1. A method for separating objects having a density exceeding a preselected density from objects having a density less than said preselected density comprising: floating said objects on a liquid having a density exceeding said preselected density; sparging a gas into a portion of said liquid in sufficient quantities to lower the apparent density of said portion to said preselected density so that said objects having a density exceeding said preselected density sink and said objects having a density less than said preselected density remain afloat; and separately removing said objects having a density exceeding said preselected density and said objects having a density less than said preselected density from said portion of liquid.

5. A method of separating articles into a plurality of groupings according to density comprising the steps of: introducing said articles into a chamber containing a liquid having a density exceeding that of at least some of said articles; removing any of said articles which sink in said liquid from said chamber as one of said groupings; sparging a gas into a portion of said liquid to adjust the effective density of said liquid to a value below the density of a second grouping of said articles but above the density of a third grouping of said articles so that said second grouping sinks leaving said third grouping afloat; separately removing said third grouping from the surface of said liquid; and causing said second grouping to move to an adjacent portion of said liquid being without sparged gas wherein said second grouping again floats to the top of said liquid for removal.

6. Apparatus for separating according to density a mixture of objects having a density exceeding a preselected density and objects having a density less than said preselected density comprising: a vessel containing a liquid having a density such that at least part of said objects having a density exceeding said preselected density will float thereon; means to place said mixture into said vessel; means to adjust the effective density of one portion of said liquid to said preselected density whereby said objects having a density exceeding said preselected density sink into said liquid and said objects having a density less than said preselected density remain afloat, said means to adjust the effective density comprising gas sparging means positioned within said vessel; and, means to separately remove said objects having a density exceeding said preselected density and said objects having a density less than said preselected density from said vessel, said means to separately remove said objects comprising a first removal means for collecting objects floating on said one portion of said liquid and means to move said objects having a density exceeding said preselected density to an adjacent portion of said liquid, being without gas sparged therein, wherein objects having a density exceeding said preselected density but less than the density of said liquid float to the liquid surface for removal from said liquid surface by a second removal means.