Towns may also sometimes be supplied with pure water by means of artesian wells, but the too prevalent plan of supplying towns with water that we know has been polluted must be condemned.

We will now pass on to consider the methods for the purification of water.

Impure water can be purified to a certain extent, that is, we can remove certain substances from water, but we cannot tell, so far as known at present, whether we have removed from the water the causes of disease. If we take Thames water that we know has been polluted, we can adopt certain methods for purifying it, and render it much more fit to drink, but we cannot tell whether we have removed from it everything that would cause disease. It must be clear to everybody that in taking impure water and purifying it we are going upon a wrong principle.

Water can be purified on a small scale by boiling it. When you boil water carbonic acid is given off, and carbonate of lime is deposited, and that is why the incrustation in boilers takes place, especially if the water contains much carbonate of lime, so that, you can reduce the hardness of water by boiling it, and kill any living .things that may be in it. This, however, can only be done on a small scale.

I may tell you that it was noticed during several epidemics of typhoid fever caused by drinking watered milk, that those who drank milk which had been boiled did not get typhoid fever. And boiling water is much more efficacious if an infusion of vegetable substances is made. For instance, in some countries where impure water is generally used, it is drunk in the form of an infusion of tea.

Water can also be softened on a large scale by Clarke's process, which consists in adding lime to the water. Some of you may be astonished at this. The salt that is generally most abundant in hard water is carbonate of lime; it is dissolved in carbonic acid, not being soluble in water alone. When you add lime it combines with this carbonic acid, and the acid is no longer able to keep the carbonate of lime in solution, and it is then deposited in the form of chalk.

Let us now consider filtration. When water containing substances in solution or suspension is passed through porous materials, it is quite clear that many of the substances that are in suspension will be arrested, and will not pass through, and so the water will be strained; but that is not what is meant by filtration,- it is more than that. When waters containing certain substances in solution are passed through porous materials, if certain conditions are observed, the organic matters that are in solution are altered ; they are converted into inorganic substances, and the salts of ammonia that are in solution are also changed ; and this change is due to the presence of oxygen derived from air. When organic substances are brought into contact with oxygen, they are oxidised-that is to say, they are decomposed, burned with oxygen, their nitrogen forming nitric acid, and their carbon carbonic acid.

In order that this may take place effectively, the water must flow downwards ; it must be poured on the top, and drawn out by pipes below. If the water is forced upwards through a filtering material, this change does not take place. When water is passing through a filter from above, it passes through that material in a large number of small streams, trickling down through the pores of the material; but when it is being forced upwards through the porous material, it rises in a continuous sheet of water, and simply displaces the air, driving it out; and this is why, when the water is passed downwards, and is brought into contact with the air, such an important change takes place; but when it is passed upwards it is not brought into such contact with the oxygen of the air in the pores of that material, and so such a change cannot take place.

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 he stored from one time to {mother; 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 maj 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 ta 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 any time.