This paper deals with the theory underlying the flotation process which is important in the separation of ores. Attention is called to a theory of adsorption and surface tension which greatly aids understanding of flotation phenomena, and which therefore should prove useful in the understanding of flotation. The author adopts the view that condensation, crystallization, surface tension, adsorption, etc., are manifestations of forces of the same kind as those involved in the formation of chemical compounds, and explains flotation by reference to these forces. In a second section of the article a series of experiments with oil films on various surfaces is described. A brief section at the ends returns to the theory of flotation, referring to the implications of his theoretical discussion and experimental results to the formation of froth, the oiling of the solid, and the contact angle of solid particles.