol-glass: \(0^\circ\)
- Mercury-glass: \(140^\circ\)
- Methyl iodide-glass: \(30^\circ\)

3. **Identify the Type of Meniscus**:
- The problem states that the meniscus observed is convex. This indicates that the liquid has an obtuse angle of contact with the glass.

4. **Analyze the Given Angles**:
- Water and ethyl alcohol both have an angle of contact of \(0^\circ\) (acute), which would create a concave meniscus.
- Methyl iodide has an angle of \(30^\circ\) (acute), also resulting in a concave meniscus.
- Mercury has an angle of \(140^\circ\) (obtuse), which would create a convex meniscus.

5. **Conclusion**:
- Since the only liquid that produces a convex meniscus is mercury (with an angle of contact of \(140^\circ\)), we conclude that the liquid in the trough is **mercury**.

### Final Answer:
The liquid in the trough is **mercury**.