The rich and highly vaporized vapors which have passed the test of this preliminary concentration, pass out of the compartment _f_ by a pipe _M_. Here again the water surrounding the pipe tends to condense all but the most highly charged vapor and send it back to compartment _f_ but the vapor which succeeds in passing over through pipe _G_ is carried downward to a condenser _H_ where it is finally condensed and drawn off as at _g_. It is necessary that the rate of mash feed be regulated so that neither too much mash shall be pumped into the mash heater _G_, or too little, and the pipe leading from the pump to the heater is therefore provided with a tap and an indicating dial. In these modern stills the following are particularly important points to be especially brought to the consideration of the distiller. It cannot be too strongly impressed that effectiveness of the distilling column depends on the plates dividing it,--that is, upon the horizontality of the plates and the form of the traps or perforations. If the plates are not horizontal the wash is not maintained at a uniform level across the entire extent of the plate and hence some of the ascending vapor will pass out without contacting with the wash through uncovered traps, while others of the traps will be so deeply submerged in wash that the vapor cannot bubble through. Again the caps should be so made as to divide the vapor into fine streams and bring it into contact with each part of the wash. Plates simply perforated and uncapped give excellent results for they molecularize the vapor ascending through the liquid contained on the plates, but they require a constant pressure of vapor, and any variations of pressure tends to discharge them. In addition these perforations gradually enlarge by the action of acids in the wash or clog up, and the apparatus soon works badly. Good forms of capped traps are those shown in Figs. 24, 25 devised by Barbet. These are provided with an interior upwardly projecting rim. Extending over the rim and down around it is a copper cap having its margin slitted. The wash carried on the plate circulates about the caps and the alcoholic vapors bubble out through the slits and up through the wash, the vapor thus being finely divided and coming into intimate contact with each portion of the wash and thus more thoroughly depriving it of its alcohol. [Illustration: FIG. 24.--Barbet Trap.] [Illustration: FIG. 25.--Barbet Trap.] Besides this there is another advantage resident in these caps, namely, that distillation may be stopped for several hours and then re-started without trouble for the reason that the wash has been retained on the plates, whereas were the plates simply perforated the wash would ooze through and the plates have to be recharged. This form of plate may be easily repaired and does not necessitate the removal or replacement of the plate itself. The caps alone need be removed.