It has been found that in the case of certain ores, residuum oil separates sulphides and metallic particles from earthy or stony materials without reference to their respective densities. In the Elmore concentrating process, which was introduced at Glasdir, near Dolgelley, in Wales, the ore is crushed in Huntington Mills, and the pulp mixed with oil in a revolving drum. The mixture is discharged into circular vats where the water and sand subside, leaving the oil carrying the pyrites floating on the top. The oil is then pumped into a “hydro-extractor,” which is run at 750 revolutions per minute, and the oil separated from the ore. The amount of oil added is equal to the weight of the ore, and the loss of oil is about two gallons per ton. The oil residuum used has a density of about 0.9, and is made more viscous, if necessary, by the addition of paraffin wax or similar materials. The oil will carry from 10 to 20 per cent, of mineral when it is fully charged. The cause of the selective action exerted by the oil is not known. It is suggested by H. F. Collins that a smooth lustrous non-absorbent particle is wetted by the oil, and adheres to it, whereas the oil will not adhere to a rough porous particle when it is soaked with water. He found that an ore containing copper and iron pyrites, and consisting mainly of crystallised minerals, wollastonite being the most abundant constituent, did not lend itself readily to oil concentration. At Glasdir, the average saving was 69 per cent, of the gold, 65 per cent, of the silver, and 70 per cent, of the copper, with a rate of concentration of about 14 to 1.