To illustrate change of phase between liquid and vapor in an confined fluid.
Hold the pulse glass so that the bulbs point up, and enclose one bulb with your hand to warm it. The heat from your hand will change some of the liquid on that end to vapor, increasing the pressure in that end and forcing the liquid into the other end.
Julius Sumner Miller, Remarks on the Pulse Glass, AJP 25, 120 (1957). The Welch Scientific Company, Instructions for use of Nos. 1665 and 1665A Pulse Glasses.