The various metal filters in which the water comes in contact with metallic surfaces, either iron, lead, tinned iron, or zinc, are objectionable from their appreciable influence upon the water retained in them for any considerable time. Pure block-tin is the least objectionable of any of the metals.
The aim of most filters is to remove impurities from the water speedily—as rapidly as it escapes from the faucet. Experiment shows that effective filtration can not be accomplished in this way, as the water does not remain long enough in contact with the filtering material used to become purified of much that might be removed by slow filtration or percolation through the same appliance. Of all the filtering materials mentioned, it seems to me that sand and charcoal are the two that accomplish the best results, and of these vegetable charcoal is the best.
Clean quartz sand will retard the passage of some impurities held in suspension, but no very careful investigation is necessary to demonstrate the presence of many impurities in water that has passed through it. The naked eye can detect them in most samples. Buck states, "The spores of algæ are not removed by the passage of water through sand," and he adds that "clean quartz sand can produce little effect" on polluted water. But he and many if not all other sanitarians assert that charcoal does purify the water and remove the odor of putrefaction. While there is no lack of authority to prove the value of animal charcoal as a filtering material, the claims of vegetable charcoal seem to me to make it more serviceable. Vegetable charcoal is "the solid residuum of the destructive distillation of wood." It is insipid and inodorous, it is insoluble in water, it is but little affected by either acids or alkalies. The ash consists chiefly of carbonate of potash, silica, lime, and the oxide of iron. Vegetable charcoal has a strong deodorizing power. Water containing sulphureted hydrogen speedily loses its odor when filtered through it. The taste of liquids, when dependent on the presence of certain organic substances, is almost or entirely removed by filtering through it. "The purifying, antiseptic power of charcoal is due to the action of its absorbed oxygen upon organic matter." A careful authority says: "Charcoal, by possessing the properties of absorption, decomposition, and combination, is eminently fitted as a filter for the purification of water, removing from it the color, odor and taste of its impurities by oxidizing and recombining them into other and inoffensive substances." A reference to chemistry shows us that the following gases are absorbed by charcoal: Hydrogen, nitrogen, carbonic oxide, marsh-gas, nitrous oxide, carbonic acid, olefiant gas, sulphurous acid, air, sulphureted hydrogen, muriatic acid, hydrochloric acid, and ammonia.
Witthaus, in his "General Medical Chemistry," says on this subject: "Its" [vegetable charcoal's] "power of absorbing odorous bodies renders it valuable as a disinfecting and filtering agent, and in the
