Flotation of Minerals. 569 



begins to increase at about 82 deg. C. up to 100 deg., the range 

 of temperature at which flotation usually takes place. Just 

 before the water began to boil some of the concentrates began 

 to tloa-t and the readino: could not be taJven. 



Behaviour inuler reduced pre>(>mre. 



Many substances when sunk in water will rise to the surface 

 when the pressure above the surface of the water is lowered. 

 Among the substances tried the following floated: — All the 

 metals in the form of foil, naphthalene, sulphur, graphite, mica, 

 and zinc blende concentrates. Particles of galena, zinc blende, 

 cerussite, river sand, and calcite came to the surface, but sank 

 again. When a mixture of particles of zinc blende and river 

 sand was put into a vacuum flask under water under reduced 

 pressure, it was noticed that the zinc blende came up more 

 persistently than the sand, which I'ose to the surface three or 

 four times, and would not do so again. The particles of mineral 

 are caused to float by gas bubbles attached to them carrying 

 them up to the surface. In some cases these bubbles are 

 strongly attached to the minerals. A burette was taken and 

 filled with mercur}' to within about an inch of the top. and 

 about 1 c.c. water was added on top of the mercury. A few 

 particles of different minerals were dropped through the water 

 on to the mercury, each having a l^ubble attached to it. A 

 rubber cap with a clip was then put over the end of the burette 

 and the lower end was connected up with a water pump. As 

 the level of the mercury w^as lowered the bubbles increased in 

 volume, ajid remained on the particles till the level of the mer- 

 cuiy was lowered considerably. Some zinc concentrates from an 

 acid float were immersed in water in a 10 c.c. graduated pipette, 

 and the expansion noted when the pressure was reduced above 

 the surface of the water. The greatest expansion noted before 

 the mineral began to float was 0.8 c.c, the original volume being 

 2 c.c. 



On some minerals bubbles will form, when subjected to re- 

 duced pressure, under water, but the minerals do not float, as 

 quartz, garnet, gypsum, haematite, wolfram, cassiterite and 

 glass. Pieces of metal in the form of foil will float even after 



