The author examines water's chemical and physical nature, its role in hygiene and physiology, and its therapeutic uses in health and disease. Chapters describe skin and mucous membrane structure, mechanisms by which baths alter circulation, temperature, and nervous function, and the effects of cold, warm, and hot applications. Historical bathing practices are surveyed, hydropathic errors and exaggerated claims are critiqued, and remedial properties such as sedative, tonic, and eliminative effects are discussed. Practical guidance and rules for many bathing methods and local applications are provided, with emphasis on rational, evidence-based use rather than sectarian extremes.