the science of the therapeutic use of baths
• (n.) A treatise on baths; the science of bathing.
Bal`ne·ol'o·gy noun [ Latin balneum bath + -logy .] A treatise on baths; the science of bathing. 
Balneology is the science of the medicinal effects of bathing and mineral springs.
balneology 1. The scientific medical study of bathing and medicinal springs. 2. The use of hot spring mineral water for healing or therapeutic purposes. This is considered to be the oldest historic use of natural geothermal waters.