Filtration must be downwards and intermittent; it must not go on continuously.

That is practically the plan which is carried out by the water companies. They allow the water to stand in tanks in the first place, and then pass it over the surface of a filtering bed; it is then collected in pipes below. When this is done it is found that a very considerable amount of purification takes place. Water, when passed through sand or gravel on a large scale, is very considerably purified of organic matters and salts of ammonia.

Water that has been sufficiently purified by being passed through sand or gravel, may become contaminated by the way in which it is supplied.

There are two ways of supplying water, the system of constant service, and that of intermittent service.

With the constant service the pipes are always full, and there is sufficient pressure to take it to the tops of the houses. With the intermittent service, the water is only turned into the pipes for a certain number of hours during the twenty-four, and the pipes during the remainder of the time have no water in them, and so foul water and foul air get into the pipes, through the joints, from the soil.

With the intermittent service it is necessary to have cisterns, so that water may be stored from one time to another; and these are liable to collect impurities, and so to render, the water impure. With the constant system the pipes are always full of water at a considerable pressure, and there is no necessity to have cisterns or butts, except for the supply of the closets, and, consequently, the risk of contamination of the water in cisterns is avoided. Neither can impurities from the soil get into the pipes. It is very important, however, that the water-closets be not supplied directly from the water-mains, for it happens occasionally that the water is necessarily supplied for a day or two on the intermittent system (as, for instance, when a reservoir has to be cleaned out or repaired), in which case foul matters may get into the pipes, not merely from the soil through leaky joints, but directly from the hoppers of the water-closets. This actually happened at Croydon last year, and was no doubt one of the causes of the very severe epidemic of typhoid fever there.

Water, then, from the causes I have mentioned, gets impure, either before it reaches us, or after it is in our houses; it is therefore advisable, as a general rule, to use extra precautions, that the water we drink and use for cooking is pure; and there are various kinds of filters that are much used for this purpose. Their action depends partly, if not entirely, upon the passage of water through some porous material, and most of them depend entirely upon porous materials to destroy the organic matters in the water.

They are made sometimes of sand and gravel, sometimes of charcoal; vegetable charcoal has been used, but it is not an efficient filtering medium. Animal charcoal is much better, while it is fresh, and so long as we are certain that it has been well burned; recent experiments have shown that animal charcoal, when it is not fresh, and is of inferior quality, from one reason or another, becomes a breeding ground for living creatures, and this also happens with a piece of sponge. Sometimes in filters, in order that grosser materials may be kept out, a piece of sponge is used, through which the water flows, and it is frequently found infested with an immense number of living creatures, - hence, these materials for filters are to be condemned.

It is advisable, then, to use as filters substances which are not, from their very nature, liable to become breeding places for animal or vegetable life; so various kinds of substances which have been thoroughly well burned are used.

The silicated carbon filter is a substance of this kind, and, unlike animal charcoal, is not liable to have in it animal matters, which have not been properly burned. Another substance, having a remarkable purifying power, and highly recommended in the. last report of the Rivers Pollution Commissioners, is what is known as spongy iron.

There is a filter, the construction of which is novel; it is called the aerating filter. The principle of it is this, that whereas in almost all other filters the air that is driven out comes up through a little pipe, which is made for it, in this instance the air that is driven out of the filtering material by the water as it goes down, is, by a simple and ingenious contrivance, driven up through the filtering material itself; when water is drawn from the filtered water-chamber, the air that has got to come back, and take the place of this water, has to come back through the filtering material, so that that material is being filled with air, whether water is passing through it, or whether water is being drawn out at the tap.

There is a carbon block, through which the water has to pass first, then it goes out below in the form of spray through several little holes, and falls upon another layer of filtering material, through that, and out into the pure water chamber. All parts of this filter can be easily got at, so that it can be cleaned at anytime.