Sulfide minerals are readily amenable to flotation, and the successful utilization of the complex sulfide ores of the northwestern part of the American continent has been made possible by the development of methods of selective flotation. Natural minerals, however, are not readily floated until the surface has been treated in some way to produce adherence of the mineral particle to the air bubble. To attain this result certain reagents known as collectors are added to the pulp, and of these, compounds of the xanthate type have wide application in dealing with sulfide ores.

The Liesegang phenomenon of periodic precipitation in gels lias iiot as yet, received a general explanation, and the four principal theories advanced have been applied with partial success only to specific systems