On evaporation of the water to dryness and heating of the residue, not more than 0.20g of one gallon of water should remain. The residue should be white or have a yellowish tint, a proof that but little organic matter is present in water, and not appear dark gray, brown or black. The more coloration the residue shows, the more organic matter the water contains. In the first case, a filtration through sand and charcoal may be sufficiently thorough; in the latter case a chemical purification with permanganate of potash should be carried out in a cistern before filtration. Besides a trace of iron it should contain no metal, especially no lead or copper.

The solution of the residue in diluted nitric acid should on addition of sulphureted hydrogen not become dark colored or be precipitated.

Lime and magnesia together ought not to exceed 0.8g in one gallon of water.

Evaporation of water should be carefully done in a porcelain dish, without bringing the water to a boil, simply by heating, otherwise the residue would get charred. Heating on a waterbath is the best method. If the residue is weighed at this stage, then heated on a platina dish to red heat, the fireproof substances remain. Weigh again and the differences in both weights give an approximate idea of the quantity of organic sub-stances that were present. It is difficult to ascertain the quantity of organic matter; the depth of coloration of the residue indicates it approximately but qualitatively.