meniscus
/ muh-NIS-kus /
Fill a narrow glass cylinder with water and look closely at the top of the liquid: it isn't flat. The water edges climb a little higher where they touch the glass, dipping in the middle into a shallow U-shaped cup. That curved surface where a liquid meets the wall of its container is the meniscus.
A meniscus is the curved upper surface of a liquid in a tube or container, caused by the contest between adhesion (the liquid sticking to the wall) and cohesion (the liquid sticking to itself). When the liquid is more attracted to the wall than to itself, like water in glass, the edges climb and the meniscus curves downward into a concave dip. When it is more attracted to itself, like mercury in glass, the edges pull away and the meniscus bulges upward into a convex dome.
Why it matters: the meniscus is the everyday signature of cohesion and adhesion, and reading it correctly is a basic lab skill — you measure the volume in a burette or graduated cylinder at the bottom of a concave meniscus, viewed at eye level to avoid parallax error. The honest caveat is that which way a meniscus curves tells you about the liquid–surface pair, not the liquid alone: water is concave in glass but would behave differently against a waxy, water-repelling surface.
Water in a glass burette forms a downward-curving (concave) meniscus, so you read its volume at the bottom of the curve; mercury forms an upward-bulging (convex) one and is read at the top.
Read a concave meniscus at its bottom, a convex one at its top.
Which way the meniscus curves depends on the liquid–surface pair, not the liquid alone. Water curves concave in glass (adhesion wins), but mercury curves convex (cohesion wins). Always read a graduated tube at eye level to avoid parallax error.