FILTER 1G7 a portiuii of the matter held in solution. On the other hand, it is often precisely such matter, when of organic origin and putrescent (and minute invisible disease-germs may here be included with the matter in solution), that it is specially desirable to remove from drinking water, as being prolific of mischief when taken into the system. In numerous instances an outbreak of virulent disease, such as typhoid fever, has been clearly traced to water so con taminated. The danger is the greater that such water may be bright and sparkling, and peculiarly palatable. That even sand has the power of removing dissolved matter from water was shown by Mr Witt s valuable experiments at Chelsea, described in 185G. In one of these, e.g., water containing 1/42 grains of chloride of sodium per gallon (70,000 grains) was deprived of 22 per cent, of that substance by filtration through a depth of 1 foot 9 inches of sand. The sand had no appreciable action on dissolved organic matter, as charcoal had, but the quantity of such matter originally present was small. It is probable that all finely porous material has such action, more or less. The efficiency of charcoal in this respect, and especially fresh animal charcoal, has been well demon strated and utilized. The mode of the disappearance of dissolved organic impurities has been a disputed point. Some say they are retained and accumulated in the pores of the charcoal ; but the experimental evidence seems to leave little doubt that they are mainly oxidized, so as hardly to impair the activity of the filter. In fact, the value of a filtering material will be found to depend chiefly on the power it has of bringing oxygen, stored in its fine pores or otherwise provided, into chemical union with the dissolved organic matter and destroying it. At the same time it is obvious that, chiefly by the mechanical action of straining, organic matter may accumulate in a filtering medium, and in course of time, through decomposition, render the water which passes through more impure and less wholesome than in its unfiltered state. We may remark here that river water and shallow well water, while extensively used for water supply, are in general the most largely polluted. Kivers commonly receive large volumes of sewage, impure surface water from cultivated land, and other contamination. The water of shallow wells, especially in large towns, or when near churchyards, stables, cesspools, &c., is often contaminated with organic matter of the worst kind, in large quantity. While rain water, collected from the roofs of houses in butts, no doubt contains organic matter, this hardly bears comparison in amount to the organic impurity in rivers (thus Dr Hassall found it a hundredfold less). Deep spring water, again, is freed from much of its organic impurity through natural filtration. The advantages of a good lake-supply for large towns has been amply demon strated in these days, notably in the case of Glasgow, which draws its water from Loch Katrine. Though the arrangements for water supply of most of our large towns include filtering processes by means of which, as a rule, excellent drinking water is abundantly provided, so that in the opinion of some chemists a domestic filter may be superfluous, while it is sometimes a source of harm (owing to lack of proper attention), it is generally thought a wise additional safeguard to employ one of those instruments, in view, more especially, of some of the possible conse quences of epidemics and floods, and the necessity of house- storage of the water received. In country places, and in various other circumstances, their use is often quite imperative if the laws of health are to be respected. The Japanese and Egyptians seem to have used water filters at an early period. These consisted of sandstone or unglazed earthenware, and were of bowl or egg shape, with small orojections at the top resting on a wooden frame. The water poured into this vessel filtered through to a vessel below. About the middle of last century, slabs of a lias found in Picardy were used as a filtering agent, being fitted as a false bottom in water cisterns ; the water was drawn off through a tap from the space below. A porous filtering stone of Teneriffe was at one time imported largely into England. The " alcarrhazas " are filter vessels of porous biscuit stoneware made in Spain. One of the earliest filters in England was that patented by Mrs Johanna Hempel in 1790. It was a supported basin made of tobacco pipe (or similar) clay and coarse sea, river, drift, or pit sand, and hardened in the fur nace. In the following year (1791) the ascending prin ciple w r as first applied by Mr James Peacock. Water from an upper reservoir was admitted through a pipe to the bottom of a box containing strata of sand, gravel, and a mixture of charcoal and powdered calcareous stone. Passing up through this, it was drawn off by a pipe at the top. The filter was occasionally cleaned with an exhaust ing and condensing pump, which sucked up water rapidly through the filtering material and then sent it back with force, washing out the dirt. The construction of filters is a matter on which inventive ness has been largely exercised. All sorts of porous substances have been called into requisition, as may be seen by a glance at the patent records. Thus, to mention some of these, we have various kinds of stone, sand, gravel, powdered glass, clay, porous sulphur, preparations of iron, charcoal (vegetable and animal), cloth, felt, horsehair, skins, paper, silicated carbon, sponge, wood, wool, cane, capillary threads, and so on. Vegetable charcoal, we may note, was first employed in 1802, animal charcoal in 1818, and solid carbon blocks in 1834. It is impossible here, and it would be tedious, to give anything like a detailed account of the changes that have been rung on the filtering principle for domestic filters. In the simplest and most familiar forms, of course, the water passes down by its own gravitation through the filtering medium to a reservoir below. The force of downward pressure has sometimes been augmented by a head of water, sometimes by a force pump, and sometimes by means of air condensed above the water to be passed through the filter. Or the air has been extracted from the vessel con taining the filtered liquid, thus adding force to the atmo spheric pressure above. The ascending principle has appeared in various applications ; and in some cases the water has been caused first to descend and then to ascend through filtering material (a vertical partition, e.g., being fixed in a vessel so as to reach nearly to the bottom, and layers of sand and vitrified limestone being placed on either side of the lower part ; the water is poured in on one side of the partition and drawn from the other side near the top). Lateral filtration has also been tried. Filters have been arranged to act in the cistern, or in connexion with the service pipe between the cistern and the water tap, or independently, and, in the latter case, either having the unfiltered water poured into them, or being placed in a vessel of it, and giving filtered water through a tube. Sometimes a series of separate connected vessels have been employed ; and for very dirty water it is often advantageous to have one system of filtration for the coarser, and another for the finer impurities. Once more, filters have been ren dered self-supplying by means of a ball-cock. These are some of the general forms which the filter has taken. The application of pressure to filters cannot as a rule be pursued very far, for it tends to derange the apparatus and render the filtered liquid muddy. Enlargement of surface is a better means of obtaining rapid filtration. Upward filtration, while it offers some advantages over downward, has not hitherto come very much into use. It is open to
