2. _Total solid residue and loss on ignition._--500 c.c. of the water under examination are introduced, in small portions at a time, into a tared platinum dish, and evaporated to dryness over the water-bath, the residue being subsequently dried for three or four hours in an air-bath at 100°. The solid residue obtained, multiplied by 200, represents parts in 100,000: or, by 140, grains per imperial gallon. It is usually considered that, unless the proportion of total solids exceeds 40 grains per Imperial gallon (32 grains per U.S. gallon, or about 56·5 parts per 100,000), the water need not be objected to for drinking purposes on this ground alone. The volatile and organic matters are determined by igniting the solid residue, which is afterwards allowed to cool. It is then moistened with a little carbonic acid water or solution of ammonium carbonate, dried to constancy at 130°, and the organic matter estimated by the decrease in weight. Formerly, this process was chiefly depended upon for determining the proportion of organic substances contained in water. It is open to numerous serious objections, among which are, that it may afford a result either below or above that correctly representing the quantity of organic ingredients present in the sample. The first case takes place when a portion of the organic matter is decomposed during the process of evaporation, and is quite liable to occur; the second case takes place when the water contains nitrates, which would be decomposed upon ignition. The method, however, possesses some value, and is still often resorted to as affording a general idea of the proportion of organic contamination present, the degree of blackening of the solid residue during the process of ignition being, at least, a useful qualitative indication.