Charcoal, simple.--All kinds of charcoal, but especially animal charcoal, are useful in the construction of filters, and have consequently been much used for that purpose. Charcoal, as is well known, is a powerful decolorising agent, and possesses the property in a remarkable degree of abstracting organic matter, organic colouring principles, and gaseous odours from water and other liquids. It has been shown that it deprives liquids, for example, of their bitter principles, of alkaloids, of resins, and even of metallic salts, so that its usefulness as a medium through which to pass any suspected water is undoubted. The one point to be observed is that it does not retain its purifying power for any great length of time, so that any filter depending upon it for its purifying principle must either be renewed or the power of the charcoal restored from time to time, and this the more frequently in proportion to the amount of impurity present in the water. A combination filter of sand or gravel and granulated charcoal is a good one; but the physical, or chemico-physical, action of such compound filters, or of the other well-known filter, composed of a solid porous carbon mass, differ in no respect from that of the simple substances composing them; that is to say, such combinations or arrangements are much more a matter of fancy or convenience than of increased efficiency. Experiments on the filtration of water through animal charcoal were made on the New River Company’s supply in the year 1866, and they showed that a large proportion of the organic matter was removed from the water. These experiments were afterwards repeated, in 1870, with Thames water supplied in London, which contains a much larger proportion of organic matter, and in this case also the animal charcoal removed a large proportion of the impurity. In continuing the use of the filter with Thames water, however, it became evident that the polluting matter removed from the water was only stored up in the pores of the charcoal, for, after the lapse of a few months, it developed vast numbers of animalcula, which passed out of the filter with the water, rendering the latter more impure than it was before filtration. Prof. Frankland reported in 1874 on these experiments as follows:--“Myriads of minute worms were developed in the animal charcoal, and passed out with the water, when these filters were used for Thames water, and when the charcoal was not renewed at sufficiently short intervals. The property which animal charcoal possesses in a high degree, of favouring the growth of the low forms of organic life, is a serious drawback to its use as a filtering medium for potable waters. Animal charcoal can only be used with safety for waters of considerable initial purity; and even when so used, it is essential that it should be renovated at frequent intervals, not by mere washing, but by actual ignition in a close vessel. Indeed, sufficiently frequent renovation of the filtering