Where fluids have to be transferred from an upper to a lower level, passing on the way over an obstacle of greater height than the upper vessel, a siphon may conveniently replace a pump, as, once the stream is started, it will continue flowing indefinitely until the level of the liquid in the supply vessel becomes so low as to admit air into the siphon.
In its simplest form, the siphon is merely a pipe bent into a U shape, with one leg longer than the other; the shorter leg is placed in the liquid which is to be drawn off. To start the siphon, it is necessary to empty it of air and fill it with water or the fluid to be siphoned; this is best accomplished by turning it end upmost and pouring the liquid in at the longer leg till it overflows, the thumb being meanwhile held over the orifice in the shorter leg. Both ends are stopped in this way while inverting the siphon into the vessel to be drawn from, and care must be taken not to remove the thumb from the mouth of the longer leg till after the shorter leg is free to draw its supply.
Figs. 66, 67 show handy glass siphons adapted for small operations, the former being without, the latter with, a stopcock c for regulating the flow. The current is started in these by applying the mouth to the end a of the tube, and employing it as an air-pump to exhaust the air till the fluid rises into the bulb 6. With harmless liquids, a simple bent glass tube may suffice as a siphon; but suction with the mouth at the end of the longer arm is somewhat inconvenient. The arrangement shown in Fig. 68 is simple, and presents certain advantages: - A glass tube g, 3/4 in. wide, and 12-16 in. long, contracted at the lower end, has, at its upper end, a cork stopper, in which the mouthpiece M and the siphon hh' are fixed air-tight. The shorter arm A of the siphon reaches nearly to the bottom of the tube, and limits the play of a glass ball A, which acts as a valve. The diameter of the ball is about 1/2 in., that of the siphon 1/4 in. The instrument thus arranged, being dipped into the vessel to be discharged, the tubes g and A become filled with liquid to the surface N N. Instead now of sucking, as with the common siphon, one blows into the mouthpiece M; and in consequence of the compression of air, the lower opening is shut by the ball A, while the liquid rises in A, and begins to flow through h' in the usual way.
If the vessel to be emptied is not full, or the column of liquid is a small one, it is necessary, before blowing into the mouthpiece, to suck it slightly, in order to obtain a larger volume of the liquid in g; as one condition for the right action of the instrument is that h h' should be filled before the column of liquid in g sinks to the mouth of the siphon at A, when one blows through M.
Fig. 69 shows a method of constructing a siphon suited for drawing off large quantities of hot or corrosive liquids (the dimensions given being adapted to sulphuric acid boiling pans). In the figure, n is a leaden siphon, 1 1/2 in. bore, through which acid is to be drawn from the pan, that lies hidden in brickwork behind the stay bar f; o is a leaden cup, 18 in. deep, 4 in. diameter, attached to a weight by a chain passing over a pulley. This cup is filled with acid; the siphon is also filled with acid, and set with one leg in the pan and the other in the cup. When the cup is lowered, the acid flows through the siphon and overflows the cup, running into p, a leaden box, 3 ft. 3 in. deep, and 9 in. diameter, whence it flows through q, a leaden pipe leading to cooler or retorts. When the cup is raised so much that the top of it is above the level of the acid in the pan, the acid ceases to flow. In the drawing, the cup is shown raised to its highest, the top being a little above the level of the top of the pan, so that were the pan quite full of acid, none would run out until the cup was lowered. The cup keeps the siphon constantly set; but if all the acid were drawn from the pan, air would enter the pan leg of the siphon, and it would become unset.
Similar siphons are used for drawing acid out of the chambers.
Fig. 70 shows a portion of another form of siphon, generally used for drawing off sulphuric acid from the retorts in which it is concentrated, but equally useful for many other purposes. The siphon a is formed of a piece of 1/2-in. bore leaden pipe, bound to a small strip of wood 6; c is a glass globe with 2 tapering tubes, the end of one tube being inserted into a leaden funnel d, in which c is hermetically sealed by a mixture of melted brimstone thickened with a little sand; e, an exhausting syringe, to the mouth of which is attached a short piece of rubber tubing. To set the siphon, one person takes a small piece of sheet rubber, say 1/8 in. thick, and holds it tightly against the mouth of the siphon, to stop the passage of air, whilst a second person takes the syringe and slips the end of its flexible tubing over the end of the upper tube of the glass globe. On working the syringe a few strokes, the air becomes exhausted from the siphon, causing the acid to flow through it, and commence to fill the glass globe. The syringe is then removed, and the piece of rubber is quickly withdrawn from the mouth of the siphon; the acid continues to flow until the retort is nearly empty.
A siphon setting apparatus is shown in Fig. 71; a is the siphon; b, a closed leaden vessel; c, an open vessel or bucket. A small metallic pipe connects the top of the siphon with the top of the vessel b, and a rubber tube connects the bottom of b with the bottom of c. To set the siphon (both legs of which must be standing in liquid), fill the bucket with water, raise it above b, and hold it there till all the water has run into the vessel 6. Then stand the bucket down, and the water will flow back into it; the vacuum thereby created in b will exhaust the air from the siphon, and set it running.
To obtain a uniform flow of acid or other liquid under varying degrees of pressure the apparatus shown in Fig. 72 is used, a is a cistern containing acid or other liquid; b, a well in the cistern having a conical mouth; c, a pipe connecting the Well and cistern with the cylindrical vessel d, which must be the same height as a; e, a delivery-pipe fitted with a tap; f, a lever working on a central pivot. One end of the lever is attached by a chain to a leaden bucket which hangs in the vessel d, and the other end is attached by a short chain to the rod g (which is of iron cased in lead), having cast on it the conical plug A, which must fit accurately the contracting mouth of the well 6. The extension of the rod below the plug serves to keep the latter in its seat. The tap in the pipe e being opened, the greater the pressure of acid in a, the higher it will rise in d, elevating the bucket and depressing the plug A, which will check the flow. Thus at whatever height and consequent pressure the liquid in the cistern may be, the pressure or flow from the delivery-pipe will be uniform.