A combination cloud chamber and ionization chamber in which the gas (argon at 51 lb in.−2saturated with iso‐amyl alcohol) is common to both chambers is described. The ion chamber, a 3¾in. diameter brass cylinder 8 in. in length open at both ends and having a116−in. central wire, is mounted coaxially with the cloud chamber, which has a useful rectangular volume 10 in.×10 in.×8 in. A burst occurring in the ion chamber generates a pulse which is used to trigger the expansion mechanism. Rapid reduction of the cylinder voltage from −1800 volts to ground, after electron collection is over, prohibits any appreciable motion of positive ions and thus permits observation of the tracks of the burst producing particles. In preliminary tests at sea level, cosmic ray bursts from cascade showers and from stars occurring in the wall (up to 10 prongs) and in the gas (up to 5 prongs) of the ion chamber have been observed.