Foremost among mechanical appliances for this purpose ranks the centrifugal machine, or hydro extractor. In principle this apparatus consists of an upright drum, which can be made to revolve with great velocity on a vertical axle. The drum may have its sides constructed of sheet metal, perforated with a multitude of fine holes, of wire game properly supported, or of basket work, according to the nature of the substances to be treated. The drum, being charged with material, is set in quick rotation. The water present is thus expelled through the perforated sides in the form of a fine shower. This process is exceedingly well adapted for removing the greater part of the moisture from cloth, yam, unspun wool, 4c.; also from crystalline and granular substances. It is not so well adapted for drying wet powders, pastes, etc., since in such cases a very considerable proportion of the solid matter is projected away along with the liquid, so the holes may get choked up. Thus it has not hitherto been found satisfactory for drying sewage mud. Its use requires, further, special niodificntloos where the liquid to useful, or hurtful. matters in solution, A recent very simple improvement has cons;derably extended the use of the hydro extractor.

The materials, instead of being put into the drum loose, are inflosed in bags of some suitable material, thus preventing the dispersion of the solids. This method has been very successfully adopted with butter, it must, however, be remembered that no substance, especially if of organic nature, can be rendered absolutely dry by the use of the hydro extractor.

Water oven.

Water oven.

Another mechanical agency for desiccation is the press, more especially that device known as the filter press, which has proved itself invaluable for separating solids from fluids when the latter largely predominate. This apparatus contains a number of cells, each consisting of a couple of cast iron plates lined, when in use, with suitable cloths. The inner surface of each plate shows a number of ridges. The liquid paste is forced by a pump, or press, into each cell through an aperture, and the water escapes through the cloth, and trickles down between the grooves formed of the ridges to the pipe at the bottom.

In Johnson's press there are several improvements. In the centre of each plate is an aperture, which places a whole series of cells in connection, so that a liquid or paste introduced through one inlet pipe fills the whole series; grooves cut in the plate facilitate the escape of the press liquor to the outlets. The number of ridges is very great. The press cloths are of different kinds, according to the material operated upon. The pressure which may be exerted by means of steam or air ranges from 50 lb. to 100 lb. per sq. inch.

The filter press, like the centrifugal machine, only expels a part of the water in mud, etc.; thus, if a sewage mud contains at the outset 90-95 per cent, of moisture, it may be reduced by the filter press down to 50-60 per cent., according to the time during which the pressure is maintained. It is only in a few cases that hydraulic presses, screw presses, etc, can be employed for desiccation.

By Cold

The concentration of saline and saccharine solutions by the abstraction of surplus water is another branch of desiccating. This is usually performed by means of heat, as described under Evaporating, but may be sometimes advantageously effected by the aid of a low temperature. Thus in several countries where severe weather predominates common salt is obtained from the ocean by exposing sea-water in shallow reservoirs to the action, of the frost. The water becoming frozen separates from the saline bodies which it held in solution, and on being removed in the form of ice the latter can be collected from the bottom of the receptacle; or repeated coatings of ice can be taken from as many freshly admitted supplies of sea-water till the solution reaches a highly concentrated form, needing but little evaporation to afford a crystalline product.

Another direction in which the concentration of solutions by cold is successfully applied is in warmer countries where sugar forms one of the agricultural products. Thus in Ohio the native women were accustomed to expose the syrup as collected in shallow pans to the night air, when the cold would suffice to freeze the water and form a crust of ice over the thereby concentrated syrup below. The bulk of the superfluous water being thus got rid of, very little further concentration by means of fire is needed to produce a solid sugar.

Distilling, (iv. 119-143.)

Mercury is now so largely used both in the laboratory and for industrial purposes, such as ore reduction, electric engineering, and so on, that a quick and efficient means of purifying it is a valuable acquisition. An apparatus for this purpose has been devised by J. W. Clark, Demonstrator of Physics in University College, Liverpool, and was recently brought before the Physical Society of London.

The usual processes for purifying mercury are either chemical, such as treatment with dilute sulphuric acid, etc, or mechanical, such as shaking and filtering through wash-leather, or distillation, either in vacuo or under the ordinary atmospheric pressure. Of all these methods the best is distillation in vacuo.

Prior to distillation it is well to filter the mercury through a cone of writing paper with a very small orifice at the apex, find to remore the lend or zinc present by chemical means; for the rate of distillation is towered by these impurities. The presence of 0.0001 part of lend is said by Gmelin Kraut to reduce the quantity of mercury distilled in a given time from 67 to 5. Gold, iridium, copper, tin, nickel, cadmium, and nrsenic do not influence the rate of distillation. The distillation of mercury at ordinary

The first apparatus for distilling ill vacuo was probably devised by Weinhold, and others hare been designed since by Weber, Shan-, Wright, and others. The arrangement of Clark, however, differs from all these in the important respect of dispensing with an auxiliary Sprengel air pump, and in, so to speak, acting as its own air pump. This is effected by supplying the mercury to be distilled from a movable reservoir in the form of a constant level regulator. On raising this reservoir, Fig. 21, the mercury is supplied to the distiller.