3. The exposure of water in divided currents to the air by passing it through a sieve has been proposed as a means of purifying water, but it is inefficient when trusted to alone. Plants in reservoirs help to absorb organic matter; and fish, by destroying small crustaceans, have been found useful. Hard waters do not bear exposure to light, as a thick green growth of chara occurs, which may block pipes, and give a bitter taste to the water. 4. =The Addition of Chemical Substances.=—(1) _Clarke’s process_ consists in adding milk of lime, _i.e._ an emulsion of quicklime with water, to the water in the reservoir on a large scale. By this means calcium carbonate is precipitated, but no effect is produced on calcium and magnesium sulphates and chlorides. The hardness of the Thames water can thus be reduced from 16° to 3° or 4° (_Clarke’s scale_). The calcium carbonate carries down with it suspended and possibly dissolved organic matter. In the _Porter-Clarke process_ lime-water, _i.e._ milk of lime diluted, and the excess of lime separated by settlement or filtration, is mixed with the water to be purified, the water being freed from the precipitated calcium carbonate either by subsidence or by being forced through a filter of stretched canvas. (2) _Carbonate of Soda_ added to boiling water throws down calcium carbonate, and possibly lead if present. Much less is required when added to boiling than to cold water. Maignen’s process consists in adding _anti-calcaire_ powder, containing chiefly carbonate of soda, lime, and alum. (3) _Aluminous salts_ are very effectual in removing suspended organic matter, if the water contains calcium carbonate. On the addition of alum, calcium sulphate and aluminium hydrate are formed, both of which fall to the bottom, carrying with them other impurities. The amount of alum required is about 6 grains per gallon of water. If the water is not hard, a little calcium chloride and carbonate of soda should be put in before the alum is added, in order that a precipitable substance may be formed. (4) _Potassium permanganate_ readily removes the offensive smell of stagnant water, but it gives a yellow tint to the water. The addition of a little alum will help to carry down the decomposed permanganate. (5) _Perchloride of Iron_, in the proportion of 2½ grains to a gallon of water, has been found to completely purify water from finely suspended organic matters and clay. (6) More recently, other substances, such as _iodine_ and _hyposulphite of soda_, have been recommended. These are supposed to act by sterilizing the water, and iodine in suitable quantities undoubtedly effects this.