FILTER, VACUUM A device by means of which liquid is forced through a filtering medium by atmospheric pressure.
A simple form of filter for viscous fluids, such as the fish-glue solution used by process workers, is that shown at A. A strong glass flask of about one litre capacity has a nipple connection blown into its side near the neck. To this nipple a Sprengel " pump " of glass is attached by means of a rubber tube. The construction of the " pump " is shown at B. The inner tube is not continuous, but consists of two tapering tubes, one being sealed to the upper end of the outer tube and the other to the lower end, their tapered ends meeting and being enclosed one within the other for a short distance. The air drawn from the flask can thus pass between them. The vertical nozzle of the " pump " is connected with a water-tap. The rubber tubes must be tightly bound on, or they will be blown off by the pres sure. The funnel containing the filtrate is
thrust through a rubber bung placed in the neck of the funnel, and it is desirable to have a per forated porcelain plate in the funnel to prevent the filtering medium, which may be cotton-wool, glass-wool, or felt, being sucked down the neck of the funnel. The filtrate having been poured in, the water-tap is turned on and the rush of water through the " pump " draws the air out of the flask, creating a vacuum in it. Thus the filtrate is powerfully sucked through. It is desirable to have a clip on the tube leading from the flask to the " pump," so that the tube can be pinched when the water is turned off, or the water in the tube will be sucked back and dilute the filtered liquid.