Thomas Clark (1801–1867) was a Scottish chemist.
He became known for the discovery of the phosphate of soda, and the process of lime softening of hard water the 'Clark process'. A Clark degree (°Clark) of water hardness is defined as one grain (64.8 mg) of CaCO3 per Imperial gallon (4.55 litres) of water, equivalent to 14.254 ppm. and 10^5 parts of water