KIND OF BLUE

摘要

HEWORLD OF WATERS IS DIVIDED BETWEEN salty and fresh. That's no accident; a great deal of en- ergy, provided free by the sun, goes into evaporating seawater and dropping it into lakes, ponds, glaciers, and aquifers. At heart this process is an electrical transformation, as sodium and chloride ions (the elements of salt) are filtered out to create reservoirs of high and low salinity. That salinity difference represents an energy gradient, a potential for ions to flow. One can think of the earth as a giant battery: one pole is called "lakes and rivers," the other is called "the sea."