Clark is best known by his hard water tests and by his process for softening chalk waters. His soap test for hardness, patented in 1841, was quickly taken by the government for waters proposed to be supplied to towns. His other major invention was the process of softening waters rendered hard by the presence of calcium bicarbonate in solution, a process that Thomas Graham took as exemplary applied science. Although the process was favourably reported on to the government in 1851 by Graham, Miller, and Hoffmann, it was opposed by the metropolitan water companies, and was adopted in only a few places.