From all we have hitherto shown on the subject of water purification, we must now form an opinion and decide upon a proper method for purifying water. If pure well or spring-water in limited quantities is used, a mere mechanical filtration is sufficient and any kind of filter or filter medium may be employed. Under these conditions a charcoal filter does excellent service and removes even the smallest traces of pollution, and lasts for some time before its activity is exhausted, which might be from a few days to not more than one month, depending on the quantity of water run through it, stated herein before as about 136 times the volume of the charcoal, and the degree of its pollution. However, we would suggest to arrange the filter for the ordinary and a reverse current, to enable the frequent washing out of the filter medium by the reverse current, and so to remove the retained solid matters, if convenient, by use of a force pump. A quite different method must be employed where impure or polluted water is our only resource, and which unfortunately, as we have seen, are in most cases the only water supply we can avail ourselves of, and when enormous quantities of water must be purified daily.

Carbonating water, solely for its purification, is too expensive and too impracticable for many a purpose, and we can give it no consideration in the way of practical purification of water.

Unquestionably the charcoal filter should take the first place of consideration; but when we consider that its time of activity is so very limited when polluted water in large quantities depends on its purifying capacity, that it is enormously expensive to be so frequently renewed as required, and further that it is impracticable as well as uneconomical to regenerate the charcoal of large water-purifying apparatus, we arrive at the conclusion, that we must look for other means of purification as a substitute for animal charcoal; or for a process that gives the same results in chemical purification, and leave it to the mechanical filter medium (sand, coke etc.,) to remove the suspended impurities as well as those separated or precipitated by the chemical purification.

We must then look upon an arrangement which combines the chemical and mechanical purification in a continuous process, acting quickly and effectively, combining the required conditions, allowing the filtering material to be easily and frequently washed out and the retained impurities to be removed in a manner that does not interfere with its continuous action.

Precipitation, aeration andjutration are decidedly the means we must adopt for a continuous, safe and effective method of purifying water. By the "Alum Process," earthy and alkaline carbonates and foreign matters and humus bodies are precipitated and even under certain conditions bacteria are destroyed. By aerating the water with special appliances, such as air compressors, etc., we substitute for the oxidizing power of the animal charcoal one of even greater energy and more continuously acting. By then filtering the water thus treated through properly prepared sand, coke, or other coarse filtering medium, we employ the proper method of purification.

If this precipitating, oxidizing and filtering method is by practical appliances and arrangements carried out to work continuously within or in conjunction with a water-purifying apparatus; and if this apparatus or filter is so arranged as to clean and wash the filtering medium and remove the out-filtered impurities by means of a periodical reverse current in an easy manner and without much interruption in the continuous filtering process; and if by mechanfbal agitation or by the force of the water the filtering medium can be agitated to assure its proper cleansing, and finally if some way is adopted to run off the washing liquid, we then have the best continuous purifying method and the most practical filter based upon scientific and practical principles that can be made.

The aeration to be effective should be carried on under pressure - simple aeration by gravitation within the filter being insufficient; and if some arrangement is combined with a filter to accomplish this missing necessity, then all filters constructed on these principles are all that can be desired of them.