The objection to filtration such as just described is found in the introduction of chemicals into the water supply, which in themselves are objectionable. Neither alum nor ferrous sulphate is desirable in water for domestic consumption. Both are injurious and in any considerable quantity are often poisonous. When used in conjunction with lime and judiciously manipulated none of these substances remain in solution; they are all removed with the material which is separated by filtration. Where applied, without care, as is often the case, in larger quantities than are necessary, some of these materials remain in the water supply; hence this form of filtration should only be used under the constant control of a skilful chemist who by his careful oversight and frequent examinations may assure the consumer that the water is free from the objectionable added materials. For these reasons the chemical process of filtration will probably never command universal confidence.