Cross flow trays usually installed in rectifying columns handling liquids free of suspended solids. The bubble caps consist of circular cups inverted over small vapor pipes. The vapor from the tray below passes through the vapor pipes into the caps and curves downward to escape below the rim into the liquid. The rim of each cap is slotted or serrated to break up the escaping vapor into small bubbles, thereby increasing the surface area of the vapor as it passes through the liquid.