Capillarity involves the behavior of liquids in narrow spaces, which can lead to confusion when comparing mercury in a capillary tube and in a barometer. In a capillary tube, mercury's meniscus is lower than the liquid surface due to its non-wetting properties with glass, while water rises because it wets the glass. In contrast, a barometer measures atmospheric pressure using a closed tube with a vacuum at the top, causing mercury to rise or fall based on external pressure changes. The diameter of barometer tubes is large enough that surface tension effects are negligible, clarifying the apparent contradiction. Understanding these principles highlights the differences in liquid behavior under varying conditions.
