"The first notice of percolation under pressure was given by Count Real, who constructed an apparatus known as Real' s solution-press or filter-press, and which consisted in a tin percolator surmounted by a tube which could be made 50 feet (15 m.) or more in length; after the fine, previously-moistened powder had absorbed all the menstruum provided for its exhaustion, the tube was filled with water, and the menstruum absorbed by the powder was thus forced out by hydrostatic pressure. The inconvenience of the long water-tube suggested its replacement by a column of mercury, which, communicating with a reservoir, forced the water through the powder Semmelbauer already conceived the idea of using compressed air for the same purpose, and a suitable apparatus for this purpose was constructed by Dr. Romers-hausen. The process was subsequently applied by Boullay to displacement under the pressure of a low column of liquid, as in the percolators at present in use. The necessity of saturating the powder with the menstruum without rendering it adhesive, and of firmly compressing the moistened powder, was early recognized as essential conditions for success. The same principle of percolating under pressure applies to the apparatus more recently constructed by Duffield, Stearns, and others, in which the air in the receiver may be rarified, and that contained in the percolator above the powder may be compressed. Rosenwasser's percolator (1881) is simply Real's press, with this addition, that the powder, by a screw arrangement, may be confined in any desired space, without the possibility of expansion on coming in contact with the bulk of the liquid percolating through it". - N. D.