is in the case of effluents from tanneries and other manufactories, or the ocherous and muddy sludges taken from the settling tanks in mines, some of which contain from 90 to 95 per cent. of water, the filter press is the best and the most economical, and it is to this particular process that Messrs. Johnson's exhibits at the Health Exhibition, London, chiefly relate. Our engravings are from The Engineer. A filter press consists of a number of narrow cells of cast iron, shown in Figs. 3 and 4, held together in a suitable frame, the interior frames being provided with drainage surfaces communicating with outlets at the bottom, and covered with a filtering medium, which is generally cloth or paper. The interior of the cells so built up are in direct communication with each other, or with a common channel for the introduction of the matter to be filtered, and as the only exit is through the cloth or paper, the solid portion is kept back while the liquid passes through and escapes by the drainage surfaces to the outlets. The cells are subjected to pressure, which increases as the operation goes on, from the growing resistance offered by the increasing deposit of solid matter on the cloths; and it is therefore necessary that they should be provided with a jointing strip around the outside, and be pressed together sufficiently to prevent any escape of liquid. In ordinary working both sides of the cell are exposed to the same pressure, but in some cases the feed passages become choked, and destroy the equilibrium. This, in the earlier machines, gave rise to considerable annoyance, as the diaphragms, being thin, readily collapsed at even moderate pressures; but recently all trouble on this head has been obviated by introducing the three projections near the center, as shown in the cuts, which bear upon each other and form a series of stays from one end of the cells to the other, supporting the plates until the obstruction is forced away. We give an illustration below showing the arrangement of a pair of filter presses with pneumatic pressure apparatus, which has been successfully applied for dealing with sludge containing a large amount of fibrous matter and rubbish, which could not be conveniently treated with by pumps in the ordinary way. The sludge is allowed to gravitate into wrought iron receivers placed below the floor, and of sufficient size to receive one charge. From these vessels it is forced into the presses by means of air compressed to from 100 lb. to 120 lb. per square inch, the air being supplied by the horizontal pump shown in the engraving. The press is thus almost instantaneously filled, and the whole operation is completed in about an hour, the result being a hard pressed cake containing about 45 per cent. of water, which can be easily handled and disposed of as required. The same arrangement is in use for dealing with sewage sludge, and the advantages of the compressed air system over the ordinary pumps, as well as the ready and cleanly method of separating the liquid, will probably commend itself to many of our readers. We understand that from careful experiments on a large scale, extending over a period of two years, the cost of filtration, including all expenses, has been found to be not more than about 6d. per ton of wet sludge. A number of specimens of waste liquors from factories with the residual matters pressed into cakes, and also of the purified effluents, are exhibited. These will prove of interest to many, all the more so since in some instances the waste products are converted into materials of value, which, it is stated, will more than repay for the outlay incurred.

Fig. 3. Fig 4.

Another application of the filter press is in the Porter-Clark process of softening water, which is shown in operation. We may briefly state that the chief object is to precipitate the bicarbonates of lime and magnesia held in solution by the water, and so get rid of what is known as the temporary hardness. To accomplish this, strong lime water is introduced in a clear state to the water to be softened, the quantity being regulated according to the amount of bicarbonates in solution. The immediate effect of this is that a proportion of the carbonic acid of the latter combines with the invisible lime of the clear lime water, forming a chalky precipitate, while the loss of this proportion of carbonic acid also reduces the invisible bicarbonates into visible carbonates. The precipitates thus formed are in the state of an impalpable powder, and in the original Clark process many hours were required for their subsidence in large settling tanks, which had to be in duplicate in order to permit of continuous working. By Mr. Porter's process, however, this is obviated by the use of filter presses, through which the chalky water is passed, the precipitate being left behind, while, by means of a special arrangement of cells, the softened and purified water is discharged under pressure to the service tanks. Large quantities can thus be dealt with, within small space, and in many cases no pumping is required, as the resistance of the filtering medium being small, the ordinary pressure in the main is but little reduced. One of the apparatus exhibited is designed for use in private mansions, and will soften and filter 750 gallons a day. In such a case, where it would probably be inconvenient to apply the usual agitating machinery, special arrangements have been made by which all the milk of lime for a day's working is made at one time in a special vessel agitated by hand, on the evening previous to the day on which it is to be used. Time is thus given for the particles of lime to settle during the night. The clear lime water is introduced into the mixing vessel by means of a charge of air compressed in the top of a receiver, by the action of water from the main, the air being admitted to the milk of lime vessel through a suitable regulating valve. A very small filter suffices for removing the precipitate, and the clear, softened water can either be used at once, or stored in the usual way. The advantages which would accrue to the community at large from the general adoption of some cheap method of reducing the hardness of water are too well known to need much comment from us.

PNEUMATIC MALTING.

According to K. Lintner, the worst features of the present system of malting are the inequalities of water and temperature in the heaps and the irregular supplies of oxygen to, and removal of carbonic acid from, the germinating grain. The importance of the last two points is demonstrated by the facts that, when oxygen is cut off, alcoholic fermentation--giving rise to the well-known odor of apples--sets in in the cells, and that in an atmosphere with 20 per cent. of carbonic acid, germination ceases. The open pneumatic system, which consists in drawing warm air through the heaps spread on a perforated floor, should yield better results. All the processes are thoroughly controlled by the eye and by the thermometer, great cleanliness is possible, and the space requisite is only one-third of that required on the old plan. Since May, 1882, this method has been successfully worked at Puntigam, where plant has been established sufficient for an annual output of 7,000 qrs. of malt. The closed pneumatic system labors under the disadvantages that from the form of the apparatus germination cannot be thoroughly controlled, and cleanliness is very difficult to maintain, while the supply of oxygen is, as a rule, more irregular than with the open floors.

IMPROVED PNEUMATIC FILTERING PRESSES.

A NEW FORM OF GAS