Flotation of Mhierah. 567 



dried or partially dried. If some ground niineral is wetted 

 and covered with water, and then is exposed to the air by 

 t-ausing the water to run to one side of the containing vessel 

 and the water is then brought carefully back, some of the 

 mineral will be seen to float on the surface of the water. This 

 form of notation takes place to a greater or lesser extent with 

 most minerals, and is apparently a different phenomenon from 

 the simple floating of particles by dropping them on the sur- 

 face. In the latter case there is an almost continuous film of 

 air or gas surrounding the under side of the mineral, and by 

 far the greater portion of it is projecting out of the surface of 

 the water. That there is not a continuous film, and that the 

 particle is partly wetted can be shown by floating some magnetic 

 substance, as pyrrhotite, or a piece of iron wire, and attracting 

 it with a magnet, when the surface of the water will be seen 

 to have been dragged up with the mineral or wire. When the 

 substance is finally lifted out of the water, water is carried up 

 on to the magnet. In the case of the wetted mineral being 

 floated by being brought into contact with the air, the particles 

 will be seen to be almost entirely submerged, only a portion 

 like a pin's point being above the surface, and, although a dis- 

 tinct depression of the surface is caused, it is not so apparent 

 as in the other case. 



When the minerals are made to sink they apparently carry 

 a bubble of some gas down with them. This bubble can be seen 

 as a rule on most particles. When the finer-grained material 

 is heaped up on the surface (as in the case of the zinc concen- 

 trates referred to) till the weight is too great, the whole mass 

 causes the surface of the water to sag; and finally to break 

 through, and carries a large amount of air down with it, the 

 inside of the envelope being quite dry. 



Hot loater. — In hot water the behaviour of the minerals and 

 metals is dili'erent. Most of them wall float but for a few seconds 

 only, becoming wetted and sinking, usually a bubble of gas 

 leaving each particle as it sinks. The fine grained material 

 can be heaped up similarly, as with cold water, a large number 

 of bubbles being observed on the under surface of the mass. 



