To assist in the development of such an important step in metallurgy is worth any effort that may be made, and it seems that accumulation of the results of practical and theoretical investigations that have been carried out by many men at many places is an important aid to further development. My part in the furtherance of ?otation is this compilation, which presents the information about the process that I have been able to ao quire by practice 'and consultation. To simplify as much as possible has been a primary object and I have endeavored to eliminate most of the speculation that has encumbered the litera ture of the process up to this time, retaining only those parts that have been definitely proved correct, or can show a reasonable basis for their inclusion. No one can claim, at this time, to know enough about ?otation to pose as a master of the art. Any attempt to be complete must include the work of many individuals and the ideas of many more. I make no claim of originality for the facts put forth in this book. They have been gathered where they could be found and have been used as has seemed most appropriate. The origi nal volume of Theodore J. Hoover has been referred to often.