When possible stagnant water should not be used for human consumption, but there are occasions when it is the only source of supply at hand. In such cases it is important that the objectionable conditions ber emoved. Various methods of removal are practised. Mechanical agitation which would naturally prevent the growth of these organisms is not applicable as a rule. Therefore recourse is usually had to chemical treatment. Various kinds of chemicals as will be shown further on added to stagnant water poison the cells of the organisms and thus apparently purify the water. The purification consists in the death and decomposition of the organisms growing upon the surface and in the body of the water. It is seen that no real purification takes place in one sense, since the remains of these organisms are simply deposited and may continue to decay as soon as the poisonous principle is sufficiently removed to permit the renewal of bacterial activity.