_Mirabilite_, (Glauber’s salt) or sulfate of soda, exists not unfrequently in the soils of the arid region and sometimes encrusts extended areas of lowlands during the dry season. When present in the soil it will commonly be seen blooming out on the surface after a rain, in light, feathery, needle-shaped crystals, sometimes to such an extent that it can be collected by the handful. Subsequently, when wafted by the wind, it is reduced to a fine white dust, which constitutes a goodly proportion and sometimes the entire mass of the “alkali dust” that is so annoying on the plains of Nevada, and in the desert regions generally, during the hot summer. Near White Plains, Nevada, it forms a thick layer of “_white sand_,” in which the foot sinks deeply, and which is carried about by the wind with great ease. Glauber’s salt is never a desirable soil-ingredient. It is largely produced as a by-product in several industries, but cannot be utilized for agricultural purposes to any extent. It is, however, much less injurious to plant growth than common salt; according to experience in California it may be considered about three times less so. It constitutes the major portion of what is commonly known as “white alkali,” which is well known to be much less injurious to crops than the “black” kind, which contains carbonate of soda. _Trona and Urao_ are natural forms of carbonate of soda or salsoda. Like Glauber’s salt, it commonly occurs as a surface efflorescence or crust in dry or desert regions; either from the evaporation of standing water, as in the case of the soda lakes of Nevada, Hungary and Egypt, or as an efflorescence on the surface of the soil, as in the western United States, Mexico (“urao”), North Africa (“trona”), and at many points in the Old Continent. In the United States it is commonly known as “black alkali,” because of the black spots formed on the surface by evaporation; practically the same name (“kara”) is given it in Arabia and Asia Minor, whence impure soda has long been imported into Europe; while in north India it forms part of the “reh” salts that incrust large areas (usar lands) in the Indo-Gangetic plain. The natural mineral always contains an excess of carbonic acid over the “normal” salt, nearly in the proportion of four parts of carbonic dioxid to three of soda; it is sometimes designated as sesqui-carbonate. In hot sunshine it may lose most of this excess for a time; while within the soil itself it may, in presence of abundant carbonic acid, become temporarily converted wholly into hydrocarbonate or “bicarbonate,” which is less corrosive than the monocarbonate or common salsoda.