The determination of the degree of impurity of spring or well water is every day becoming of more and more importance. In order to discover how much foreign matter is contained in a sample of water, the chemist is under the necessity of drying up a quantity of it, and of weighing the residue. This operation is tedious, and gives a trust-worthy result only when the evaporation has been carried on in closed vessels from which atmospheric impurities have been carefully excluded; and, therefore, any assistance which can be obtained from mechanical measurements must be acceptable as affording a test for confirmation of the results.