Rigid Axis is like Wall, but it rotates around its center-point. It is often used to make a hammer or other rotating device by putting Jet inside one end of it. This makes it spin wildly. Rigid Axis doesn't melt, burn, float or sink, but it does mix with other elements. For example, it can be used to filter water from other liquids by mixing it with Heater.
Axis can exist in a liquid state, either by pressing Shift+X to Pour some, or by mixing it with Inflow. Liquid Axis does not seem to have any special properties.