A good many vacuum pumps, worked by a flow of water, have from time to time been introduced to the notice of the public, but the majority of them are Dot completely satisfactory. Bun-sen's is perhaps the best; but its production requires the aid of skilled workmanship, and the outfall tube mast be at least 32 ft. in length. This altogether forbids its use except upon the upper floor of a building.

The pump to be described was designed by A. P. Smith some years ago. The upon that of Giffard's injector. A cistern of water, such as is to be found in every house, is all that is needed for a water supply; but the greater the head of water, the greater the power of the pump. If the water can be laid on from the main supply, nothing further could be desired.

Although a good head of water - and, therefore, pressure - is desirable, it by no means follows that a large quantity of water is required: It is desirable to attach a screw pinch-cock on the rubber tube which connects the pump with the water supply, so as to govern the quantity flowing through - a condition easily acquired by a little observation and practice.

Automatic rapid fiiter

Automatic rapid fiiter.

The greater the pressure of water, the greater the power of the pump, which is capable of lifting a column of mercury equal in height to that of the barometer at the time being minus the tension of aqueous vapour (the colder the water, therefore, the better). However, the pump will work very well, and lift 15 in. or more of mercury with a head of 10 ft,

The construction of the pump offers no difficulty to anyone who can bend a piece of glass tubing, draw out a jet, and bore a hole in a cork; this last is perhaps the most difficult of the three. Procure a glass lamp chimney about 23 cm. in length, and some glass tubing of an internal diameter of about 6 mm. Fit two sound corks to the ends of the lamp chimney. Through the centre of the upper cork pass a glass jet with a short nozzle (Fig. 36). Through the lower cork pass another jet, having a long sloping nozzle. The diameter of the holes at the ends of the jets may be about 1 1/2 or 2 mm. (The size really depends upon the water supply.) Care should be taken that the hole in the lower jet is not smaller than that of the upper jet; they ought to be the same size. These two jets are placed diametrically opposite each other, and nearly in contact, so that water flowing down may pass smoothly out of one into the other, without striking the edge and spurting off into the chimney glass, A vacuum is produced at this point. This is really easy to accomplish, however difficult it may appear on paper.

Adjust an exhaust tube through the upper cork, and, to make the whole affair like an instrument that is intended to work, and not like a model, mount it as In Fig. 37, where A is the pump, BCD the eahauat tube; but at It insert a three-tray metal tube, and attach a barometer tube (same tubing as before), which dips into a vessel of mercary. The junctions with the metal tube may be made either by good corks or rubber tied on with wire (the glass tube must project inside the metal tube in any case), and all the junctions well covered with several coats of shellao varnish. The glass tubes may be filed to a board in the manner shown in Fig. 38, by cutting a groove in a piece ol cork, and screwing a strip of tin or brass over the whole.

Methods of folding paper filter.

Methods of folding paper filter.

The final adjustment of the jets can only be made while the water is flowing, and the barometer tube, or the exhaust tube, is dipping under mercury in order to ascertain when the pump is doing its best. When the proper position has been found (generally obtained by twisting the jet, as the point is sure not to be quite central), the corks may be covered with electrical cement, or several applications of shellac varnish.

The filtering bottle requires no special description. Care must be taken to have a sound cork, and it is as well to soak it in melted paraffin to fill up the pores. The rubber which connects the bottle with the exhaust must be as thick as possible, and the ends of the glass tubes must be placed in contact, or the rubber will be flattened by the pressure of the atmosphere, and close the tube. A particular kind of rubber tubing is manufactured specially for such purposes as these, and cannot, well be squeezed Bat, as it has a diameter of 2 cm., and a bore of only 4 mm. However, ordinary black rubber tube will do very well for most purposes. - (Yearbook of Photography.)

Vacuum pump for hastening filtration.

Vacuum pump for hastening filtration.

(h) Some precipitates are so exceedingly fine tuat the best filtering paper is incapable of retaining them. In such cases the difficulty may be overcome by stirring up with the liquid to be filtered a little finely-powdered French chalk (or paper pulp obtained by dissolving filtering paper in aqua regia and reprecipitating in water); this settling on the filter closes the pores of the paper still further, and prevents the passage of the precipitate. When filtering hot liquids which are very acid, or have a high specific gravity, much annoyance may be caused by the repeated breakage of the filtering paper. This can generally be prevented by supporting the apex of the filter on a strip of muslin laid across the funnel, or by using papers which have been steeped in 1*42 nitric acid for a few minutes, washed and dried, whereby the papes is greatly strengthened. For this purpose, also, an extra strong variety of filter paper has been introduced commercially, the peculiarity consisting in a network of linen threads interwoven with the substance of the paper during manufacture.

The ordinary funnel with sides at an angle of 60° is not adapted for very rapid filtration. The long French form, having a length about twice that of the widest diameter, yields much better results, and used in conjunction with a plaited filter paper gives the greatest rapidity of filtration which it is possible to obtain with the simple paper and funnel. Two forms of funnel, each the subject of a patent, have been introduced with the view of lessening the disadvantages of the ordinary 60° funnel. The first is furnished with straight projecting ribs on the interior, which to a great extent keep the paper from close contact with the sides, and this certainly aids filtration considerably. The second is of more recent introduction, and may be described as a funnel of the ordinary shape enclosing the body of a slightly smaller funnel perforated all over with small holes, and kept from contact with the outer funnel by 8 projecting ribs, the whole being made of earthenware in one piece. The space between the inner and outer bodies of the funnel is closed at the top, and a circular hole is provided in the latter, which may be closed airtight by a stopper.