How to start a siphon running is sometimes quite a problem. If the liquid that is to be siphoned off is harmless, the siphon tube may be filled by suction with the mouth at the end of the longer arm. But this is not always very pleasant, and sometimes it is even dangerous if the liquid is of a poisonous nature. The accompanying engraving illustrates a method by which the siphon may be started by compression instead of suction. The idea is so old that probably it is new to many. The device consists of a large test tube, in the open end of which a cork is fitted, while in the opposite end a small hole is cut by means of a file. A glass marble is placed in the tube, and serves as a valve to close the opening. Through the cork the shorter leg of the siphon is run, and also a small bent tube. The outer end of the latter tube is placed in the mouth, and on blowing into the test tube the compression serves to close the valve and at the same time force the liquid through the siphon. As soon as the pressure is relieved, the liquid in the vessel will flow up through the opening in the test tube, and continue running off through the siphon. The shorter leg of the siphon extends to within a short distance of the ball valve, so as to limit the motion of the latter, and prevent it from striking the glass with a blow sufficiently hard to break it.

Fig. 161 - Starting device for siphons.