Illustrated here is an old example of precipitation in liquid phase. A solution of antibody is carefully layered on top of a solution of antigen, such that there is no mixing between the two. Over the course of a few hours, at the interface where the two layers meet, antigen-antibody complexes form a visible precipitate, as seen in the tube on the right. The other two tubes are negative controls, containing only antibody or only antigen plus an irrelevant protein in the second layer. (Note that the tubes are illuminated by a beam of light coming at an angle; the precipitate scatters the light - soluble protein does not - and becomes visible as a bright band on a dark background.)