The formula for cesium sulfide is Cs2S. It is formed when two atoms of cesium join with a single atom of sulfur and is a very mild acid that can be dissolved in water. It is also a salt that is associated with sulfuric acid.
Cesium sulfide is used to prepare dense aqueous solutions, which aid in separating dense substances from lighter ones in a process known as isopycnic centrifugation. Isopycnic centrifugation is also known colloquially as "density-gradient" because during the process molecules or elements bearing a greater density move toward the bottom of the solution, while those which are less dense float toward the top.