CLEAN DRINKING-WATER. 247 



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 ma- 

 terial used to become purified of much that might be removed by slow 

 filtration or percolation through the same appliance. Of all the fil- 

 tering materials mentioned, it seems to me that sand and charcoal are 

 the two that accomplish the best results, and of these vegetable char- 

 coal is the best. 



Clean quartz sand will retard the passage of some impurities held 

 in suspension, but no very careful investigation is necessary to demon- 

 strate 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 algae are not removed by the passage of water through sand," 

 and he adds that " clean quartz sand can produce little effect " on pol- 

 luted 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 char- 

 coal as a filtering material, the claims of vegetable charcoal seem to 

 me to make it more serviceable. Vegetable charcoal is " the solid re- 

 siduum of the destructive distillation of wood." It is insipid and inodor- 

 ous, 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 : Hydro- 

 gen, 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 sub- 

 ject : " Its " [vegetable charcoal's] " power of absorbing odorous bodies 

 renders it valuable as a disinfecting and filtering agent, and in the 



