Flotation of Minerals. 569 



(using H^SOj and oleic acid) was immersed in boiled water 

 and boiled under reduced pressure for three hours, as much of 

 the air being pumped out as possible. All the mineral was 

 then on the bottom of the flask. The top of the flask was 

 then closed. On being brought into contact with the vapour 

 above the water, a film was again formed. When examined 

 with the aid of a lens the film was seen to be almost ent'irely 

 submerged, only fine points of the particles pi'ojecting above 

 the surface. A film of magnetic sulphide of ii-on on the surface 

 of water when attracted by a magnet drew up the surface of 

 the water with the mineral. Copper foil will not float in air- 

 free water under reduced pressure, but if allowed to come in 

 cont-act Avith the vapour above the water will float, similarly 

 to the film float obtained with minerals. 



Another sample of zinc concentrates from a film float was 

 floated on the surface of water in a flask under reduced pres- 

 sure, and exhausted as far as possible by a water pump. Most 

 of the particles forming the film sank, although the remainder 

 persistently floated. With a lens bubbles could still be seen 

 attached. It was frequently noticed that under reduced pres- 

 sure some mineral with oil would rise to the surface in agglo- 

 merated masses, and then spread out to form a film on the 

 surface. Sometimes a mass of sulphides clinging to a bubble 

 on coming to the surface and the bubble bursting, the sul- 

 phides would spread out as a film, the free surface of the water 

 taking the place of the surface of the bubble. 



Action of Acids and Oils in Film Flotation. 



The ores used in the following tests were freshly broken and 

 tested as soon as crushed, so as not to allow the surface of the 

 minerals to become altered. They were all crushed through a 

 40-mesh sieve, and flotation as a film was tried in water alone, 

 with oil, and with oil, acid and water. 



