When limestone (CaC03) contains clay, the process of calcination produces a compound which, due to the silica in the clay, gives the product the power of solidifying or "setting" when wet, either under water or in the open air. The degree of heat employed in the calcining process, and the relative proportions of clay and limestone, determine whether the product is hydraulic lime, quick-setting cement, or Portland cement. These cements were calcined formerly from deposits of the constituent materials just as they were found mixed in the earth, and the varieties of these mixtures were such that the resulting cements were not uniform, but the application of chemistry within recent years for the purpose of securing exactly the amount of each material required, and of avoiding deleterious materials, has brought about the extensive manufacture of a high grade of Portland cement. This has for most uses displaced other kinds of building cement, but has not displaced the use of lime for mortar, and there are cases in which a quicker setting cement may be more advantageously used than Portland cement. The name "Portland cement" comes from the supposed resemblance of the dry cement to Portland stone, in England, and was given it by its discoverer, in 1824.