Re: Why does mercury 'bubble' upwards when water bubbles downwards?

When a liquid is placed in a cylinder a meniscus forms. The curvature of the meniscus depends upon surface chemistry, the same science involved in soaps and detergents. What you refer to as "bubbling downward" refers to the edges of the meniscus being higher in the cylinder than is the liquid in the center. This is frequently referred to as capillary rise and results from the water wetting the surface of the cylinder.

Mercury "bubbles downward" or forms a capillary depression. In other words, the mercury does not wet the glass surface of the cylinder, so that the outside edges of the mercury are lower than that in the center. The effect of capillary rise and capillary depression become more obvious as the diameter of the cylinder is decreased. If you put a small glass cylinder (between a centimeter and a millimeter in diameter) into a pool of either water or mercury, you could actually see that the water within the cylinder would be higher than in the surrounding pool of water. Likewise, the mercury within the cylinder would be lower. This is all related to surface tension or surface free energy.

Imagine that the molecules of water or mercury are like a group of teenage boys that want to be accepted by their friends. When you put the boys all by themselves, then tend to congregate together to form a spherical drop. All liquids attempt to minimize surface energy by forming a drop, and groups of small drops will want to get together to form larger drops. That way nobody stands out, nobody is alone.

Now, say that you confine this drop within a cylinder; you are going to make all of the boys stand in a hallway. If we can line the hallway with something attractive, say a row of very attractive young ladies, then the boys will be attracted into the hallway and will spread out, with more boys attracted to the walls of the hallway than left in the center. If we increase the attraction on the walls of the hallway, say by putting all of the girls in bikinis or lingerie, then the boys will spread out even more (to get a better look) and boys outside of the hallway will attempt to enter the hallway. (Those boys with same sex orientation might be analagous to filling the hallway with mercury rather than water).

On the other hand, we could line the hallway with something unattractive to boys, say maybe fresh cow manure. Now the same boys will resist going into that hallway, and those near the walls are going to be pushing toward the center.

So in our example, a clean glass cylinder will attract water, whereas, if we line the cylinder with something like wax, the water will be repealed. In the case of water versus mercury, water likes to wet clean glass surfaces, whereas mercury is not attracted to clean glass.

Energy is required to increase the surface area of any material. Generally a small amount of heat is generated when a clean solid surface is immersed in liquid (called the heat of immersion). If the heat of immersion is greater than the energy to create new surface area (increase the surface free energy) in the liquid, then the liquid will wet the solid surface and spread over it. If the heat of immersion is less than the increase in surface free energy required to generate additional surface area, the liquid will attempt to not wet and spread over the surface. Surface chemistry can get pretty complex and is usually covered only later in college or graduate school. It is increasingly important because it applies to problems ranging from catalysts to molecular biology. My favorate reference is Arthur W. Adamson's book "Physical Chemistry of Surfaces" published by John Wiley & Sons (although he doesn't use the same type of examples involving teenagers as I do).