Balneology is the branch of medical science concerned with the therapeutic value of baths, especially those taken with natural mineral waters. Balneotherapy may involve hot or cold water, massage through moving water, relaxation or stimulation. Many mineral waters at spas are rich in particular minerals (silica, sulfur, selenium, radium) which can be absorbed through the skin. Public baths originated from a communal need for cleanliness at a time when most people did not have access to private bathing facilities. In ancient times public bathing included saunas, massages and relaxation therapies. Members of the society considered it as a place to meet and socialize. Public bathing could be compared to the spa of modern times. Woodcut from "Kalender des Doctor Kung!", 1508.