gas containing granules of a resiliently compressible material having a density variable with pressure are used as a solid phase support in liquid phase reactions such in immunological reactions . varying pressure causes the density of the granules to change and is used for mixing and separating the granules in liquid phase . in an immunological reaction , the granules having an attached antibody are combined with a solution containing an antigen , then varying pressure is applied to the solution to mix the granules in the solution to fix the antigen to the antibody and then pressure on the solution is varied to force the granules to the top or bottom of the solution whereby the solution can be separated from the granules .