1. Boiling. 2. Filtration. 3. Distillation.

Boiling is a certain method of destroying bacteria. If there is the slightest doubt as to the purity of the water, it should be boiled for twenty minutes and then be quickly cooled. It should be put into perfectly clean bottles, tightly stopped, and be placed on or near the ice.

Filtration

The modern filter contains a hollow tube of unglazed porcelain, which is porous and through which the water percolates slowly. All mechanical impurities of the water are deposited on the surface of the tube. These impurities usually carry the bacteria, which are thus removed.

The tubes should be washed often, and occasionally be re-moved from the filter to be baked or steamed.

Charcoal is used in many niters, as it removes all color, odor, and taste from the water by holding back the organic matter; but the charcoal may soon become a source of con-tamination, as bacteria will grow rapidly in the organic matter present.

Many cities purify the water supply by precipitating the organic matter present by the addition of iron sulphate, alum, or calcium hydrate. The bacteria are entangled in the precipitate, which is then filtered out by mechanical filtra-tiqn through sand and gravel.

Distillation is the process of condensing steam. The water thus obtained is the only chemically pure water.