Finally, to make sure that these have been all got rid of, the water traverses a filter, and then it is for all practical purposes as soft as rain-water. Some people are frightened of this artificially softened water, on the ground that chemicals have been added to it, supposing, apparently, that when they use such water they are really employing a chemical solution. That is quite wrong, however, for the added chemicals, combining with the "hardness," form substances which are quite easily extracted from the water altogether. If we liken the hardness to a number of pickpockets in a crowd, and the added chemicals to a number of policemen who come in to arrest the said pickpockets, finally leaving the crowd free from both pickpockets and policemen, we get a simple illustration of what takes place. But almost equally important as the provision of pure water is the effective dealing with the drainage of a large town. Much offensive matter flows under the streets of our towns and cities, and if it is not to become a nuisance it must be scientifically dealt with. Years ago the drains of London simply emptied themselves into the Thames, until, in 1864, two large drains were constructed, one on each side of, and approximately parallel with, the river, to intercept the old drains and to carry their contents to points many miles down towards the sea. Even that, however, by no means abated the evil, for it simply transferred it to a new place. The river was as foul as ever. William Morris, in _News from Nowhere_, pictures the catching of salmon in the Thames off Chelsea, while one of London's prominent citizens, referring to what was being done in the direction of purifying the river, jocosely promised the members of Parliament a little fly-fishing at Westminster. Equally remote, it is to be feared, from actual accomplishment, these two prophecies do certainly indicate the tendency of events, for science has enabled the authorities to relieve the long-suffering river of much of the pollution which they used to thrust into it. The first great step was the introduction, in 1887, of a treatment in principle very like that just described for softening water. The liquid from the drains is gathered into large reservoirs, where chemicals are added to it, causing the heavier matter to be precipitated in the form known as "sludge." The liquid portion, or "effluent," as it is called, which is left is discharged into the river just as the tide is ebbing, so that it is carried right away, and, being comparatively inoffensive, it pollutes the river very little indeed. The sludge, on the other hand, is pumped into special steamers, which carry it down to a certain spot off the Thames Estuary, where they drop it into the sea. The currents at the particular spot chosen are such that none of it returns to the river.