purity in most instances, sometimes considerably, that domestic filters are useful, but because, as I have before remarked, especially where the intermittent system of supply is in vogue, the water, even if delivered pure, is rendered impure in the houses themselves by being stored in filthy receptacles. The majority of the filters in domestic use rely upon the principle of downward filtration. In a few, the water is passed upwards through a filtering material. The chief materials used are animal charcoal

vegetable charcoal is not a good material for filtering purposes-silicated carbon, carbide of iron, spongy iron and sand. When animal charcoal is used, it must be spe

cially prepared and well-burned. If any of the animal matter be left in it, it becomes as has been shown by the Rivers Pollution Commissioners, a breeding place for myriads of small worms which pass into the water. With the other materials mentioned, there is, of course, no risk of this, as they are made of burnt shale, or taken from the interior of blast furnaces. Some filters are placed inside the cisterns, so that all the water that is drawn off has to pass through them. Others are placed on the main water pipes themselves, or in the taps. One of the former kind is known as "the self-cleansing filter," in which the suspended particles in the water are prevented from getting at the filtering material by a ring of compact silicated carbon, and the water itself is made to wash the outside of the block of filtering material through which it has to pass. My experience goes to prove, that filters that are always under water cease to purify the water after a time, unless means are taken for aërating them, and in many instances I have known water to be rendered more impure by its passage through a filter which has been used. in this way for a considerable time. Of forms of domestic filter, the glass decanter with a solid carbon or silicated carbon block has the great advantage that every part of it can be seen, so that it can be kept scrupulously clean. These filters go on working perfectly well for an almost unlimited time, scarcely anything being necessary beyond cleansing the surface of the block once now and then with a hard brush. It is a very good plan to have a kind of double filtration. Sometimes the water is made to pass

through a piece of sponge before falling on to the filtering material with the view of arresting the coarser suspended matters. It is far preferable, however, to use the carbon

block for this purpose. In Prof. Bischoff's spongy iron filter the filtering material is always under water, and the action which goes on in it is certainly quite different to that which I have explained and is as yet little understood. The Rivers Pollution Commissioners have expressed the highest opinions of this substance as a filtering material. On account of the fact that the water dissolves a little of the iron on its passage through the spongy iron, it is made to pass through layers of pyrolusite (an oxide of manganese) and of prepared sand afterwards, with the view of removing this, and then, in order to aërate it, it is delivered through a very small hole in a fine stream into the pure

water receiver.

It will thus be seen that it is rather more complicated than some of the other forms of domestic filter. The slight trace of iron that remains in the water can hardly be considered a disadvantage, at any rate in large towns.

Lastly, I must notice the filter known as the "aërating

filter." In this by an ingenious contrivance, the air passes to and from the filtered water chamber through the filtering material itself, and not by means of a small chan