How is water to be supplied to places? The best plan of supplying it is, as the ancient Romans did, to go to a considerable distance from a place and find water which has not been contaminated, and bring that water to the place where it is wanted.

In the year 92 after Christ, Frontinus, the Roman engineer, described the aqueducts bringing water from various distances to Rome. They brought waters of different degrees of purity, and Frontinus tells us that the purest waters were used for drinking purposes, and the less pure waters for washing the streets, and for the various other purposes for which pure water was not necessary.

One of these aqueducts brought water for fifty-four miles. Some of them have been repaired from time to time, and still supply the city of Rome with water. And if you go to Rome to-day the first thing that will surprise you is the immense abundance of pure water they have there, and that immense quantity of pure water is brought by some of the aqueducts made by the ancient Romans. They brought spring water from a distance, collected it in reservoirs, let the suspended matters settle, and then distributed the water by pipes to the different parts of the city. They constructed aqueducts not merely for Rome, but for many other great Roman cities. We have been told that the Romans did not understand the first principles of hydrostatics because they did not use inverted syphons for the aqueducts which supplied Rome. They understood them well, for when they had to traverse deep valleys, as at Lyons, they took the water across by means of inverted syphons, and they used precautions which would perfectly astonish you if I had time to tell you about them.

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.