First at the University of Glasgow and then at Edinburgh, William Cullen and his students used chemical affinity to analyze mineral waters in their courses on materia medica, chemistry, and later natural history. These chemical analyses provided information about the constituency, medical efficacy, and the natural and artificial formation of spa waters. In turn, the precision required in isolating dilute salts in mineral waters drove the advancement of affinity theory and its analytical techniques. The recognition that gases and various non-soluble earths contributed to the medical efficacy of mineral waters but could not be studied in samples that had been bottled and shipped led to the rapid development of in situ experimentation. Mineral water literature was a medium of advertising, for the water as a natural commodity, the spa as a medical resort, and the doctor as an authority. Doctors wrote pamphlets, articles, and monographs to gain the patronage of the gentry who owned or controlled the spas and to attract clients. In a period with no formal system of medical accreditation, these publications served as credentials demonstrating formal education, knowledge of a particular spa, and (through the dedication and list of subscribers) the patronage of the local gentry. I use the correspondence and mineralogical publications of Cullen and his students to show how affinity chemistry superceded the battery of tests that had been used in the sixteenth and seventeenth centuries to identify the constituents of mineral waters. I will also detail how these doctors sought and gained patronage through their various publications.