"To begin percolation, the rubber tube is lowered and its glass end introduced into the neck of the bottle previously marked for the quantity of liquid to be percolated if the percolate is to be measured, or of a tared bottle if the percolate is to be weighed; and by raising or lowering this recipient the rapidity of percolation may be increased or lessened as may be desirable, observing, however, that the rate of percolation, unless the quantity of material taken in operation is largely in excess of the quantities, shall not exceed the limit of ten to thirty drops in a minute. A layer of menstruum must constantly be maintained above the powder, so as to prevent the access of air to its interstices, until all has been added or the requisite quantity of percolate has been obtained. This is conveniently accomplished, if the space above the powder will admit of it, by inverting a bottle containing the entire quantity of menstruum over the percolator, in such a manner that its mouth may dip beneath the surface of the liquid, the bottle being in such shape that its shoulder will serve as a cover for the percolator, (illustrated in Fig. 390).