The plates, as before explained on page 55, are each formed with a dropping tube _O_ (see Fig. 23), which extends above the plate to an extent slightly less than the desired thickness of the layer of liquid on each plate, and with perforations each having an upwardly projecting rim, and each covered with a cap _A_. This rim and cap form a trap. The ascending vapors pass up through the perforations, down between the rim and the edge of the cap and thus out through the layer of wash contained on the cap. The wash remains constantly level with the top of the tube _O_, the excess running off through the tube _O_ to the compartment or plate beneath. To return to Fig. 22, the wash by the pipe _a_ enters the distilling portion of the column at the uppermost plate thereof and, as described above, drops down from plate to plate. A steam pipe _S_ enters the bottom compartment of the distilling portion of the column and the steam as it rises through the little traps, bubbles out through the layer of wash and in each compartment enriches itself with alcohol. Thus the rising column of vapor is constantly becoming richer and the downward current of wash constantly weaker until at last it passes away as spent wash at the very bottom of the column by the pipe _D_. The hot vapors, as before described, pass upward and enter the rectifying portion of the column _B_. This consists of a series of compartments having perforated bottoms and dropping tubes. The vapor passes upward through these perforations of the plates,--the condensed portion of it dropping back again on to the lower plates or on to the distilling plates to be again vaporized and concentrated and the more highly vaporized portion passing out at the top of the column through the pipe _E_ to the concentrator _G_. The concentrator consists of a tank containing water within which is supported a vessel _F_ having double walls. The interior of this vessel is likewise filled with water. Between the double walls and surrounding the coiled pipe _c_ passes the vapors from pipe _E_. At the bottom of the vessel _F_ is a compartment _f_ connected by a pipe _F¥_ with the upper compartment of the rectifying column. The less highly heated vapors will be condensed by the passage through the double walls of the vessel and the condensation will collect in the compartment _f_, and from there pass off by pipe _F¥_ back to the rectifying column, to be again vaporized and strengthened by the descent from plate to plate of _B_.