leaves are submerged in water, but if necessary, the operation may be performed with the box empty.

This dropping of the cakes into an empty box, however, is not so satisfactory but can usually be accomplished by using a large volume of low pressure air suddenly admitted to the interior of the leaves. The use of water for this purpose does not answer since it at once throws off a line of cake along the bottom of the leaf and thus obtains an exit so that the water level never rises within the leaf and the upper two-thirds of the cake remains undetached. The best way to effect a discharge of the cake into an empty box is to use a specially constructed leaf having a perforated pipe along its upper edge inside the canvas and attached to the wooden header, one end being closed by a plug and the other passing out through the header at the opposite end from that of the usual vacuum pipe. For dropping the cake water is admitted through the upper pipe and compressed air through the lower or usual suction pipe, the result being that the water flowing from above moistens the canvas from top to bottom, acting as a lubricator and rendering the cake easily detachable by the compressed air acting uniformly over the whole interior of the leaf.

If the leaves are kept in good repair and thoroughly pervious to the solution, this filter gives good service not merely as a dewaterer but also as a means of removing the rich pregnant solution from the residue, so that with medium grade ores no washing by decantation is needed before filtration.

The principal drawback lies in the time and expense consumed in repeated transfers of pulp, solution, and water, into and out of the container. There is also a tendency to salt the barren-wash and water-wash storage tanks making such washes less effective for their purpose, but the latter drawback can be largely overcome by using what is known as the complete gravity system and having a separate pump and pipe line each for pulp, solution, and water wash.

At the mill of the Butters Divisadero Company owing to the excessive deposition of lime in the filter canvas and the high cost

of muriatic acid for cleaning, the writer introduced a different form of leaf that met the special circumstances very satisfactorily. It consisted of a wooden frame on which the cloths were fastened by merely driving in thin wooden cleats. The centre of the leaf was formed of a sheet of cocoa matting and the filtering medium was good quality twill cotton sheeting. No stitching was used but the whole was held together by vertical strips of wood about ten inches apart and easily removable. After running about a month each leaf was removed and instead of being acid treated was entirely dismantled, an operation requiring about twenty minutes. The old filter medium was thrown away and the cocoa mat put out to dry, to be subsequently beaten to free it from deposited matter and later to be inserted in a new leaf. The leaf in hand was then refitted with new sheeting and a clean mat and clamped up again for replacement in the filter box. The cost of the new sheeting was found to be considerably less than the former cost of acid for treating the canvas and in addition to this the leaf after renovation was far more efficient than the old acid treated leaf.

There seems to be an opening for the use of such a leaf in localities where muriatic acid is expensive or difficult to transport and also in cases where the deposition of lime is excessive and accompanied, as it was at Divisadero, by substances much less soluble than lime in muriatic acid.

The Moore filter is similar in principle to the Butters but in this case the leaves are made removable and are transferred successively from the pulp box to the wash solution and from there to the water wash and finally to the dumping hopper. This is done by aggregating a number of filter frames into a unit or basket and raising, transferring, and lowering by means of an electric overhead crane.

The Oliver Filter. This consists essentially of a slowly revolving drum, the lower half of whose circumference is submerged in the pulp to be filtered. The filtering medium is applied all around the periphery, which is divided up into a number of separate segments, more or less isolated from one another.

The vacuum is applied through the axle bearing by means of a header valve which is so arranged that at a certain point in the revolution of the drum the vacuum connection is severed and

compressed air admitted under the filter medium which serves to detach the cake and allow it to drop onto a metal deflector or scraper, whence it slides down to the residue discharge launder. As the newly formed and adherent cake rises from the bath of slime pulp it comes under the action of a number of atomizing spray nozzles by means of which the wash liquor is distributed